Field meetings to the Western USA 1981 & 1982

SPECIAL REPORT Field Meetings to the Western USA 1981 & 1982 The Directors: K. L. Duff & T. D. Ford

DUFF, K. L. & T. D. FORD. 1984. Field Meetings to the western U.S.A. 1981 and 1982. Proc. Geol. Ass., 95 (2), 97-148. The Association's first field meeting to the United States took place during September 1981 and was repeated (with some changes in the itinerary) in JulyfAugust 1982. This report gives details of the itineraries which were followed and which covered over 3,000 miles in the geologically-varied states of California, Arizona, Utah, Colorado and Nevada. Localities visited included Oak Creek Canyon, Meteor Crater, the Painted Desert, Petrified Forest, Sunset Crater, Grand Canyon, Mesa Verde, San Juan Mountains, Monument Valley, Bryce Canyon, Zion Canyon, Death Valley, Mono Craters, Yosemite and Point Reyes. A general geological introduction to the area prefaces the field meeting reports, and is followed by detailed itineraries which it is hoped could be used by individual members of the Association in the future. The itineraries provide route details, information on the geology to be seen while travelling and specific details of geological and geomorphological features to be seen at the various stops. Suggestions on accommodation are given in the itineraries.

K. L. Duff, Nature Conservancy Council, Foxhold House, Thornford Road, Crookham Common, Newbury, Berkshire RGI5 BEL T. D. Ford, Department of Geology, The University, Leicester LEI 7RH

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K. L. DUFF & T. D

1. INTRODUCTION Late in 1980, at the Field Meetings Committee Meeting which was considering the programme of meetings for 1981, it was suggested by our then President, Professor Alec Smith, that the Association should consider the feasibility of a 'geological holiday' in the western United States, designed to combine geology and sightseeing. The idea received enthusiastic support and, after detailed investigations by Dr. Duff, then Field Meetings Secretary, it was decided that such a meeting should take place during the late summer of 1981. As demand for places on the trip proved greater than could be satisfied in 1981, it was decided to repeat the tour in the summer of 1982, so that members bound by school holidays could take part. The 1981 trip was led by Keith Duff, and the 1982 trip by Trevor Ford assisted by Tony King, who acted as Local Secretary. The two tours were basically identical, except that the 1982 visit lasted 22 days instead of 17, and included an extra portion in the middle part of the trip, in southeast Utah and southwest Colorado. The 1981 visit took place between 16 September and 2 October, whilst the 1982 visit ran from 30 July to 21 August. The two trips each

FORD

covered about 3,200 miles, and took in such famous sights as Oak Creek Canyon and Sedona, Meteor Crater, the Painted Desert, Petrified Forest, Sunset Crater, Grand Canyon, Mesa Verde, San Juan Mountains, Monument Valley, Bryce Canyon, Zion Canyon, Death Valley, Mono Craters and Lake, Yosemite and Point Reyes. It also included the cities of Los Angeles, San Francisco, Las Vegas and Phoenix. The 1981 party (Fig. 1) consisted of 43 members, who travelled by Laker Airways to and from Los Angeles, and then used hired Greyhound and Trailways buses to convey them around the 3,200 mile loop to San Francisco, before flying back to Los Angeles. It was necessary to use two bus companies, since Greyhound refused to travel through Death Valley at that time of year-a decision which later proved very wise! Of the two, however, Trailways were far more efficient and courteous, and gained the general approbation of the party. The 1982 party contained 22 members, and flew to Los Angeles by British Airways, Laker having gone into liquidation in the meantime. After two nights in the city they flew on to Phoenix, where four self-drive hire cars were collected and used to convey the party to San

Fig. 1. The 1981 party at Mather Point, Grand Canyon.

USA FIELD MEETINGS 1981 & 1982

Francisco, from where the party flew back to London. The hire cars were fitted with CB radios, to enable communication between cars and to allow all members to hear the leader's running commentary en route; it also enabled some drivers to engage in colourful conversations with the local populace, to the general enlightenment of both! The drivers were Trevor Ford, Tony King, Ron Roberts, Dan Griffin and Keith Mantle. Several wives acted as reserve drivers and took the wheel from time to time. The geological structure and history of this part of the United States is varied and spectacular, and it is not possible in the course of this report to go into these aspects in very great detail. Numerous good and comprehensive text-books and guides exist on this subject, and we seek here only to give an overall impression of the geology of the region, together with information on particular features of note. More detailed guides were prepared by Keith Duff for the 1981 tour and by Trevor Ford for the 1982 visit, whilst the GA library contains a good selection of books, guides and maps relating to the western United States. In the report which follows, the daily itineraries are combined for that portion of the route which was common to both parties, whilst the different parts of the routes followed by the 1981 and 1982 parties are set out separately. As a result, we have not used calendar dates in the report, but have divided the itinerary into geographic sections (e.g. Phoenix to Petrified Forest). Where the 1981 and 1982 routes differed, we have indicated which tour took which route. For the help of members who may like to follow all or part of these itineraries in the future, we have included the names of the hotels or motels at which we stayed, and which we recommend, with one possible exception as noted later. The report consists of a general description of the overall geological context of the western United States, defining and describing the main physiographic provinces into which it may be divided, and relates these to features which were seen during the visits. This is followed by more detailed information on the itineraries, together with descriptions of the main localities. Although the report is intended to be largely self-contained, anyone wishing to follow it personally may obtain greater benefit if they read it in conjunction with the appropriate geological highway maps produced by the American Association of Petroleum Geologists, and with more detailed geological maps which are widely available in the U.S.A. In addition, a wide range of maps, guides, pamphlets and books on geology and natural history are available at all the Visitor Centers in the National Parks and it is recommended that such sources be used to supplement the information given in this report; the material available at the Visitor Centers is usually very well-produced, comprehensive and inexpensive. Reference to books or papers which provide further details of specific sites or areas appear in the reference

99

list at the end of this report, which also contains general references not otherwise mentioned. 2. THE OVERALL GEOLOGICAL CONTEXT A geological map of North America reveals that the continent falls readily into three major geotectonic units (Fig. 2), which constitute a central shield and platform, flanked to the east and west by mountain belts. Our itineraries lay in the southern part of the western mountain belt, usually known as the Pacific Cordilleran Region. The southern Cordilleran region can be sub-divided into four major 'Provinces', each distinguished by distinctive physiographic and geological characters, with a further four smaller subdivisions of the Pacific Cordilleran Province being shown to the west of the Basin and Range Province. Our routes took us over three of the Provinces and the 1982 tour just touched the Rocky Mountains Province. Study of the Pacific Cordilleran Region reveals that the western U.S.A. has been the site of diverse geological events ranging from the Precambrian to the present day. During the Palaeozoic, a major north-south-trending geosyncline occupied the area, and was subjected to progressive deformation in later Palaeozoic times. Further sediments were then deposited in Mesozoic seaways, with marine sedimentation being terminated by orogenic activity in late Mesozoic and early Tertiary times. These events are frequently referred to as 'Laramide' in the Rocky Mountains area, and 'Nevadan' in the Pacific coastal region, although it is now believed that there is some overlap in time between the two. These younger deformations, together with subsequent erosional, depositional, volcanic and tectonic events, have determined the visible physiographic pattern of the western U.S.A., and obscured much of its earlier history. This complex geological history has arisen because, since late Precambrian times, the western margin of the continent has been the leading edge of the American plate which has gradually and obliquely overridden the complementary mid-ocean ridge, the East Pacific Rise. This has led to repeated subduction alternating with very extensive shearing along NNW-SSE faults, of which the San Andreas Fault system is currently active. The Cordillera thus consists of a series of small 'plate-lets' or terrains which have been shuffled laterally as well as being deformed by subduction and obduction, so that many of the terrains are no longer in juxta-position with what were the associated parts of the relevant structural complexes. The terrain concept is a new one only recently proposed (Coney et al. 1980; Dickinson, 1978) and far from fully understood as yet, particularly in the southern part of the Cordillera. (a) The Pacific Cordilleran Province This consists of a number of different orogenic belts, containing intensely deformed rocks ranging in age

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Fig. 2. Principal geological units of the western United States, with the routes of the two Geologists' Association tours in 1981 and 1982. LA-Los Angeles; Ph-Phoenix; GC-Grand Canyon; Du-s Durango; LV-Las Vegas; Y-Yosemite; SF-San Francisco.

from Precambrian to Cenozoic; extensive areas of igneous intrusions, mainly Mesozoic, also occur. The current form and tectonic fabric of the area has been determined by Cenozoic deformation and volcanism, which have obscured much of the earlier deformation. Physiographically, there are two main mountain

zones, a rugged and seismically active belt-the Coast Ranges-and an interior high relief belt comprising the Sierra Nevada and the Cascade Mountains. Between the two belts are lowland troughs filled with late Cenozoic sediments or lavas, known collectively as the Great Valley. The Coast Ranges are split

USA FI ELD MEETINGS 1981 & 1982

longitud inally by the San Andreas Fault System , a dextral strike-slip with highly active contemporary seismicity. Movements totalling some 375 miles have taken place since the Miocene. (i) The Coast Ranges The bulk of the Pacific Coast Range s consists of strongly deformed sediments and meta sediments , depos ited in a series of N-S fore-deep trough s and island arcs bordering the North American craton and initiated in Precambrian times. Sedimentation in this basin was essentially continuous throughout much of the Palaeozoic and Mesozoic , but with major phases of tectonic deformation and volcanism having occurred in the Precambrian, late Palaeozoic, Perm o-Triassic and late Mesozoic. In Cretaceous times, there was regression leaving only a relict sea , but marine deposition was resumed in the Cenozoic in California, before being terminated by Miocene deformation and wrench faulting , which also had major tectonic effects in the Sierr a Nevada and the Basin and Range Province . The Coast Ranges contain the most interesting and spectacular stru ctures in the entire region , owing to the intensity and youthfulness of the shearing which took place along a plexus of broad NW-SE tran scurrent fault zones . The y also contain the famous 'Franciscan Complex ' , a very widespread monotonous mixture of thick greywackes, basaltic-spilitickeratophyric lavas, bedded chert, and their metamorph osed equivalents. The whole comple x has the appe arance of a tectonic melange , comprising a mixture of originally more or less separate rock suites , deposited upon oceanic crust. Dominated by greywackes, terrains of these unfossiliferous rocks of uncertain age have been brought into juxtaposition with othe r terrains of similar rock suites by the many wrench faults in such a way that the original relationships of anyone terrain are now obscured. The Franciscan Complex is generally regarded as being no more accurately datable than 'Mesozoic' , though a few section s have yielded microfossils of various ages from late Tri assic to Cretaceous. The intim ate mixture of terrains has been caused by subduction of the Pacific Ocean Plate beneath the continental margins in Mesozoic and earl y Cenozoic times. Associated with the thrust and wrench fault zones is a melange of sheet-like masses of ultramafic rocks such as serpentinites , which are probably parts of the mantle stru cturally interleaved with the Franciscan greywackes durin g plate interaction. Most of the shearing in these melan ges apparently predates the Cenozoic San Andreas Fault System , which has been superimposed on the Franciscan structures. It is now believed that the San Andreas Fault is an immense tr ansform fault , connecting the northern end of the E ast Pacific Rise spreading axis, located near Baja Californi a , with its

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continuation west of Oregon and Washington, some 950 miles to the north. (ii) The Great Valley This very marked topographic basin is now largely the flat alluvial plain of the San Joaquim Valley in central Californi a, which was an island sea during most of the Cenozoi c, and contains several thousand feet of marine Pliocen e, Miocene and Eo cene sediments beneath a surface cover of Quatern ary deposits. At the eastern limit of the region these sed iments overlap onto the western flanks of the Sierra Nevada . As a result of the recognition of the 'suspect terrain' concept (Coney et al. 1980) a closer look at the deep structures of the Great Valley is being undertaken by oil geologists and a complex of fold and fault-bounded structures, some containing oil, has been found in the earlier Tertiary rocks beneath the cover of almost undisturbed later Tertiary and Quaternary sediments.

(iii) The Sierra Nevada The Sierra Nevada is a large block-faulted mountain range , with the block broadly tilted toward s the west , showing a relatively gentle western face (deeply dissected by canyons ) and a precipitou s fault-scarped east face overlooking Owens Valley and the Mammoth LakelMono Craters area. The mountains consist of deformed and metamorphosed Palaeozoic , Mesozoic and early Cenozoic rocks, laid down in the axial zone of the Cordillera, and intruded by a complex series of major deep-seated igneous bodies. The main phase of deformation took place in the late Mesozoic (the Nevadan Orogeny), and the batholithic intrusions of the Sierra Nevada seem mainly to postdate much of the deformation . Most of the intrusions are granites, quartz diorites and granodiorites, and constitute a comple x series of different igneou s masses, emplaced at different times throughout the later part of the Nevadan Orogeny. Recent radio-m etric dating of the granitic domes has reve aled that they range in age from late Tri assic to late Cretaceous and thus they can be regarded as diapirs rising at intervals from a long-lived subducti on zone throughout the Mesozoic. Many xenoliths and metamorphic inclusions occur within the batholiths, and extensive areas of metamorphics flank the western side of the intrusive belt. Subsequent to the phase of batholithic intru sion the roo f rocks have been removed by powerful erosion . Tertiary lava flows and associated volcanics cover exten sive areas toward s the north ern end of the Sierra Nevada , with volcanoes and lavas domin ating the terrain in the Cascade Mountains to the north . Tertiary volcanics are also welldeveloped toward s the eastern edges of the massif, to the east and north of Yosemite. Inde ed , acidic eruptive vulcanicity has occurred in Mono Crate rs almost into historic times, and seismic activity hints that sub-surface intrusion is continuing.

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K . L. DUFF & T. D . F O RD

(b) Basin and Range Province This occupies a vast area of the arid to semi-arid region of the western U.S.A., and is characterised by a landscape of sub-parallel elongate mountain ranges alternating with valleys floored by Quaternary and late Cenozoic sediments. The ranges are mostly between 40-90 miles long, 3-6 miles wide at their bases, and are spaced at about 6-25 miles from crest to crest. In southwest Arizona , basin deposits have encroached upon the ranges , and erosion has reduced many of them to small remnants of once more extensive mountain masses. Shallow playa lakes intermittently pond in the undrained basins during periods of heavy rain, and salt-encrusted salinas develop locally, as in Death Valley . In some places, dunes occur on the lee side of basins and migrate upslope. The southern part of the province is drained by major rivers rising in the Rocky Mountains or along the rim of the Colorado Plateau, and these include the Colorado River. The province has a very characteristic structural pattern, largely dating from the late Cenozoic times , which consists of NNW-SSE trending fault blocks, giving rise to a series of uplifted mountain range s and downfaulted basins . The fault pattern is characteristic of an area under structural tension , the main direction of stretching being in an east-west direction , and normal faulting predominates. This belt of rifting extends from the Rocky Mountains to the Sierra Nevada , with geophysical evidence indicating that crustal thinning is well-developed within the province . The province is also characterised by high heat flow and much seismicity . The rocks present within the upfaulted mountain ranges are parts of terrains which range in age from Precambrian to Miocene and represent the remnants of Palaeozoic and Mesozoic orogenic belts which are no longer coherent tectonic entities. Study of the structural units within the upfaulted mountain ranges indicates that there was a complex and long Precambrian history within the source area, with early Precambrian gneisses, schists and granites being overlain unconformably by extensive deposits of late Precambrian and Cambrian quartzose sediments with subordinate dolomites and a tillite-evidence of a late Precambrian glacial episode. At least three major episodes of metamorphism and deformation have been recognised. During the Palaeozoic, the Cordilleran terrains accumulated great thicknesses of shale , chert and quartzite in western Nevada, and carbonate rocks in eastern Nevada and western Utah ; two major phases of deformation , in the middle and late Palaeozoic, have affected these rocks. The early Mesozoic was marked by a widespread marine transgression and marine sedimentation dominated throughout the era . However , in Middle Jurassic and again in Lower Cretaceous times , the western part of

the Great Basin (see below) was intensely deformed , and an episode of batholithic intru sion took place. Few Cretaceous rocks now remain , although some volcanics were extruded during this period , accompanied by more deep-seated intrusions. Towards the end of the Mesozoic , and extending into the Cenoz oic, Laramide orogenic movements caused thrust faults to develop in a belt running from southern Nevada through western Utah , over which eastward plate movement took place, with local reactivation of ancient faults resulting in reversed displacements partly cancelling the earlier throw. Post-Laramide sedimentation seems to have been on a very small scale, only being represented by local accumulations of terrestrial deposits and eruptive volcanics. A major surge of andesitic volcanism during the late Eocene seems to indicate a fundamental change in the upper mantle beneath this province. This volcanism lasted for about 20 million years and terminated abruptly approximately 18 million years ago, in the early Miocene. It was followed, starting about 17 million years ago , by a phase of basaltic volcanism over large parts of the region , which heralded the phase of crustal fragmentation into the blocks which were to become the Basin and Range block mountains and basins . The basins formed by this process have , in most cases , accumulated vast thicknesses of poorly consolidated sediments of late Cenozoic age ; geophysical studies have indicated the presence of over 10,000 feet of such sediments beneath the floor of Death Valley. Most of these sediments are derived from the adjacent ranges, with fan and playa lake deposits being particularly characteristic. Volcanic activity has also been abundant , especiall y in the north and west of.the province . The Great Basin , mostly lying in Nevada , is a particularly large intermontane basin , with totally internal drainage. Southwards, in Arizona and New Mexico, the fault structures tend to be rather older (early Tertiary), the mountains are more subdued and drain to the sea. Although, over most of the province, adjacent fault blocks appear to have moved mainly vertically relative to each other, lateral movement (especially strike-slip faulting) is especially pronounced in the southwestern Great Basin area and in the Mojave Desert, and is known to have right-lateral strike slip of over 375 miles. (c) The Colorado Plateau Province This is a major intermontane province between the Rocky Mountains and the Basin and Range Province. It occupies an area of approximately 181,000 square miles (about twice the area of Great Britain) , and is generally at a relatively high altitude , with individual plateaux reaching between 5,000 and 10,000 feet. The

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USA FIELD MEETING S 1981 & 1982

individual plateaux are nor mally separated by scarps ('cuestas') formed in part by canyon -cutting and the stripping-back of less resistan t layers, and partly by displacement along major faults. In general terms, the Colorado Plateau consists main ly of Palaeozoic and younge r sedimentary rocks which are nearly horizontal, but which nevertheless have been uplifted , faulted , and in places subjected to monoclinal folding above reactivated basemen t faults. Volcanicity has been especially common towards the margins of the plateau, whereas structural upwarps are scattered throughout. Igneous structures presen t include volcanoes, volcanic necks, lava-capped mesas and complex intrusive laccoliths . The Colorado River system has deeply dissected the region, resulting in a netwo rk of scenic canyons separating plateaux. The western margin of the Province is still seismically active and moder ate ea rthquake shocks are quite frequent. The main structural grain of the plate au region is nort h-south. The area of greates t uplift and denudation is the Grand Canyon section , in the sout hwestern part of the Province , which is

characterised by plateau features on late Palaeozoic rocks (mainly Permian ), modified locally by Tertiary volcanic feat ures such as the San Francisco Peaks (Upper Tertiary and Qu atern ary) in the Flagstaff area. South of the Grand Canyo n the plateau surface is fairly uniform ly at the level of the top of the Permian Kaibab Limestone, and the southern margin of the province is marked by a pronounced escarpment known as the 'Mogollon Rim' . To the north of the Grand Canyon section lie the high plateaux of Utah , where the strata dip gently northwards with steps preserved on the more resistant Mesozoic rocks; the culminati on of this 'staircase' is found in Tertiary sediments which cap the highest levels , notably in northern Utah and in the Br yce Canyon areas. Fa ults, many of which continue nor thwards from the Grand Canyon section, separate the plateaux, and give rise to linear valleys. Erosion alon g the margins of the plateaux has prod uced the spectacular scenery of the Utah high plateaux, especially Zion and Bryce Canyons . The province has been an area of tectonic stability

TABLE 1. Generalised stratigraphic sequence in the southwestern part of the Colorado Plateau (with an outline of the main lithologies and depositional environments). Geograp hic distribution of units is shown by vertical bars Eocene

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since late Precambrian times , two main tectonic phases having been recognised within the Precambrian rocks , an early phase of folding , metamorphism and the intrusion of granites and pegmatites at about 1,700-1 ,400 Ma , and a later phase of major faulting and block tilting about 700 Ma . A major unconformity separates the Precambrian basement from the 'platform' deposits above , with Cambrian sandstones and marine shales giving way to marine carbonates. Channels cut into these are filled with Devonian dolomites, thickening to a massive carbonate sequence westwards. Unconformities bound the massive Redwall Limestone of Mississippian age both above and below. Pennsylvanian fluvial red sandstones and Permian dolomites complete the Palaeozoic. The Mesozoic and Lower Tertiary are dominantly clastic sediments, with late Triassic vertebrates and plants preserved in the Petrified Forest, and coals in the Upper Cretaceous in several downwarped basins (Table 1). After the long absence of igneous activity it recommenced in the Miocene with the intrusion of laccoliths of intermediate rocks (diorites and monzonites), followed by late Miocene and more recent activity which included vast quantities of basaltic lavas , and great accumulations of intermediate and silicic tuffs and lavas, together with the intrusion of local plugs and necks of minette , trachybasalt and monchiquite. The basalts are normally the olivine tholeiites typical of the western U.S.A. , and are probably related to spreading phenomena centred beneath the Basin and Range Province . Scattered kimberlite plugs bring up xenoliths of high grade basement metamorphics.

3. ITINERARIES (a) Los Angeles The 1981 party spent two days here and the 1982 party one , the 1981 group staying at the Gala Intowne Motor Hotel and the 1982 group at the Figueroa Hotel, adjacent to each other on South Figueroa Street. No formal fieldwork was arranged, but members took the opportunity to visit such attractions as Disneyland (with its memorable mine train and dinosaur replicas) , the Universal Film Studios (where members came face to face with 'Jaws'), Marine World and La Brea Tar Pits. Just off Wilshire Boulevard crude oil ('brea') still oozes out of the ground . The tar pits contain vast numbers of Pleistocene and Recent fossilised vertebrates, including saber-toothed tiger , mastodon , giant ground sloth , giant vulture , elephant and camel. The adjacent George C. Page Museum house s the collections of fossils made from the pits (over H million specimens!) , as well as reconstructed skeletons , and provides an opportunity to watch palaeontologists at work cleaning specimens.

Other members of the 1981 party visited the Natural History Museum (in Exposition Park) , the San Andreas Fault Zone in the Palmdale area , or paddled in the Pacific at Santa Monica , finding many similarities to Blackpool! In addition, several members visited the publications section of the US Geological Survey (in the Federal Building on North Los Angeles Street) and the California Division of Mines (Room 1065, 107 South Broadway) , where geological maps , books and guides were bought. The 1981 party were delighted to be welcomed to Los Angel es by an ex-member of the Association now teaching at the California State University at Long Beach. Professor John Dennis, an American who used to be a student at Imperial College, spent two hours outlining the geology and scenic attraction of the Los Angeles area , and provided members with specific suggestions of how best to make use of the limited time which was available to them. He also proved to be a shrewd source of information on local restaurants! (b) Los Angeles to Phoenix The 1981 party spent the first field day driving 400 miles from Los Angeles to Phoenix , Arizona, by chartered Gre yhound bus . The route was via Interstate highway 10 (110) across low ground of the Coast Ranges as far as Indio, where the trace of the San Andreas Fault was crossed (the eye of faith was needed here !). The remainder of the day was across the somewhat monotonous topography of the Basin and Rang e Province , with Jurassic intrusives , Jurassic and Triassic metamorphics and Precambrian intrusive rocks forming the main range areas , whilst Pleistocene and Recent alluvial sediments occupy the basins. The party also became familiar with the abundant and diverse forms of large cacti which characterise the desert in this area, principally the saguaro and the cholla and the thickets of creosote bush . The classic desert 'washes' , wide dry river courses which become torrents during the infrequent phases of heavy rainfall, were frequently seen, the strength of the torrents being indicated by the substantial nature of the bridges which carried the road over them. Entering the out skirts of Phoenix a marked change in scenery was apparent , as the dry desert gave way to green fields of cotton , lettuce , melons, vegetables and alfalfa, with groves of oranges, grapefruit, lemons and olives. This was clearly seen to have been made possible by an intensive system of irrigat ion which had reclaimed this land from the desert. The part y spent the night in the Holiday Inn Central in Phoenix, a well-appointed hotel with a much appreciated swimming pool. In spite of an early start the following morning , four of the party rose at the crack of dawn to visit the Desert Botanical Garden in Papago Park. This involved high-speed taxi rides and convincing the

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USA FIELD MEETINGS 1981 & 1982

park staff that they should be allowed in three hours before it officially opened! The 1982 party saved a day by flying direct from Los Angeles to Phoenix, and then collecting their hire cars for the remainder of the journey. Their overnight stop, later that day, was at the Kings House Hotel, Flagstaff.

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(c) Phoenix to Petrified Forest The route out of Phoenix on 117 took us across the northern flank of the Sonoran Desert part of the Basin and Range Province, towards the Colorado Plateau. After some 50 miles of scattered outcrops and road-cuts of Precambrian metamorphics locally intruded by granite bosses and veins, an extensive cap of Tertiary basalts gave rise to long road-cut sections in lavas and tuffs. A long descent through these led into the Verde Valley, floored with later Tertiary and Quaternary limestones and subordinate evaporites, which accumulated in a lava-dammed depression until the dam was breached in early Pleistocene times (Fig. 3). A short diversion led to the Indian ruins of Montezuma's Castle (neither a castle nor anything to do with Montezuma!) where an alcove in a cliff of Tertiary lacustrine limestones was utilized by the Sinagua Indians about 1200 A.D. for building a complex of 'safe' storage rooms for grain and for retreat during inter-tribal strife. A detour west of 117, via State 179 on to US 89A, passes through Sedona and Oak Creek Canyon, one of the most scenic parts of the Mogollon Rim, used extensively for making 'western' films. Approaching Sedona from the south, the airport occupies a prominent position on top of a lava-capped mesa, whilst to the west of the town lies the well-known landmark, the Coffee Pot, formed of Pennsylvanian Schnebly Hill Beds. The red sandstone buttes and mesas here form part of the Mogollon Rim of the Colorado Plateau and a section through the Rim from the Verde Valley to the Grand Canyon is given in Figure 4. Driving north along the bottom of the valley north of Sedona, the road follows Oak Creek, which has been excavated largely along the line of the Oak Creek Fault, which runs almost due N-S and has a down throw to the east of between 700 and 1,500 feet. In the area between Sedona and Midgley Bridge the inner gorge of the river shows Mississippian limestones at the bottom of the canyon, unconformably overlain by red, juniper-covered slopes and then cliffs of red Pennsylvanian to Permian Schnebly Hill sandstones (Fig. 5A). The Oak Creek Fault is crossed by the road just over 1 mile north of Midgley Bridge. At the northern end, the road climbs out of the valley up a magnificent series of switchbacks, which provide excellent exposures of Permian Coconino Sandstone and of Tertiary olivine basalt lava flows

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(often with well-developed columnar jointing), with red cinder layers and baked earths also being prominent. The switchbacks also twice cross the line of the Oak Creek Fault, which can be seen to bring Coconino Sandstone into contact with Kaibab Limestone or Tertiary basalts. At the summit of the pass, the Oak Creek Canyon Overlook gave a panorama of Tertiary basalts resting on the yellowishgrey dolomites and dolomitic sandstones of the Coconino Formation, with the reddish-orange sandstones of the Schnebly Hill Formation forming the base of the sequence. Travelling north towards Flagstaff on US 89A the road crosses the surface of the Coconino Plateau, one of the larger subsections of the Colorado Plateau, here formed by the Kaibab Limestone, covered by a patchy

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K . L. DUFF & T . D . FORD

Fig. 4. Simplified geological section from the Verde Valley northwards across the Mogollon Rim to the Grand Canyon (based on a diagram drawn by W. J . Breed).

veneer of basalt, well seen in a number of road cuts (Fig. 4). From Flagstaff, we turned east on 140 towards Winslow and Holbrook. Flagstaff has an impressively scenic location , being overlooked on the northern side by the San Francisco volcanoes, and on the east by Elden Mountain, a dacite lava dome cored by a laccolith . Some of the dacite was so viscous that it solidified befor e reaching the base of the mount ain ; good exposures of the upp er flows, showing notable longitudinal jointing , are seen overlooking the junction of US 89 and US 66. R oad cuts in Quaternary basalt lavas are seen for a few miles east of Flagstaff. The road then pro ceed s across the Kaibab Limestone which surfaces the whole of the Grand Can yon area, until shortly before the turn south to Mete or Crater , when it crosses onto the basalt Triassic Moenkopi Form ation red -bed s (largely dune-bedded sandstones in this area) . Meteor Crater is one of the two designated US Natural Landm arks which are in private ownership. The crate r (Fig. 5B) is 4,150 feet across and 570 feet deep , and was formed by a nickel-iron meteorite estimated to have been 81 feet in diameter , which arrived about 25,000 years ago. It was originally thought to have been volcanic but , although meteorite fragments were discovered in 1891, it was not finally accepted as an impact crater unt il about 1930. The crater has been dated by means of Pleistocene fossils contained within the lake sediments at its bottom. The meteorite exploded on impact , throwing vast blocks of rock high into the air , and as far away as six miles; some of the blocks weigh over 4,000 tons. Only about 10 per cent of the original mass of the meteorite remains, and has been located beneath the crate r, off centre to the southeast, where the rim rocks are arched up by over 100 feet . The residue of the meteorit e was located , by drilling, at a depth of 1,376 feet below the plateau surface somewhat to one side of the Crater ; drilling has also revealed that 620 feet directly below the crater floor the rock layers are undisturbed . In the floor of the crater white mound s of pulverised Coconino Sand stone are visible, marking

the site of an early attempt to locate the meteorite by a mine-shaft. The parties gained a good view of the profile of the Crater with its upturned edges from a low ridge about a mile north of the Crater. Continuing east on 117 beyond Winslow, the shallow semi-dry sediment-l aden cours e of the Little Color ado River was crossed. Floodgate s are needed to control the effects of flash flood s afte r summer thundersto rms. After Holbrook the Petrified Forest National Park is reached . Best entered from the south entra nce, off US 180, this is a part of the Painted Desert named from its profu sion of silicified tree-trunks. The Painted Desert obt ains its name from the brilliantly colour ed rock formations which occur, all belongin g to the late Trias sic Chinle Form ation , and consisting of red , pink , purple , bro wn and grey shales, clays and sandstones (Fig. 6A), forming a shallow synclinal basin. In the vicinity of the National Park the Chinle Form ation increas es in thickness to over 1,500 feet and the tree s are scattered through much of this, along with a variety of reptile remains. The y accumulat ed during the late Triassic when a highland area (known as the Mogollon ) existed in SE Arizona and New Mexico. Streams flowed northw est from that area to an inland sea which lay to the west of the present Colorado River. In addit ion to river flood-plains over the Painted Desert area , there were also marshes and swamps. The climate was very warm, with a luxuriant vegetation domin ated by tall trees , closely related to A raucaria, many of the se Triassic tree s were over 200 feet high, as may be seen in the Long Logs area. In addition to petrified wood , the Chinle Form ation also yields fossil amph ibia , reptil es, fish, bivalves and snails, which inhab ited a continuall y changing landscape of river chann els, mud-banks and sand-bars, ponds and marshes. The vast amounts of sediment which were brought in by the rivers ensured that dead animals and fallen tree s were rapidly buried , with tree trunks ofte n having been swept down river chann els to pile up as great log ja ms against the sand bars. The swampy conditions rapidly wate rlogged the wood , and since this encouraged a redu cing environment to develop ,

USA FIELD MEETINGS 1981 & 1982

Fig. SA. Towers of Schnebly Hill Sandstones (Pennsylvanian-Permian) above Oak Creek Canyon.

Fig. 58. Meteor Crater, Arizona.

107

108

K. L. DUFF & T. D. FORD

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Fig. 6A. Clays and caliche layers of the Triassic Chinle Formation in the Painted Desert between Cameron and Tuba City, northeastern Arizona.

Fig. 6B. Silicified logs in the Petrified Forest National Park. Scarps of Chinle Formation can be seen in the background.

USA FIELD MEETINGS 1981 & 1982

this decreased the rate of decay . Two hypotheses have been proposed to account for the silicification of the wood: one suggests that the wood was not petrified until Tertiary vulcanism provided silica in the waters of a temporary lake , while the other proposes that silicification took place during early diagenesis of the sandstones in late Triassic times . The latter hypothesis is favoured nowada ys as it is difficult to see how the tree-trunks could have been preserved with both cell structures and their shape undeformed for perhaps 150 million years before silicification in the Tertiary era . Also traces of uranium mineraliz ation, thought elsewhere on the Plateau to have been deposited in the Cretaceous, are found in cracks in the silicified tree-trunks . All manner of colouration is present in the logs with the range of colours depending upon the chemical composition of the minute traces of other elements which were contained within the groundwaters. Iron gives rise to the brown and red, uranium to yellow; manganese or carbon to the black, blue and purple; copper occasionally adding blue or green ; and the white or grey of pure quartz. Cavities within the logs occasionall y remained open long enough to allow gemstone crystals of the semi-precious varieties of quartz to grow, forming geodes of amethyst, rose-quartz and smoky-quartz. It was the blasting of logs in search of these crystals , and the building of a mill to crush logs to make abrasives, which led Arizonan citizens to petition Congress in 1895 for the establishment of a national park. Over 40 species of plant fossils are recorded from the Chinle Formation, but over 75 per cent of these occur in forms other than as petrified wood , mostly as soft parts (such as leaves and stems) which are preserved as plant compressions in the more shaly horizons ; well-preserved ferns are part icularly characteristic. There is also a diverse vertebrate fauna from these beds , including lungfish (Ceratodus) , salamander-like creatures (Eupelor) , crocodiles, other armoured reptiles and the small dinosaur Coelophysis. The road through the Park is some 20 miles long and stops with geological interest include the Museum at the Visitor Center with the Long Logs half mile walk nearby. Agate House is an Indian ruin built of blocks of petrified wood . Agate Bridge is a log 100 feet long spanning a ravine eroded beneath it. At Blue Mesa and the Teepees the Chinle Formation clays have been eroded into a badland topography with many of the hillocks capped by disintegrating logs. Caliche beds are present as layers of nodules. Newspaper Rock is a fine example of where prehistoric Indians made use of flints to scratch drawings in fresh rock beneath the veneer of iron- and manganese-rich desert varnish coating the sand stones . The petroglyphs show a wide variet y of animals such as deer and antel ope , plus stylized coiled snakes and other abstract designs. Human figures with elaborate

109

headgear and cloaks may indicate a connection with the Aztec s of Mexico. Occasional designs with men on horseback date from after Columbus' voyages as the hor se had become extinct in North America in the late Pleistocene. Lacey Point and Kachina Point pro vide vistas over the Painted Desert showing classic gully erosion on badlands cut into the Moenkopi Formation with patches of Pleistocene lavas. Bentonite layers in the Moenkopi Formation show disturbed ground due to their swelling in intermittent rainstorms. Both parties stopped at most of these localities, and were able to study the petrified logs (Fig. 6B), the enclosing sediments and the desert ero sion features which are so well-seen towards the northern end of the National Park , where the Painted Desert extends away into the distance . The 1981 group spent the night at the Rainbow 8 Motel in Holbrook , whilst the 1982 party returned to the Kings House Motel in Flagstaff. (d) Petrified Forest to Grand Canyon Since the route from the Petrified Forest to Flagstaff retraces the route described in the previou s section , the details which follow start from Flagst aff. State Highway 89 leads northwards through the suburbs of Flagstaff , towards Page and the Grand Canyon. The road passes blowing dust localities , produced from glacial and volcanic ash deposits. Te stimon y to the strong winds which can blow here are the weights on the power lines , designed to prevent sta nding waves forming on them . Near here a 'lunar crater' landscape was created by NASA to allow the astron auts to practice with the lunar rover. The road provide s exten sive views of the San Francisco volcanic field. (i) The San Francisco Peaks Volcanic Field South of the Grand Canyon and the Colorado River lies a little-dissected area of the Colorado Plateau, capped by a series of volcanic peaks which tower up to 5,000 feet above the level of the plateau (Figs. 4 & 7), and which culminate in Mount Humphries , the highest point in Arizona (12,633 feet). This mountain is the most impressive feature of a volcanic field covering 3,000 square miles, and containing over 250 craters. Geologically , the volcanoes range from Pliocene to Pleistocene in age, the most recent recorded eruption being at Sunset Crater in 1065 A.D . The major peaks around Mount Humphries are high enough to maintain snow cover , and were glaciated during the Pleistocene ; some of the summits contain corr ies, and glacial outwash deposits occur locally, especially in the Sunset Crater area. Eruptions at many of the craters have been of Strombolian type , giving rise to cinder cones with steep-sided craters, which dot the country around Flagstaff. Howe ver , the princip al mountain group is composed of composite polygenetic volcanoes , produced by a series of eruptions (many of them

110

K. L. DUFF & T. D

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violent), which gave rise to deposits of ash flows, base surges and lahars, as well as of lava. The volcanic history of the San Francisco field shows a grouping into three broad periods during the last 10 million years; an early basaltic phase, a middle silicic phase (ranging from andesites to rhyolites), and a return to basaltic activity during the Pleistocene. Twelve miles north of Flagstaff a loop road leads to Sunset Crater and Wupatki National Monuments, which were visited by the 1982 party. (ii) Sunset Crater National Monument Sunset Crater is a basaltic cinder cone of Strombolian type, in which cinders and bombs expelled from the vent fell back near it and cascaded down the steep slopes until they came to a stable angle of rest. The crater also has associated lava flows, one of which

FORD

blocked off the former valley drainage of Bonito Park, and acted as a dam behind which an alluvial flat has been laid down. The last activity of the volcano was in 1065 A.D. and comprised the eruption of vast quantities of ash and cinders, which built up a cone over 1,000 feet high, as well as lava flows which issued from three or four vents near its base. The deposits overwhelmed Indian dwellings, and it has been possible to date the eruption by means of archaeological techniques and tree-ring chronology. The final phases of activity included the emission of steam and volatile gases which altered and reddened the core of the volcanic pile and gave Sunset Crater its pinkish top and its name. The colouring is due to fumarolic gypsum, sulphur and limonite. It is the latest-known eruption of the volcanic field. Access to the summit of Sunset Crater is now forbidden, because of the cumulative erosional effect which the feet of visitors were causing. However, there is a good lava trail near the western foot of the cone. Geologically, the trail displays an excellent range of volcanic features, such as spatter cones, fumaroles, collapsed lava tubes, lava bubbles and domes, gas vesicles and xenoliths of Kaibab Limestone. The basaltic lavas are mainly of aa type, and pressure ridges and lava squeeze-ups are well seen. On the opposite side of the paved road to the lava trail, the Bonito lava flow can be viewed, still with little vegetation on it. Its dark surfaces and rough flanks reveal broken up lava crusts and levees; it combines ropy pahoehoe features with the rubble appearance of aa lava. The peak lying north of Sunset Crater is O'Leary Peak, a series of rhyodacite domes dating from 0.23 million years ago. The 1982 party were fortunate enough to have a guided walk up Lenox Crater, an ash cone adjacent to Sunset Crater, led by a young lady Ranger. Beyond Sunset Crater the Wupatki loop road leads off the margin of the volcanic field across red sandstones of the Triassic Moenave Formation to the ruined Indian pueblo village of Wupatki, abandoned when volcanic ash showers got too hot for comfort! Small canyons hereabouts are margined by collapsed joint blocks where the clays beneath the sandstone scarp have been washed out. An unusual feature is the Earth Crack, asmall hole leading down into a vast dry joint system (the water-table is at least 1,000 feet down) which 'breathes' air in and out as it tries to equalize with atmospheric pressure changes outside; a howling gale blows in or out at different times. The road continues up a small fault scarp capped by a cinder cone, Doney Crater, with many fragments of the underlying Kaibab Limestone thrown out amongst the ash. Returning to 89 North, the route takes one down the dip-slope of the Red Mountain monocline on to Triassic strata at Cameron, the eastern gateway to the Grand Canyon.

USA FIELD MEETINGS 1981 & 1982

111

One mile west of Cameron, Highway 64 crosses Tappan Wash, a valley filled by a basalt lava flow -'--r-J.....,~ ..,-',O---'-,\ {grey.vhIt el which originated far to the south, and continued north :lTor eo Ls Ire d buff} into the Little Colorado valley, where it ran westwards Co·,:oo.lo.o. ~~( .: ~:.:.'<.: (buff) PERMIAN for 9 miles. The wash has now been re-excavated = ermit Sho e (red) through the flow, which has been dated to 500,000 BP; river gravels above the flow have yielded the fossil Ired) remains of Elephas columbi. Beyond, the Kaibab monocline is visible to the left, MISSISSIPPIAN with steeply dipping Kaibab Limestone and 'flat-irons' DEVONIAN along the base of the slopes. The projecting spur visible towards the right-hand end is a landslide mass CAMBRIAN of Permian Kaibab Limestone which has slid over the upturned edges of the Triassic beds below, giving a tectonically-reversed succession. zs Nearby is the Little Colorado Canyon overlook, o a: w from which both parties gained an excellent view of o a: the Little Colorado Canyon, virtually dry at this time 0.. w of year, which is 1,000 feet deep and much narrower "" -' than the Grand Canyon. The walls of the upper 400 feet are Kaibab Limestone, beneath which is massive I o Coconino Sandstone and Toroweap Formation, here >: barely distinguishable as a result of facies changes. Seven miles further on, the road climbs steadily uphill and the parties had an impressive view northward over Fig. 8. Pictorial stratigraphic column of the rocks visible in the entrenched course of the Little Colorado, cut into the Grand Canyon (based on a diagram drawn by W. J. Breed). the surface of the Marble Platform. ~

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(iii) Grand Canyon National Park-General Geology Measured along the Colorado River, the Grand Canyon is 277 miles long, ranges in width from 600 feet to 18 miles and part of it is 1 mile deep. Within it is revealed the geological record of over 2,000 million years. The generalised sequence present within the part of the Grand Canyon around the South Rim village is shown in Fig. 8. The most noticeable cliff-forming horizons are the Kaibab Limestone, Coconino Sandstone and Redwall Limestone, whilst the Cambrian Tapeats Sandstone forms a prominent and widespread platform (the Tonto Platform) above the Inner Gorge; this platform is 1,200 feet above the river. The recorded geological history of the Grand canyon (Breed & Roat, 1978) starts about 2,000 million years ago in the early Precambrian with the deposition of a sequence of shales, sandstones, lavas and limestones. A phase of orogenesis metamorphosed these into gneisses and schists (the dark Vishnu Schist of the Inner Gorge), and also involved the intrusion of pink and white granitic rocks, some as dioritic plutons and swarms of pegmatite veins. These veins and sheets stand out very clearly within the dark-coloured metamorphics, especially if visiting the Inner Gorge or flying through the Canyon. A long period of erosion followed, wearing down the mountains to expose the metamorphosed and intruded core, and eventually producing a broad peneplain. Variations in land and sea level produced a new

sedimentary cover, with up to 12,000 feet of sediments. Part of this cover can be seen in the dipping later Proterozoic (younger Precambrian) sediments of the eastern areas of the inner canyon (Fig. 9). The lower part of the later Proterozoic is the Unkar Group-dominantly red beds overlain by basalts and by the argillaceous Chuar Group, which has yielded late Precambrian fossils. The fact that these sediments are tilted indicates that another orogenic episode occurred before the deposition of Cambrian sediments, complete with a subsequent phase of erosion and denudation. Above this second major unconformity there accumulated the great thickness of horizontal strata which are such a feature of the Grand Canyon. The thick Cambrian sequence was deposited under marine conditions and yields a distinctive fauna of trilobites, brachiopods and primitive molluscs. No Ordovician or Silurian rocks occur, and the Devonian is only locally present, usually in the form of channel infills, represented by the Temple Butte Limestone, which yields a sparse fish fauna. It appears likely that the remnant outcrops of this formation infill erosional hollows and channels cut into the surface of the Cambrian Muav Limestone during the erosional phase of late Cambrian, Ordovician and Silurian times, and that post-Devonian erosion has subsequently removed such of the Temple Butte Limestones as were deposited at higher levels.

112

K. L. DUFF & T. D. FORD

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U S A FJELD MEETI N G S 1981 & 1982

This phase of erosion, which has almost completely obliterated all record of the presumably exten sive Devonian Sea, was followed by the deposit ion of the massive Redwall Limestone , broadly equivalent in age to the Carboniferous Limestone, and which contains the characteristic Lower Carboniferous marin e faunas. It is about 680 feet thick and the red colourati on is due to surface staining and water leaching out iron compounds from the overlying Supai Form ation. The Supai Form ation is broadly equi valent in age to the Coal Measures and extends abo ve the CarboniferousPermian boundary; it consists of a variet y of sediments, laid down in fluvial conditions, with brief marine intercalations and several disconform ities. The red Hermit Shale was largely laid down under fluvial conditions and consists of alternations of shales, siltstones and sandstones with fossil plants. Above lies the beautifully aeolian cross-bedded Coconino Sandstone , up to 400 feet thick . The uppermost levels of the Can yon indicate a return to marine condition s, with the limestones and dolomites of the Toroweap Formation and the Kaibab Limestone , the latter forming the plateau surface over a wide area around the Grand Canyon. No higher beds occur in the canyon , but the presence of Triassic sediments forming Cedar Mesa, east of De sert View, and Echo Cliffs, north of Cameron, indicates that a considerable thickness of Mesozoic sediments once overlay the canyon, but have been stripped away by erosion . About 65 million years ago the region was subjected to the phase of uplift which eventu ally created the Rock y Mountains, and which also created the Color ado Plateau Province . The whole canyon area is extensi vely faulted , and monoclinal structures are also well-develop ed . Two distinct phases of faulting have been recognised , both of Cenozoic age, with an early phase of N-S faulting and a later phase of NE-SW faulting. The N-S faults and the associated monocline s all overlie fault zones that were active in the Prec ambrian, and indicate that the ancient Precambrian fault zones were rejuvenated by reverse movement and accompanying monoclinal flexing of the overlying Palae ozoic rocks . The NE-SW faults have much lower thr ows, postdate the N-S faults , and appe ar to be the result of simple tensional stresses. The canyon also cuts across the southern end of a bro ad plunging N-S ant icline , which accounts for the higher elevation of the north rim (8,000 feet ) than the south rim (7,000 feet). It is unclear when first the Colorado River began flowing over the Plateau , but the initiat ion of the phase of downcutting which gave rise to the Canyon is generally thought to have occurred about 10 million years ago. The origin of the canyon is thought to have been either by headward erosion of the protoColorado, or by antecedent dra inage by the Colorado River with incision as uplift on the Kaibab axis occurred , or by superposition from the former cover of

113

Mesozoic strata. A combination of all three proce sses is likely to be the true explanation . (iv) Desert View At Desert View, just within the park , the parties ' first sight of the canyon was most impre ssive and the sheer scale took some time to accept (Fig . lOA). The viewpoint gives good views upstream and across eastwards to Cedar Mesa and Echo Cliffs. The Kaibab Limestone forms the rim , Toroweap Formation shale s form the ledges below, Coconino Sand stone forms the conspicuous white cliff, the thick Hermit and Supai shales have numerou s ledges , Redwall Limeston e forms the lower major cliff, and the Bright Angel Shales extend down to the Tapeats Sandst one platform immediately above the unconformity at the top of the Inner Gorge. The Inner Gorge here is in eastward-dipping sediments of the late Precambrian Unkar and Chuar groups. The canyon is 4,500 feet deep her e. The leaders gave talks here and the GA party was joined by a variety of tourists and even a couple of Park Rangers. Three miles to the west , Lipan Point overlook pro vided a fine view down the Inner Gorge , with the early Precambrian Vishnu Schist being clearly visible dipping beneath the Unkar Group , and both being capped by the Cambr ian Tonto Platform. In the next 20 miles the parties stopped at several viewpoints which provide further panoramas ; in particul ar Yaki Point gave a view of the Kaibab Trail down into the canyon , and is worth a visit for the spectacle of changing colours and shadows at both sunrise and sunset. Members of the part y who were intending to walk down the Kaibab Trail the following day were particularly interested to view the steep switchbacks at the top! Dioramas are used to condense the geological histor y of the Canyon in the Visitor Center adjacent to the accommodation and restaur ants in the South Rim village. Both parties stayed at the Mushwip Lodge. (v) Grand Canyon A ctivities The 1981 party spent one compl ete day here , and the 1982 party two. The range of activities undertaken was varied, some members takin g helicopter or light aircr aft flights from the nearby airport at Tusayan into and over the Grand Canyon, whilst others followed the West Rim Dr ive along the canyon rim. The mor e energ etic walked the tr ails down into the canyon , with the 1982 party being rather less rushed than the ir colleagues of the previous year. Four of the more energetic members of the 1982 part y arose early to walk the seventeen mile circuit to the Inner Gorge , down the Kaibab Trail , along the Inner Gorge and up the Bright Angel Trail, no mean feat considering th at the journey involved 5,000 feet of climbing and temperatures of over lOO°F in the inner canyon. The Kaibab Trail pro vided the parties with the

114

K . L. DUFF & T . D . FOR D

Fig . lOA .

..

Fig. 108 .

USA FIELD MEETINGS 1981 & 1982

better views and geology and also provided the choice of either short or long walks (Fig. 11). The short walk, taken by most of the 1981 party, took them to just below Skeleton Point, a descent of some 2,500 feet in a distance of 3 miles, i.e. about halfway to the bottom. Keith Duff was unfortunate to suffer a jammed camera, which could not be repaired until we returned home, during this walk, but the kindness of Harold Grieve, who lent him a spare camera, was greatly appreciated, and ensured that the high consumption of film continued! During this walk, the parties were able to examine a detailed section through the Permian and Carboniferous succession down to a Devonian channel. The trail begins with a 200 foot drop via a series of switchbacks in the Permian Kaibab Limestone, which contains some chert and brachiopods, then levels off across the mixed lithologies of the Toroweap Formation, where fallen blocks of Kaibab Limestone lie scattered on the slopes. The trail then steepens again with more switchbacks through the yellow dune-bedded Coconino Sandstone. Hermit Shales, containing some sandstones, then extend down to the level of the platform on Cedar Ridge. Off to the left is a display case containing specimens of Permian fossil ferns, and O'Neill Butte ahead is an outlier of the Esplanade Sandstone, which forms part of the Supai Sandstones. The trail then steepens again and crosses onto the Pennsylvanian sandstones and siltstones with thin limestones, then levels out across a platform at the top of the Redwall Limestone, and reaches Skeleton Point at the edge of the Redwall Limestone cliff. Over the edge members could see that the Redwall was stained by leaching from the Supai Group above, and some solution cavities were seen in the limestone. From the third switchback past the summit of Skeleton Point, an excellent view of the Tonto Platform and the Inner Gorge was gained; near the bottom of the switchbacks there is a section through a Devonian channel cut into the Cambrian Muav limestones. A longer walk with a further 1,000 foot descent takes one to the Tip-Off on the rim of the Inner

Fig. lOA. The eastern Grand Canyon seen from Desert View. In the left centre near-vertical cliffs of Precambrian Cardenas Lavas are covered with argillites of the Chuar Supergroup (Precambrian), unconformably capped by the scarp-forming Cambrian Tapeats Sandstone. To the right the Canyon is deeply incised through Palaeozoic strata. Fig. lOB. The Grand Canyon downstream from South Rim Village, seen from a helicopter. The Inner Gorge is incised into Vishnu Schists and capped by Cambrian Tapeats Sandstone. Rising above the wide bench of Bright Angel Shales are cliffs of Mississippian Redwall Limestone. Sandstones of the Supai Formation appear on the extreme left.

115

Canyon, crossing the Cambrian sequence on to the 'great unconformity' where the Tapeats Sandstone rests on a series of downfaulted blocks of Unkar redbeds among Vishnu Schists. The final 1,200 feet into the bottom of the Canyon crosses these Precambrian fault blocks giving good sections. The footbridge at the bottom is 7 miles from the Rim and 5,000 feet lower. A riverside trail downstream links the Kaibab Trail to the bottom of the Bright Angel Trail, nearly 11 miles long, but with several rest places with water available. Other members walked part of the Bright Angel Trail which starts with a steep section and a tunnel in the Kaibab Formation before an easing over mixed Toroweap sediments. The Bright Angel Fault, downthrowing 189 feet to the east, is seen before the second tunnel which, with the next switchbacks, is in the dune-bedded Coconino Sandstone. At H miles there is a resthouse and water, and the rocktype is Hermit Shale, followed by a long descent over Supai Group sandstones, siltstones and limestones. Three mile rest house also has water and is on top of the Redwall Limestone, the descent through which is very steep. Below is the Cambrian Bright Angel Shale, and Indian Gardens here are fed by springs from the overlying limestones; water is again available. From Indian Gardens a left fork leads across the Tonto Platform to Plateau Point, situated on the Tapeats Sandstone at the edge of the Inner Gorge, overlooking the Colorado River 1,400 feet below. Precambrian red Hakatai Shale and Vishnu Schist can be seen below the unconformity. A right fork leads down the narrow Pipe Creek Canyon across the Vishnu Schists to the river trail, linking to the foot of the Kaibab Trail. The helicopter and light aircraft trips give a choice of a traverse of the eastern Grand Canyon to the confluence of the Little Colorado, or a downstream flight to the west and into Havasupai Canyon. Most members of the parties took advantage of these options, with the majority opting for the Havasupai flight. The Little Colorado flight provides fine views of the Inner Gorge (Fig. lOB), the inclined Unkar and Chuar Group sediments and intercalated lavas, with faulted monoclinal structures of the East Kaibab monocline and Butte Fault system, whilst the Havasupai flight provided members with better views of the Palaeozoic strata, particularly the Supai Group Esplanade Sandstone. Faults of 100-200 feet throw look puny in the scale of the cliffs. The flight passes over the tributary Havasupai Canyon where an isolated Indian reservation is bounded by 1,000 foot cliffs. Downstream Supai Creek tumbles over high waterfalls caused by tufa dams backing up silt infills. Massive lands lips were seen around Thunder Falls, with substantial streams jetting out of caves in the Muav Limestones. Above all, these flights gave members an opportunity to appreciate the gigantic scale of the processes of cliff retreat and fluvial

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'0"

USA FIELD MEETINGS 1981 & 1982

incision. A few members took a light aircraft flight instead, which covers the same ground as both helicopter flights, but much more quickly; it was also cheaper! The 1982 party had a considerable advantage over their predecessors of 1981, in that their possession of self-drive cars gave them greater mobility within the area. Within the Grand Canyon village vicinity, the only transportation which is permitted (other than hire coaches which are only allowed to bring groups to and from the viUage) is private cars, official National Park courtesy buses or bicycles! This meant that getting out to viewpoints to see the sunrise or sunset, or taking the West Rim Drive, was very difficult for the 1981 party, whilst the 1982 party had more flexibility with cars and were herded out at both sunset and sunrise by a camera-mad Tony King! Later the West Rim drive by courtesy 'tram' provided the party with more fine overlooks of the Canyon, beginning with the Trailhead Overlook which showed potential walkers what to expect along the Bright Angel Trail; the view clearly showed the Bright Angel Fault, the amount of movement on which could easily be seen. The West Rim drive also passes Orphan Mine, which was worked between 1893-1966 for copper, and then for uranium. The ore is in a massive pipe at the level of the Hermit Shale, filled with Coconino Sandstone and formed by karstic collapse into the Redwall Limestone. Half a mile further on is Hopi Point, which has excellent views both ways along the Canyon, and is one of the best viewpoints for the sunset. The Abyss, to the west, has an unusual vertical wall extending down over both the Kaibab and the Coconino Beds. Pima Point provides the best view of the river at Granite Rapids and Falls, where it tumbles over the Vishnu Schist. In 1981, Fay Hicks was adventurous enough to take a whole-day mule trek from the canyon rim down the Bright Angel Trail to Plateau Point and back and discovered that there were more comfortable ways to travel! It did, however, provide her with a memorable and unforgettable experience, but one which no-one cared to repeat the following year! After the Grand Canyon, the routes taken by the 1981 and 1982 parties diverged. The 1981 party went via Page and the Glen Canyon Dam across the northern end of the Kaibab uplift through Kanab to Mount Carmel Junction and Bryce Canyon, whereas the 1982 party went further east through Kayenta and Monument Valley to Durango in southwest Colorado, taking two extra days before rejoining the 1981 party's route. The two routes are now described separately, and the itineraries then link up again at Bryce Canyon. (e) Grand Canyon to Bryce Canyon (1981)

From the Grand Canyon the party retraced their earlier route back to US 89, and turned north towards

117

Cameron across the rocks of the Lower Triassic Chinle Formation. Here, clays of the Petrified Forest Member contain bentonites whose swelling characteristics gave an uneven road surface. Travelling north, Echo Cliffs, ahead and to the east, gradually appear, and their monoclinal form becomes apparent. The ground to the west is generally flat-bedded, but the dip increases rapidly towards the east, so that steeplyinclined rocks representing the whole of the Triassic are seen in the massive cliffs, capped by the Triassic/Jurassic Navajo Sandstone. At Bitter Springs, the party turned east on State 89 towards Page, climbing towards a huge road-cutting through the summit of Echo Cliffs. A pull-in just before entering the cutting allowed us to study the marvellous view west over the Marble Platform, the wide plateau surface developed on the surface of the Kaibab Limestone, into which the 'Marble Canyon' of the Colorado has been deeply entrenched. The surface of the Marble Platform was noted to be remarkably extensive, its level surface being broken only by isolated remnants of the former Triassic cover, or where outcrops of post-Permian sediments have been preserved beneath younger basalt lava flows. The only other features seen on it were the basalt and cinder piles of Tertiary and Quaternary volcanoes. The party then continued towards Page and crossed the bridge by the massive Glen Canyon Dam which holds back the vast water area of Lake Powell. This artificial lake is 180 miles long and was created by the building of the dam between 1956 and 1964. Brick-red Navajo Sandstone forms the river cliffs by the dam, and its permeability and patchy calcite cement look like causing serious problems in the future, in addition to the steady infilling of the upper reaches of the dam by silt brought in by flood waters from the Rocky Mountains some 600 miles upstream. The spillways were found to be inadequate to cope with the heavy run-off in early 1983 and water topped the dam! Numerous rock-bolts can be seen in the canyon walls, inserted to improve stability and reduce spalling. A sample rock-bolt was seen on the wall of the powerhouse; 2 inches in diameter some extend in as much as 75 feet. Past Glen Canyon Dam the party continued along US 89 towards Kanab, across the rather featureless ground formed by the Jurassic Carmel Formation, before dropping down again to run along the foot of the very impressive Vermilion Cliffs, so called because of the dominant colouration of the Triassic Chinle, Moenave and Kayenta Formations which form them. These cliffs form the lowest step of the Giant Staircase of successive escarpments (Fig. 12). Members eventually realised that the feelings of deja vu which many of them had experienced along this part of the route were due to the area having been used extensively by the film industry in the making of westerns. This was further emphasised as we passed

118

N

K . L. DUFF & T . D . FORD

Pink

Cliffs

Fig. 12. The 'Giant Staircase' of successive alternations of hard and soft Mesozoic strata on the Colorado Plateau.

through Kanab and saw a rather rundown-looking motel prominently advertising that 'Ronald Reagan slept here'; clearly his fortunes had changed in the meantime! This part of the journey was marked by a torrential rainstorm, which turned out to be the only rain encountered during the trip. North of Kanab our route ascended the Vermilion Cliffs via Kanab Canyon, and members were able to see clearly a spectacular sequence of two massive sandstone units, the Navajo and Wingate Sandstones, separated by bedded red sandstones of the Kayenta Formation. Very large-scale cross-bedding was evident in the Navajo and Wingate Sandstones , with very impressive folded and crumpled laminae being seen in places. The lower beds of the Navajo Sandstone in this area are red , and the road continues to climb across these towards the spectacular White Cliffs, cut in the massive dune-bedded bleached white sandstones of the upper part of the Navajo Sandstone. Locally, the Navajo Sandstone can be seen to be capped by the striped shales and marine lime stones of the Middle Jurassic Carmel Formation. Four miles before reaching Mount Carmel Junction the road descends towards the valley of the Sevier River and crosses the trace of the Sevier Fault , which down throws some 1,500 feet to the west, bringing the Carmel Formation to ground level along the Sevier Valley; the White Cliffs along this portion of the valley take the form of a fault scarp. The Sevier Fault runs N-S and is one of the major tectonic features of the Colorado Plateau which divides southern Utah into structural blocks; it can be traced for about 300 miles. Good exposures of the limestones and shales of the Carmel Formation were seen in road cuts between the White Cliffs and Mount Carmel Junction. The party spent two nights at the Best Western Thunderbird Lodge Motel at Mount Carmel Junction , the first having arrived from the Grand Canyon and the second after spending a day at Bryce Canyon National Park . There was still time for us to make a brief visit to the Zion National Park Center on the afternoon of our arrival at Mount Carmel , but this section of the journey is described later . On returning to Mount Carmel, some of the party spent an interesting half-an-hour in the company of our Greyhound bus driver, Bill Fitzgerald , who insisted on showing them the 'luminous fossils' which were supposedly to be found in the road cuttings adjacent to the motel. In view of the fact that it was dark , and of

the warnings which the party had received about the night-time activities of the reptilian inhabitants, this was perhaps not one of the recommended excursions , but in view of the Ca llovian age of the rocks, the Director felt compelled to attend! Luckily , no mishaps occurred, but we failed to locate the 'luminous fossils' , which we later discovered to be fragments of crinoid stems whose 'luminosity' was caused by light reflecting from the calcite crystal cleavage surfaces . Some of our more sensible colleagues had spent the early evening on the motel 's 9-hole golf course! The 1982 party spent one night here, en route from Bryce Canyon to Zion National Park. Both parties travelled the route between Mount Carmel Junction and Bryce Canyon, the 1981 group making a return journey, whilst the 1982 group travelled south only in their journey from their previous overnight stop at Panguitch . In order to reduce duplication , the route is described here. The route from Mount Carmel to Bryce Canyon continued the ascent of the 'Grand Staircase' of southern Utah (Fig. 12) and took the parties from the Middle Jurassic at Mount Carmel Junction through the whole Cretaceous sequence and up to Eocene sediments at Bryce . The road (US 89) runs northwards , parallel to the Sevier Fault , ascending the Grey Cliffs (formed largely of Cretaceous Wahweap Sandstone) and the Pink Cliffs (formed of Eocene Wasatch Formation limestones) . Bryce Canyon is a N-S trending line of east-facing cliffs and amphitheatres cut in the Wasatch Formation of the Paunsaugunt Plateau; the Sevier Fault divides this plateau from the Markagunt Plateau to the west, which lies considerably lower as a result of the fault throw . Along US 89 the party noted nearly-continuous exposures of Jurassic and then Cretaceous sediments, including coal beds in the Dakota Sandstone, once worked at Glendale. Just north of Glendale, a series of Tertiary basalt lava flows were seen capping the hills east of the road, and there is evidence that lava-damming of streams in this area formed lakes in which lacustrine sediments were deposited . This is best seen north of Hatch, where the flat valley floor is covered by a veneer of Quaternary sands , gravels, and volcanic ashes . Eight miles north of Hatch, the routes of the 1981 and 1982 parties met at the junction of US 89 with State 12 to Bryce Canyon. The road climbs east , towards Red Canyon , crossing the trace of the Sevier Fault which is excellently exposed on the north of the road at the mouth of Red Canyon. Here, dark grey Tertiary basalts to the west are downfaulted against the bright pink limestones of the Wasatch Formation , which show some of the erosional features seen to the extreme at Bryce Canyon. The parties expended much film here , in spite of a reluctance on the part of the Gre yhound driver to allow us off the bus! He seemed to think that the busy road represented a major danger

119

USA FIELD MEETINGS 1981 & 1982

to pedestrians, although to anyone used to London traffic, this was incomprehensible! The remainder of the route to Bryce Canyon, along State 12, provided tantalising glimpses of the range of features to be seen in Bryce and also demonstrated the art of the highway engineer by means of the many tunnels and natural bridges which had been cut through the soft Wasatch

Formation, some of which seemed perilously small for the passage of our coach!

(0 Grand Canyon to Bryce Canyon (1982) Retracing our route from the South Rim Village to Cameron, we turned north on US 89 for 14 miles before turning east on 160 towards Kayenta. After 3

MONUMENT VALLEYGOOSENECKS AREA.

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120

K. L. DUFF & T. D. FORD

Fig. 14A.

Fig. 14B.

121

USA FIELD MEETINGS 1981 & 1982

miles a short digression was made to the Dinosaur Footprints, where a young Navajo Indian guide demonstrated his knowledge of the best prints, and tried to sell the traditional Navajo turquoise and silver jewellery. The Chinle Formation here forms one of the 'treads' of the giant staircase of Mesozoic formations. A few miles eastwards the party made a short diversion to view the Hopi Indian village of Moenkopi, before crossing a rather featureless area to the foot of the Black Mesa, a tableland of Cretaceous strata with marine clays overlain by coal-bearing deltaic sands. The coal is now being strip-mined on a vast scale to feed power-stations many miles away. North of Black Mesa a short visit was made to the Navajo Tribal Monument, a small National Park based on canyons cut into the Lower Jurassic Navajo Sandstones. The canyon walls have suffered springhead sapping and stress release spalling to form large arch-like caves once occupied by the Anasazi Indians, who abandoned the sites about the 13th century. A short walk across the aeolian sandstones gave a fine overlook view of the Betatakin cliff village in one such archway. Continuing eastwards to Kayenta near Tsegi, the Comb Ridge Anticline was crossed, bringing the Entrada and Navajo sandstones up. Highway 163 leads north from Kayenta and after a few miles crosses into Utah close to the prominent eroded volcanic plug of Agathla. This is the gateway to Monument Valley where scarp recession across the 90 mile-long Monument Upwarp has left the impressive series of scarps and residual towers-the Monuments (Fig. 13 & 14A). These are formed of Permian De Chelly Sandstone on sloping pedestals of Organ Rock Shale. The most impressive sections are in the Navajo Tribal Park with its rough gravel road winding between the Monuments and along accumulations of sand dunes. The drivers decided not to risk the heavily laden cars on the rough road and the party had to be content with the spectacular view from the overlook. Afterwards, US 163 descended the eastern flanks of the Upwarp and the party noted Alhambra Rock, an eroded Tertiary dolerite dyke, standing high on the left. Beyond, Mexican Hat stands on a crossing of the San Juan River, a deeply incised tributary of the Colorado, where the party spent the night at the Canyonlands Motel, memorable chiefly for its faulty air-conditioning and long delayed services in the restaurant.

Mexican Hat lies in a syncline in Pennsylvanian cyclothems with an unusual oilfield. Only about 600 feet below the surface the oil lies in a syncline for the simple reason that there is no water to float it up into the adjacent anticlines. A few miles north are the Goosenecks of the San Juan (Figs. 13 & 14B). Here the party observed the intricate meanders which have been superimposed from a former cover of Mesozoic sediments, into Pennsylvanian cyclothems on the east flanks of the Monument Upwarp. They also had views to the Mexican Hat Rock and to the 'flat-irons' on the near flank of the Raplee Anticline, perhaps one of the most striking structures to be seen in the area. The road then climbs across the anticline and descends to a valley in the Chinle Formation before going through a deep cut in the Navajo dune sandstones of the Comb Ridge Monocline. In Bluff, the overlying Jurassic sandstones are eroded into the Navajo Twin rocks. North of Bluff, highway 262 crosses the Four Corners area. This is a rich oilfield region, though now approaching exhaustion; production is mainly from Pennsylvanian reef limestones some 5,000 feet down. A sucession of reef belts parallel the margins of the almost totally enclosed Paradox Basin which developed in late Palaeozoic times between complementary uplifts of parts of the Colorado Plateau. The basin filled with cyclothems of black shale, limestones and evaporites, thereby providing ideal conditions of source rocks and traps for oil generation (Fig. 15). An overlook above the Aneth oilfield provided an opportunity for a talk from the leader and

UTAH

•

HENRY

"MTNS

NEW MEXICO

ARIZONA

Fig. 14A. One of the Mittens, a 'monument' of Permian De Chelly Sandstone on a pedestal of Organ Rock Shale, in Monument Valley (photo by D. L. Baars). Fig. 14B. Oblique aerial view of the Goosenecks, showing the incised meanders cut into Pennsylvanian cyclothems by the San Juan River (photo by D. L. Baars).

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122

K . L. D UFF & T. D . F ORD

a discussion on the oil-well installations, the nature of Juan river. Two nights were spent at the General the Parad ox Basin and the structures which we would Palmer House hot el , adjacent to the railroad station, see within it dur ing the next few days. where the stea m engine buffs were showered with Ent er ing Colorado the Ute Moun tain Ter tiar y smuts and ashes from the low grade coal. laccolith was passed on the left with the Mesa Verde Durango lies in the valley of the An imas River plateau rising to the east. Beyond Cort ez the party downstream of Pleistocene terminal moraines with turned off to the south and climbed into the Mesa exten sive outwash terraces. On a high terr ace is the Verde National Park , crossing the Cret aceous Mancos campus of Fort Lewis College , and a viewpoint there Shale (Fig. 16) with its spectacular landslips. The provided the part y with one of the most complete overl ying Middle Cretaceous Mesa Ver de Sandstones stratigr aphic sections visible anywhere. The section (locally with coal seams which cannot be worked in the ranges from Precambrian crystallines in the distant Park ) form the cliff top s and have been undercut in a Gren adier Mountains through Cambrian , Devonian , number of places providing the Anasazi Indians with and continuousl y up to the Lower Terti ary , some sites for their cliff 'palaces'-actually villages dating 16,000 feet of beds altogether with a fairly uniform from around 1,000 A.D. sited in undercuts caused by southerly dip (Fig. 16). To the north lay the La Plata spring-sapping of the jointed sandstones (Fig. 17). Mount ains , a Tertiary laccolith complex similar to Afte r the inevitable visit to the Visitor Center and several others in the region. We then drove up the bookshop the party toured the nearby Spruce Tree U-shaped glacial Animas Valley descending the Hou se and then drove round on to the adjacent spur stratigraphic section as we went , and reaching of the plateau in a gathering storm. A few hardy souls glaciated scenery at 10,000 feet in Coalbank Pass (Fig. descended the ladders to Cliff Palace to be trapped 18), a marked contrast to the desert of the previous there by torrential rain and violent 'son et lumiere ' week . A basal Penns ylvanian conglomerate rests effects from the storm before climbing the ladders unconformabl y on folded and faulted rocks from back out again! Returning to 160, the part y continued Precambrian to Mississippian in age. Roadcuts give east for some 30 miles in heavy rain to Durango , good sections in Penns ylvanian cyclotherms (no coal !) which lies on the Animas River , a tributary of the San and through the margin of a Cenozoic laccolith at Coal GENERALI ZE D ST RAT I GRAPHIC COLUMNAR SECT I ON ALONG THI:: ANI MAS VALLEY

TERT IARY

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400 ', g r ay-g reen to b rown sst and sh , with 3 coal zones

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LE\olIS SHALE :

200 ' , liqht. gray marine sst

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Fig. 16. Pictorial stratigraphic diagram for southwestern Colorado.

USA FIELD MEETINGS 1981 & 1982

Fig. 17. A cliff 'palace' of the Anasazi Indians beneath overhanging Cretaceous Dakota Sandstone in Mesa Verde National Park, Colorado.

Creek. Just north of the Molas Pass there are good sections in Pennsylvanian down to Precambrian (Fig. 16), with an interesting palaeokarst developed on the Mississippian Leadville Limestone (Fig. 18). Red Pennsylvanian shale lies on hummocks of Mississippian limestone. The party then took a gentle walk past Molas Lake which crossed much of the Palaeozoic in less than a mile. Halfway down the steep descent to Silverton the road crosses a ring fault marking the boundary of the Silverton Caldera with its Tertiary volcanics dropped down by at least 3,000 feet (Fig. 19). The old mines around Silverton worked various metals in highly altered rhyolites. Old mines and patches of oxidized pyritic rhyolite dot the hills. A Victorian steam railroad covers the 45 miles from Durango to Silverton following the Animas Gorge instead of climbing the hills. Some members rode the train one way and drove the other. After a second night at the General Palmer House the party retraced their route to Cortez where they

123

turned north on US 666 towards Monticello in Utah. There we turned north on US 163 with views of the Abajo Mountains, another complex Tertiary laccolith, to the left. Road cuts showed sections in the sandstones and shales of the Morrison Formation. A diversion was made west along State 211 in the southern part of Canyonlands National Park to Newspaper Rock to observe both Indian and modern petroglyphs (Fig. 20A) which have been scratched into the desert varnish on the Wingate Sandstone. The party then returned to 163 and noted the isolated Church Rock, built of Entrada Sandstone, which gave a taste of things to come. The parallel valley to the east (Lisbon Valley) contains oilfields and uranium mines. Continuing northwards on 163 the La Sal mountains to the east are another Tertiary laccolith complex. Moab lies in a faulted salt intrusion valley-the strata in the opposing cliffs do not match (Fig. 21). A series of sub-parallel NW-SE salt intrusions cross this part of Utah, where Pennsylvanian salt has been mobilised and risen as diapirs through the Mesozoic cover with spectacular results (Fig. 22). Horsts and graben cut the canyon walls and gypsum-halite domes breach the surface. In the subsurface the upturned margins of porous strata are sealed by salt and form oil traps. Escaping fluids have dissolved uranium salts and these have been concentrated in fossil sandstone channels in both Chinle and Morrison Formations, where a combination of porosity and carbon content (in the shape of wood) has given conditions favourable for the precipitation of carnotite and associated minerals. The evaporites contain extensive potassiumsalts which are produced as brines from wells west of Moab. The party continued across the Colorado River north of Moab and entered Arches National Park which presents bizarre scenery cut into Entrada and Navajo Sandstones with a thin and undulating siltstone between. Close jointing has led to erosion causing fins, towers and arches to develop roughly parallel to the flank of the Salt Valley Anticline (and salt intrusion) to the northeast. The party visited the Windows Section with its spectacular row of arches eroded through sandstone fins (Fig. 20B) and then drove down into the eastern end of Salt Valley, noting the highly disturbed strata along this salt intrusion structure, before taking the short walk up the aeolian sandstone slabs to get a distant view of Delicate Arch (sometimes known as the Old Maid's Bloomers). A return was then made to Moab where the night was spent at the Ramada Desert Lodge Inn. The following morning the party drove east on 128 along the river bank in the Colorado Gorge to see the fault and graben structures affecting the sandstones (Fig. 22). Time did not permit a visit to Fisher Towers or to Onion Creek where a gypsum cap on a salt intrusion is seen to intrude the Pennsylvanian Cutler

124

K. L. DUFF & T. D. FORD

Fig. 18. Molas Pass, near Silverton, southwestern Colorado, showing a palaeo-karstic pinnacle of Mississippian Leadville Formation projecting through Pennsylvanian shales in the foreground. Beyond Molas Lake are dipslopes of Mississippian limestones, and the distant mountains are Precambrian quartzites of the Uncompahgre Group.

Formation conglomerates. Returning to 163 the route took us north for a few miles and then west on the branch road to Dead Horse Point State Park at the northern end of Canyonlands. This impressive view is of a sweeping meander in the Colorado River (Fig. 23A) where it has been incised through the salt-cored Shafer Dome, exposing Permian to Jurassic strata in the canyon walls. To the south potash-evaporating lagoons are the result of brine-pumping from Pennsylvanian evaporite beds. Potash is extracted but the unwanted halite is dumped into dams in side canyons, whence one day a flash-flood may wash it into the Colorado! The party then drove north to Crescent Junction where 170 was taken westward through Green River. The road then crosses a major anticline, the San Rafael Swell, cored by Permian limestones and showing a full sequence up to Cretaceous on each limb. At Salina, the party turned south on US 89 towards Sevier and Panguitch with road-cuts showing sections in granites, diorites and volcanics, some greatly altered and colourful as could be seen in the Big Rock Candy Mountain, 8 miles south of Sevier.

The rocks here have been altered by the action of hot waters during the Tertiary volcanism, which have broken down the volcanic rocks into the rather colourful assemblage of iron-bearing minerals which occur. From Sevier almost to Panguitch the party's route lay across a complex suite of Tertiary volcanics, and the road cuts and cliffs provided excellent exposures of volcanic breccias, tuffaceous conglomerates, welded tuffs and obsidians. Approaching Panguitch, the route crossed the upper end of the extensive Sevier Basin of Quaternary age dammed by lava flows north of Hatch, and to be seen the following day whilst driving from Bryce to Mount Carmel Junction. The party spent the night at the Best Western Motel in Panguitch, before driving the few miles south to Bryce Canyon, where their route rejoined that of the 1981 party. (g) Bryce Canyon National Park

Bryce Canyon is not really a canyon at all, but a series of natural amphitheatres cut into the Eocene

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limestones (Wasatch Formation) of the Pink Cliffs, the eastern edge of the Paunsaugunt Plateau. The edge has been worn back westwards from the Paria River valley as the river incised its course along the line of the Paunsaugunt Fault, which separates the Paunsaugunt Plateau from the Aquarius Plateau to the east. The park owes its beauty to the fact that much of it is a badland area, a most impressive concentration of spires, grottos , walls and arches, deeply-sculpted by erosion (Fig . 23B). The 1981 party 's first view of the 'canyon' was from Fairyland View just outside the Park entrance , but the 1982 party arrived in a thunderstorm with low cloud swirling round and drove to the Visitor Center first and then to Rainbow Point at the south end of the Park to give the clouds time to disperse . From Fairyland View overlook the broad valley of the Paria River could be seen cut along the line of weakness created by the Paunsaugunt Fault , and beyond it the Aquarius Plateau, edged by the pink rocks of the Table Cliffs. These are the same rocks as seen at Bryce , but are obviously at a higher elevation, revealing the amount of movement along the fault to

125

be about 2,000 feet. The fault crosses the low ground below, but was not an easily visible feature. The foreground provided a first view of the badlands so characteristic of Bryce and of the Wasatch Formation limestones and sandstones whose varying strengths which give rise to the rock spires, often known as 'hoodoos' (Fig. 23B) . It is because of the success of the Paria River in eroding its valley that the tributary canyons have been able to create the badlands of Bryce , eating back into the plateau edge and gradually capturing the headwaters of the slow flowing plateau-top drainage of the Sevier River system . The steeper the slope which is available , the more vigorous and powerful the drainage becomes, and this could be well seen at Bryce. From the scarp edge it takes only a couple of miles to drop nearly 1,500 feet eastwards towards the Paria River, whilst west towards the Sevier, it takes 20 miles to drop that amount. Another interesting effect of the progressive erosion by slope retreat at Bryce is the manner in which retreat takes place. It does not cause the slopes to become gradually more gentle and buried beneath their own debris , but instead , their steep slopes are maintained. Evidence of this can be seen on many of the trails along which the rocks often display scour marks, lines parallel to the present slopes but slightly above them. These are the remnants of formerslopes , and since they parallel the existing ones they prove that , as Bryce retreats back into the plateau, its slopes maintain their steep angle. The climate of the area has much to do with the steep slope angle, for long peri ods of heavy rain create vigorous runoff, with streams rushing down the steep ravines and flushing away the debris to maintain the steep angle . A number of other weathering effects depend upon the individual strengths of rock layers and their variable resistance to erosion. A characteristic feature of the Wasatch Formation is that it is well cleaved by vertical joints, which allow the isolation of individual spires and control the development of walls. The features produced are called 'hoodoos'. The parties noted that none of the hoodoos remained smoothsided , as erosion soon picks out the harder and softer beds to give them their ribbed appe arance. A hard bed usually forms the upp ermost section of a hoodoo, acting as a protective cap-rock. This may be worn down by erosion , but more often it is toppled becau se the softer bed below has been reduced to the point where it can no longer support the capping . Cap-rocks may fall and begin to protect fresh columns . This could be well-seen at The Hat Shop, on the 'Under the Rim Trail' from Bryce Point, where the blocks protect pillars of debris eroded from the scarp face of an earlier time. After visiting Fairyland View, the 1981 party returned to the main road and continued on to the Park Entrance and Visitor Center, where they were

126

K. L

DUFF & T. D. FORD

Fig.20A.

Fig.20B.

127

U SA FI ELD MEETI NG S 198 1 & 1982

Fig. 21. Simplified geological sketch map of the ArchesCanyonlands-Moab area, Utah (based on a compilation by P. L. Williams, US Geological Survey Misc. Invest. Map I 360) .

dismayed to be handed notices warning all visitors to avoid the local rodent population (chipmunks and ground-squirrels) in which plague was endemic! The receipt of this information caused fundamental change s in the attitude of many members towards these inqui sitive and hungry creatures! The Visit or Center supplies infor ma tion on the various trails which exist at Bryce and the parties decided to follow the Navajo Loop Trail, which runs from Sunset Point to Inspiration Point, and includes the Queens Garden Trail. This took members down over th e rim of the plateau and in amongst the hoodoos, and enabled them to appreciate for themselves th e scale and complexity of the features. A leisurel y dr ive to the southern end of the park follow ed , with stops at most of the viewpoints on the way, most notably Bryce Point, Farview Point, Natural Bridge, Ponderosa Can yon and Rainbow Point. Th e 1982 group did these in reverse order. Travelling north to south members also noted the larger- scale changes which occurred , and which are related to th e stage of de velopment which different parts of the park have reached. In general, the northernmost amphitheatres are still at an earlier stage of their development than tho se to the south and few hoodoos are th erefore to be seen at Fairyland View , whe re the slop es tend instead to be

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Fig. 20A. Petroglyphs scratched through the desert varnish on Triassic Wingate Sandstone at Newspaper Rock, Canyonlands National Park, Utah. Fig. 20B. 'The Spectacles' a double arch in the Entrada Sandstone, Arches National Park, Utah.

128

K. L. DUFF & T . D . FORD

Fig. 23A.

Fig. 23B.

USA FIELD MEETINGS 1981 & 1982

benches. Erosion has been proceeding for longest at the southern end of the park, and it became clear that three different types of amphitheatre could be recognised; a northern section showing young features, as at Fairyland View; a central section, from Boat Mesa to Bryce Point, which is fully-developed erosionally with gullies excavated in the full depth of the Wasatch Formation and many hoodoos, and a long southern section where the height of the plateau increases. The increase in height adds new massive beds to the top of the succession, and also brings in the underlying Cretaceous beds towards the base of the cliffs. Many springs issue from permeable layers in these rocks and seepage along these layers is an important agent in the undermining of the rocks above, which causes the rockfall activity which itself produces the characteristic landforms of the southern amphitheatres. The southern area is thus characterised by vertical cliffs, kept fresh by rockfalls, and there is no chance for hoodoos to form. The parties then retraced their route out of the park, and drove to Mount Carmel Junction, before continuing on to Zion National Park the following day. (h) Mount Carmel Junction to Zion National Park

The route towards Zion leads westwards along the 'Mount Carmel Scenic Highway' mainly across the Middle Jurassic Carmel Formation and higher beds before dropping down the sequence onto the Triassic/Jurassic Navajo Sandstone and underlying units. Nearly continuous exposures of these units were seen along the roadside. The higher beds of the Carmel Formation were well-exposed a few miles west of Mount Carmel and were seen to consist of a sequence of red and grey sandstones, locally gypsiferous (the Entrada Group) overlain by massive gypsum beds and conglomerate (Curtis Group) and thin-bedded sandstones (Winsor Group). Looking back towards the east, the parties gained their best view of the White Cliffs. The highest point reached was at Coal Hill, a southward extension of the Cretaceous plateau which lies to the north, where exposures of Cretaceous Dakota Sandstone and Tropic Formation shales, lignites and coals could be seen. West of Coal Hill, the road begins the long descent to Zion National Park.

Fig. 23A. An incised meander of the River Colorado at Dead Horse Point, near Moab, Utah. The wide bench in the lower centre is of Permian marine limestone, with cliffs of Triassic and Jurassic sandstones behind. Fig. 23B. View north from Sunset Point across Bryce Canyon National Park towards Boat Mesa. The extent and scale of 'hoodoo' development is well-seen.

129

(i) Zion National Park

Just beyond the park entrance the parties stopped for a spectacular view of the famous cross-bedding at Checkerboard Mesa (Fig. 24). Here, the welldeveloped dune cross-beds in the bleached white Navajo Sandstone were seen to be cut by a series of parallel vertical joints, giving the whole rockface an intricate net-like pattern. Past here, the road loops along the twisting valley of Clear Creek, and a short tunnel section occurs at one point. Clear Creek eventually joins Pine Creek and the valley then becomes so narrow that the road has to abandon it and bypass the restriction by means of a remarkable tunnel, just over a mile long, complete with 'windows' though the wall nearest the valley side. Immediately before the tunnel entrance is the Zion Canyon Overlook Trail, visited by both parties. The trail is quite short (400 yards) and traverses ledges on the north side of Pine Creek. Members observed many plants, birds and lizards, as well as seepage-lines, cross-bedding, the overhang cave and slickrock features such as desert varnish on their way to the overlook. At the far end of the trail is one of the most impressive views in Zion, a gigantic plunge into the lower part of Pine Creek Canyon as it runs down to join the Virgin River in the distance. Enormous cliffs of desert-varnished Navajo Sandstone were seen towering above the mudstones, silts and sandstones of the more gently sloped Kayenta Formation near the valley floor. Distant peaks, such as the Beehives, Meridian Tower and the Altar of Sacrifice, revealed the rounded weathering of the Navajo Sandstone. The view also showed the switchbacks of the road after its emergence from the tunnel, which lies behind the massive rockwall to the left of the viewpoint. What the parties did not appreciate, until they had passed through the tunnel, was that the Canyon Overlook Trail ends on the top of a huge natural arch held up against the valley walls-the Great Arch of Zion. Our route continued down through the tunnel; looking back at the Great Arch we emerged onto the floor of Zion Canyon, carved by the Virgin River North Fork. The parties first stopped at the Visitor Center, where an excellent array of local cacti were observed, before driving north into Zion Canyon. One of the most obvious features of Zion is the fact that the major cliff-forming element-the Navajo Sandstone-is not red, in marked contrast to a great deal of the Permian and Mesozoic sequences of the Colorado Plateau. It is dominantly a bleached white colour, compared with the red and orange colours which it had displayed at previous outcrops. This white colour is particularly characteristic of the Navajo in the Zion region, as is its tendency to weather into typically rounded hilltops. The overlying rocks had been well-seen in our drive along the Mount Carmel scenic highway, and include variable deposits of the

130

K. L. DUFF & T. D. FORD

Fig. 24. Checkerboard Mesa, Zion National Park. Strongly cross-bedded Jurassic Navajo Sandstone permeated by welldeveloped subvertical joints.

Carmel, Entrada, Curtis and Winsor Formations. From this succession it is immediately apparent that, although there are red coloured rocks at Zion, the colours are usually much more varied and it is this contrast of white, tan and red layers which gives Zion much of its beauty. The white upper slopes and hilltop cappings are eroded outliers of the White Cliffs, and the lower slopes cut down through the Kayenta and Moenave Formations seen in the Vermilion Cliffs. Zion Canyon itself is a deeply-incised gorge cut by the North Fork of the Virgin River, and is characterised by massive near-vertical cliffs towering up to 2,000 feet above the valley floor. Although several formations outcrop in these vertical cliffs, portraying them on a geological map is virtually impossible as the various boundaries lie on top of each other! Like so many of the other scenic features of the American west, the canyon is a relatively young feature, having been formed by the enormous erosive power of the river, combined with the agency of mass wasting. Once the river had cut the gorge, mass wasting then proceeded to enlarge it, aided by the nature of the Navajo Sandstone and the underlying

Kayenta Formation. The Navajo is permeated by vertical joints and fractures, which assist the passage of water descending through it from the plateau above. When the water reaches the base of the sandstone beds it has to migrate sideways, being unable to pass readily through the Kayenta mudstones beneath, and emerges as valleyside springs. Erosion on the spring lines, coupled with water pressure and the jointing in the Navajo Sandstone, enables large masses of the canyon walls to collapse, sometimes forming arches, or else falling as detached columns or spires. The debris of all this activity usually covers the lower slopes of the canyon, obscuring the outcrops of the Kayenta Formation. Driving north into the main canyon, the parties noted that the route passes first between two remnants of a great landslip which once blocked the canyon floor. Twenty feet of Holocene lake sediments built up behind this obstacle before the powerful Virgin River was able to cut through it. Some of this sediment still remains, whilst the landslip material is now divided into two, forming detached klippen resting on slide planes. From the Court of the Patriarchs rest area members

131

USA FIELD MEETINGS 1981 & 1982

had a good view of the west side of the valley, with three peaks of the Patriarchs visible over the river. They soon found, however, that it was difficult to get all three in one photograph, even with a wide angle lens, since the scale of Zion's cliffs is so enormous. From the Grotto car park, a little further north a footbridge crosses the river and provides access to the west bank trails. The two main trails available are the Emerald Pools Trail and The Angels Landing Trail, and members took advantage of both, depending on their fitness and adventurism! The Emerald Pools Trail leads to pools formed below two massive waterfalls over the Navajo Sandstone, but unfortunately the shortage of water during the summer meant that there was a disappointing change of colour from emerald green to muddy brown! The Angels Landing Trail was very much more strenuous and involved a 5 mile round trip with an ascent of 1,500 feet, much of it in very exposed situations over the last half mile. Those members who ventured on this route soon

Fig. 25. Angels Landing, Zion National Park. The route to the top follows the fin of rock in the foreground, with sheer drops of some 1000 feet to the valley floor on either side. The figure in the foreground gives a scale.

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Fig. 26. Relationship of the stratigraphic sections seen in Bryce, Zion and the Grand Canyons.

discovered that the upper parts involved climbing up narrow paths over bare rocks, holding onto chains anchored to the rock, and often with sheer drops of over 1,000 feet immediately behind them (Fig. 25). Those who reached the summit agreed that it was an experience they would never forget, especially since superb views over the whole canyon area were their reward. Other members of the parties visited Weeping Rock where they noted the tufa-encrusted channels and the springs seeping out of the base of the Navajo Sandstone above; a very rich flora was seen to inhabit this moist environment. Further up the valley from the Weeping Rock, members followed the course of a deeply-incised meander around the foot of The Organ, and reached the entrance to The Narrows, a huge cleft extending northwards away from the open land of the canyon floor. The cliffs tower above the river, which is totally confined between sheer rock walls often no more than 20 yards apart. The Narrows exist in their present form because they are walled entirely by Navajo Sandstone, and have not yet been cut down to the level of the weaker Kayenta Formation. Once they reach that level, undermining and collapse will start to occur and they will gradually widen out. The Narrows demonstrate what Zion must have looked like all the way down the valley when the river first began to cut the gorge. As with the Colorado gorge across the Kaibab Uplift, which became the Grand Canyon, it is generally believed that Zion Canyon was formed through progressive downcutting by a river which existed here before the uplift of the plateau. Viewed together Bryce, Zion and Grand Canyons represent the successive stages of river incision into the Precambrian to Lower Tertiary stratal 'layer-cake' of the Colorado Plateau; the whole stratigraphic succession is exposed in their walls (Fig. 26). The 1981 party left Zion in the mid-afternoon and drove straight to Las Vegas, whilst the 1982 group spent all day at Zion and stayed overnight at the Pioneer Lodge, Springdale, only a few miles down-valley from Zion. (j) Zion to Las Vegas

Leaving Springdale and driving westwards, the parties observed good exposures of the Chinle Formation

132

K. L. DUFF & T. D. FORD

along the roadside with the entire formation in plain sight as a broken slope that leads upward towards the base of the towering cliffs of Navajo Sandstone. Just south of Springdale, to the left of the road, a prominent mountain known as The Watchman demonstrates a clear section of the Triassic and Jurassic rocks of the area, with high vertical cliffs of Navajo Sandstone towering above the talus-covered slope of the Kayenta and Wingate, and falling to a smaller vertical cliff in the Springdale Sandstone Member of the Chinle Formation, with more talus slopes beneath. The lowest unit seen is the Petrified Forest Member of the Chinle Formation, which consists of an upper sequence of irregularly-bedded red, grey, yellow and green sandstones, conglomerates, limestones, shale and gypsum, resting upon a lower sequence of shales, soft sandstones, weathered volcanic ashes and various calcareous rocks coloured yellow, green, purple, pink, lilac, grey, red, blue and brown. The units constitute the most richly coloured rocks in Utah! The grey Shinarump Conglomerate forms a level bench beneath the Chinle Formation, and the lowest Triassic Moenkopi Formation appears at Rockville and is magnificently exposed along both sides of the Virgin River between Rockville and Hurricane Cliffs. Just west of Grafton, the hills on the north side of the road for about 5 miles between Coalpits Wash and North Creek were seen to be capped by Tertiary basalt lava flows, possibly associated with the Hurricane Fault. Approaching Hurricane, there is a narrow outcrop of Permian Kaibab Limestone immediately to the east of the Hurricane Fault, which is a major N-S trending fault, with an estimated down throw of about 8,000 feet to the west. The fault forms a massive line of cliffs known as Hurricane Cliffs, and to the west, Jurassic rocks are exposed, often overlain by Quaternary basalts and alluvium. The Virgin River is deeply entrenched into the lavas and there are hot springs close to the highway bridge suggesting that volcanic activity is only dormant! On entering Hurricane, the 1982 party took a side road to the south which climbs back up the lava flows of the fault scarp to an overlook whence the structure of the Virgin River Anticline and its cap of Quaternary volcanoes with their ashes and lavas can be appreciated (Fig. 27). Even the youngest lavas are displaced by faulting, though in the Grand Canyon to the south the displacement on the Hurricane Fault in the Precambrian is in the opposite direction. Clearly it has moved several times. West of Hurricane highway 9 crosses the Virgin River Anticline and the familiar sequence of Triassic and Jurassic sandstones and shales was clearly seen on both sides. Tertiary lavas blanket the red sediments westwards until the parties joined I15 near St. George. The road then follows the Virgin River as it cuts a progressively deeper gorge westwards down through

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the Triassic, Permian, Carboniferous and into the Cambrian, rather like the Grand Canyon in miniature. This entrenched canyon lasts for about 15 miles, and cuts through a sequence largely composed of dolomite limestones. Very suddenly, both road and river cross the Grand Wash Fault and emerge onto a wide flat plain covered by Quaternary alluvium, with a major cliff-line extending away to north and south. These are the Grand Wash Cliffs, a fault-scarp formed along the line of the Grand Wash Fault, and marking the junction between the Colorado Plateau Province to the east and the Basin and Range Province to the west. Both parties made a brief stop off the freeway at the next intersection, at Littlefield, to obtain a view back to the cliffs. The fault downthrows about 3,000 feet to the .west, causing back-tilting of blocks of Palaeozoic and Mesozoic sediments on the downthrown side. Soon after entering Nevada on I15, the 1982 party took a left turn at Glendale on to State 12 to the Valley of Fire State Park, which provided a miniature equivalent of much of the tour, with Palaeozoic limestones and Mesozoic sandstones tilted and faulted, and Tertiary volcanics and gravel sheets on top, all within a few square miles. The tilted Navajo Sandstone has been eroded into a fantastic array of vari-coloured pinnacles and canyons with abundant petroglyphs scratched into the desert varnish. State 12

133

USA FIELD MEETINGS 1981 & 1982

continues round the south of the Valley of Fire and along the north shore of the artificial Lake Mead, held up by the Hoove r Da m . The road crosses wide expanses of alluvial grave l filling basins between fault blocks, mainly of Mississippian limestone with patches of Tertiary lavas, and joins highway 93 just nort h of Hoover Dam . Built in the 1930s the wall of concrete fills the Colorado Go rge to a height of nea rly 700 feet , providing hydroelectric power for much of southe rn California and Nevada. The dam is founded on granite with dolerite dykes. Guided tours and automatic lectures illustrate the nature of this engineering marvel. Las Vegas lies some 30 miles to the north . Both parties stayed at the La Quinta Motor Inn, on Las Vegas Boulevard. This lies on 'The Strip' , the road upon which all of the famous Las Vegas hotels and casinos are located, and ensured that members were well-served with evening entertainme nts! In 1981, the Director made the mista ke of hand ing all members free copies of a local paper which he had been given by the motel management , and did not discover until later (when tactfully button-holed by several members) that its contents and advertisements were somewhat more direct than had been expected! The nocturnal att ractions of the city were sampled , to varying degrees , by all members and both Direc tors were greatly relieved to discover that the parties which left the following morning contained full complements! Clearl y the GA will not forget Las Vegas; I wonder if Las Vegas will forget us? Las Vegas marked the 1981 party's change from a hired Greyhound bus to a Trailways bus, Gre yhound having refused to drive through Death Valley in September. Our new driver , Max Baker, proved to have a much more accomodating temperament, which turned out to be most fort unate! (k) Las Vegas to Bishop via Death Valley This was expecte d to be 'a long, very hot, but spectac ular day, with memorable heat in Death Valley' . In fact, the 1981 party found this to be an uncannily accurate pred iction , since a bus breakdown left them stranded on the flanks of Death Valley for over eight hours! Early star ts from Las Vegas were essential for this journey, the 1982 party (having learnt from the previous year's experience) leaving at 6 a.m ., some two hours earlier than in 1981. The two groups took different routes into Death Valley, but saw similar features , since the whole day's journey lay within the Basin and Range Province , with desert environments as the main feature of interest . The 1981 party travelled out on US 95 towards Reno , turning south on State 29 at Lathrop Wells towards Death Valley and then west at Death Valley Junction on 190 down to the valley floor (Fig. 28). To the sout h of 190 the party observed the Greenwater Mountain Range,

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consisting of Tertiary volcanics capped by Quaternary volcanics, whilst to the north lie the Funeral Mountains, consisting of folded Palaeozoic s. Feat ures

134

K. L. DUFF & T. D. FORD

Fig. 29A.

Fig. 298.

USA FIELD MEETINGS 1981 & 1982

135

such as this, which bring rocks of very different ages Nearby, the salt-flats have been corroded into a into close juxtaposition, are due in part to the major pseudo-karst of clints and grykes known as the Devil's faulting which characterises the Basin and Range Golf Course. The party noted that some low points on Province. Descending the pass, the party noted that the road had muddy puddles and scattered boulders the solid geology was extensively mantled by a thick here, the result of a 'storm' of 0.1 inch of rain the cover of superficial deposits, derived from rapid previous day. Even such a small amount of erosion of the surrounding uplands; occasionally, precipitation can move surprisingly large rocks when hydrothermal mineral veins could be observed to cut funnelled down bare rock canyons on to alluvial fans. these recent deposits, suggesting the youthfulness of After admiring these effects, the party continued to the volcanicity in this area. The party were interested Furnace Creek grocery store and the Visitor Center, to stop and examine a large noticeboard setting out where they rejoined the route of the 1981 party. Death Valley Survival Hints and at the same time had In 1933 almost 3,000 square miles of southeastern a good view of the numerous small borate mines in the California and western Nevada were set aside as uplifted and folded Pliocene sediments. The main stop Death Valley National Monument. It is the hottest, on the descent into the valley was at Zabriskie Point, driest and lowest area of any of the American parks from where an excellent view was gained of gulley and has an annual precipitation rate of only 2 inches erosion in folded Pliocene clays with borates and per year. Topographic relief within the area ranges gypsum belonging to the Furnace Creek Formation. from over 11,000 feet at Telescope Peak to 282 feet The main floor of Death Valley was visible in the below sea level at Badwater, less than 20 miles to the distance, with 10,000 feet high mountains beyond, and east. This topographic relief is much less than it was in large alluvial fans spreading onto the salt floor of the earlier times, since in this enclosed basin all of the valley. The party drove on down-valley, to emerge on erosional debris carried down out of the mountains is the main valley floor and continued the short distance deposited on the floor of the valley. The total northwards to Furnace Creek Ranch, where the thickness of these basin deposits is very great, and has grocery store provided cold drinks, which were much been estimated at over 10,000 feet on geophysical appreciated, as was the old steam engine used to haul evidence. Death Valley is a major structural basin within the borax out of the valley. A limited range of books and publications was available here, but the best source Basin and Range Province with the upfaulted blocks was the nearby Visitor Center, which has an excellent forming the bounding mountain ranges. Faulting museum and geological display. The elevation here is began in the late Tertiary, and was mainly normal 192 feet below sea-level, and daytime temperatures faulting that gave rise to block-faulted mountains normally reach about 120°F; the record is 134°F, and bounded to the west by faults (giving steep fault at night it cools to a summer average of about 85°F. scarps), and sloping down more gently towards the The 1982 party took a different route into Death east. This configuration is well seen in both the Valley via 160 to Pahrump across typical alluvial basin Panamint Ranges and the Black Mountains, with fill, south into California at Shoshone, and then on 178 tilting of the upfaulted blocks being responsible for the through the Salisbury and Jubilee Passes into the gentle eastern slopes. Death Valley is still a southern end of Death Valley (Fig. 28). The long downward-moving block today, as is amply evidenced descent down an alluvial fan from nearly 4,000 feet to by fault scarps which may be seen across recentlybelow sea level brought home the significance of formed alluvial fans. Volcanism was also an active desert piedmonts. The road turns northwest along the process here, with basaltic lava flows being particularly foot of the Black Mountains, crossing a series of widespread in the northern areas. The most impressive and characteristic features of alluvial fans, some broken by recent faulting, to Badwater (Fig. 29A), the lowest point in the western Death Valley, however, are those which demonstrate hemisphere at 282 feet below sea level. Freshwater weathering and erosional processes in arid environsprings emanating from the fault scarp here are very ments. There is a great variety of features which soon contaminated by the salts of the valley floor. combine to make Death Valley a classic area for the study of such processes. Alluvial fans, most of them very large, are widespread in the valley, since conditions are optimum for their formation. Large amounts of coarse, weathered materials which Fig.29A. Some of the 1982 party at Badwater Springs on the accumulate in the mountains are flushed out by the edge of the salt flats in Death Valley National Monument, periodic torrential floods and, since this is a California, the lowest point in the western hemisphere at 282 tectonically active area, with continuous uplift of the feet below sea level. mountain blocks, there is a vast and continued source Fig. 29B. The Tufa Towers revealed by the shrinking Mono of supply for the fans. In the stable areas, pediments Lake, California. The Sierra Nevada can be seen in the (erosional features) are normally developed at the base of mountains. Research has shown that some of distance.

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K. L. DUFF & T. D. FORD

the smaller pediments have been almost completely buried by alluvial fans, suggesting that the pediments were formed prior to the most recent phase of sustained tectonic activity, and that they are now being covered by alluvial fan deposits. So extensive and closely spaced are the alluvial fans that they overlap each other, with a series of coalescing alluvial fans forming alluvial slopes known as bajada. In Death Valley, bajadas are now the dominant landforms, but where the canyons are more widely spaced, individual fans have maintained their own identities. The effects of Pleistocene climatic change can be detected in the Death Valley area, albeit in a manner different to that in areas where large glaciers developed. During periods of higher precipitation the excess water formed large lakes in the enclosed basins of the province. In Death Valley, Lake Manly was formed at this time, and grew to be about 90 miles along and 585 feet deep at its maximum stage. When the lake level remained constant for an extended period, wave-cut terraces were notched into the sides of the mountains, and these ancient shorelines can be clearly seen today. About 11,000 years ago, with the warming and drying that culminated in the Thermal Maximum, Lake Manly disappeared. By evaporation of the lake waters, all of the various salts were precipitated to form the thick playa deposits which floor the valley and, of these salts, borax was the most abundant. The Devil's Golf Course, between Furnace Creek and Badwater was visited by the 1982 party and was found to consist of jagged salt ridges and pinnacles in the central part of the salt pan. Detailed discussions on the processes involved in the formation of the salt pan features, such as desiccation cracks, salt wedges and salt pools, will be found in Hunt (1975). Playa lakes form during storms, to vanish in a few days, mainly by evaporation. The playa deposits consist of silts and clays, interbedded with salts which were formed by precipitation as the lake water evaporated. Sand dunes are found in certain places in this desert area, but are nowhere near as widespread as might be expected, in view of the high winds and sparse vegetation. The main dune areas are in the vicinity of Stovepipe Wells, where the dunes are composed mainly of quartz sand. Continuing north from the Visitor Center, the 1981 party pulled in by the roadside some four miles later, where the salt flats make their closest approach to the road, and made an expedition onto the flats. Their first discovery was that distances are highly deceptive in this terrain, with the flats being rather further away than was anticipated. On reaching them, it was discovered that the area consisted of a complex pattern of small dried-up salt pools surrounded by more marly deposits, all showing classic desiccation features, and with the sediments feeling surprisingly damp only a few inches below the surface. Members also appreciated the low humidity in the valley, which

made temperatures of 120°F seem bearable in 1981, though the 1982 party enjoyed a cool 95°F. Continuing north, both parties stopped near Stovepipe Wells to admire the sand dunes to the north. All the way through Death Valley, the huge alluvial fans and pediments were seen to emanate from deep canyons cut into the bare rock surfaces of the mountain ranges with such apposite names as Funeral Range, Spectre Range, Ghost Range and Black Mountains. The best views of the alluvial fans came at the start of the long ascent out of Death Valley up an enormous alluvial fan system derived from the Panamint Mountains to the west. The climb to the summit of Townes Pass involved an ascent of 5,000 feet over a steady rise lasting 22 miles, before the road dropped into the Panamint Valley, the next valley west. Towards the top of the climb there were clear indications that the 1981 Trailways bus was having difficulty and shortly after having reached the summit the worst fears were realised, as it ground to a halt overlooking the Panamint Valley. After a diagnostic examination by the driver, ably assisted (at the cost of a pair of trousers and a shirt) by our resident motor engineer, David Bryant, it became obvious that a replacement bus was needed. With true British stoicism the party settled down to wait, some remaining in the bus (until the heat became too much), whilst others wandered off to explore the nearby desert! Tony King, having commandeered a passing car, was carried away to find a telephone (close by, or so we thought), to reappear some 5 hours later in the custody of a California Highway Patrol Officer who had brought him back from the nearest telephone, some 60 miles away in Lone Pine! The last two hours of our wait was spent in the dark, with members examining scorpions (absorbing the heat from the road surface) by means of torch-light and wondering about the rustling noises which we kept hearing in the brush; after receiving a visit from one of the Death Valley Rangers, who observed that there was no way that he would wander about this road after dark, the party retreated to the bus. Eventually, at about 11 p.m. the replacement bus arrived from Las Vegas, and the party continued on its journey to Bishop, arriving at the motel (Best Western Holiday Lodge) at about 2 a.m. All who experienced this unplanned surprise were agreed that it was the most memorable part of the tour! Probably the most surprised, however, was the bus-driver, who could not believe the calm way in which the situation was accepted; he clearly expected that if it had been an American party, he would have been lynched! The 1982 party made a brief and nostalgic stop at the breakdown point, four of their number having experienced the previous years excitement. This seemed to tempt providence, since a few miles further one of the cars suffered a most unusual breakdown as one of the plastic wheel-trims melted with the heat

USA F IELD MEETINGS 1981 & 1982

from the brakes and began to cut into the tyre walls. Delays were caused whilst a replacement was fitted at Lon e Pine and the desert was henceforth treated with more res pect ! As a result of these mishaps, the 1981 party saw noth ing of the geology between Death Valley and Bishop and the 1982 party were unable to appreciate it properly. The route lay across the margins of the Basin and Range Province as far as Lone Pine , with similar fea tures to those see n in Death Valley . At Lone Pine the parties turne d north along State 395 towards Bishop , up the valley of the Owens River, the western most basin of the Basin and Range Province. Immediately to the west the high granite bath oliths of

137

the Sierra Nevada rise stee ply above the valley, rising to a height of nearly 14,500 feet at Mount Whitne y nearb y. Th e margin of th e Sierra Nevada is fau lt-bounded , and movement still occurs along it, the most recent major earthquake having flattened Lone Pine in 1872, and given rise to a fault-scarp 18 feet high in alluvium, much of it still visible. Continuo us views of the Sierra Nevada were visible for the remainder of the journey to Bishop. (I) Bishop to Yosemite

Travelling north from Bishop on US 395 the par ties' rout e was largely over Pleistocene volcanics at the

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Fig. 30. Simplified geological sketc h map of the Mono Craters and lake area, California (based on parts of the Geo logic Map of California, Walker Lake and Mariposa Sheets).

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K. L. DUFF & T. D. FORD

northern end of the Owens Valley (Fig. 30). The Sierra Nevada towers on the left, with the light granite cupolas contrasting with dark roof pendants of gneiss. Sheet jointing and much granitic veining are visible closer to the road from time to time. Five miles out of Bishop terraces of the Bishop Ignimbrite lie to the right, representing some 35 cubic miles of acid tuffs which filled this part of the Owens Valley. Small vents lie along the marginal faults. The Tungsten Hills, to the left, consist of Palaeozoic roof pendants in the edge of the granite, and include the dark red folded masses on the skyline. Scheelite mineralisation here is metasomatic in origin and occurs in limestone pendants which have been altered to skarn. Many workings and mine roads were visible on the hillsides; the ore reserves are 10 million tons, with 0.7% tungstate. The long steep upgrade to Sherwin Summit has roadcuts in Pleistocene lavas, pumice and tuff. At 21 miles, roadcuts on the right near the road junction showed both till and Pleistocene ash. Ash and lava dating has shown that some till in this region is 3 million years old, and provides evidence of the earliest glacial advance in North America; these exposures lie at only 8,000 feet on Deadman Pass, and so it is clear that the glaciation in this area started much earlier

than it did in Europe. At 23 miles, a large cut on the right revealed ignimbrite of the Bishop tuff lying on 750,000 years old boulder clay. At 43 miles State 203 leads left to Mammoth Lakes and the Devil's Postpile, with Mammoth Mountain to the left being a silicic plug dome. There was insufficient time for us to visit this well-known and spectacular National Monument. At 50 miles Deadman Pass at 8,040 feet was crossed and one mile past the pass, Obsidian Dome could be seen as a series of broken ridges through the trees to the left. A short diversion here to the left led onto a dirt road for less than a mile to the steep margin of the obsidian dome (Fig. 31). The obsidian shows excellent banding features on both a macro- and microscopic scale. At 60 miles the road crosses onto a plain formed of Quaternary alluvial sediments. Mono Craters, to the right, are a series of 6,000-year-old fault-aligned volcanic vents which gave rise to obsidian ash cones and rhyolite plugs. The 1982 party turned right on State 120 for about 6 miles, to the point where the road passes close to the north end of Mono Craters, and where several circular domes form the highest parts of the volcanic mass. One of the craters is only a few hundred feet from the road, and a short walk took the party up the flank of a

Fig. 31. The front of the obsidian flow at Inyo Craters, Deadman Pass, Owens Valley.

USA FIELD MEETINGS 1981 & 1982

cone of loose fine pumice particles blown out by a series of explosive eruptions. The cone contains a crater with a late obsidian dome in the centre . This is covered with blocky lava rubble, much of it consisting of dark volcanic glass (obsidian) , which in places displays fine flow laminae and contains layers of frothy pumice. The 'craters' are not craters in the ordinary sense. Instead , the Mono volcanoes represent a relatively unusual type of volcanism which took place in a repetitious sequence of events in different parts of the extensive Mono Craters complex . Three main phases can be discerned; the first involved development of explosion pits consisting of conical depressions whose sides slope inwards. The material forming the walls and rim of the pits consists of pumice and fine volcanic fragments, although large fragments of obsidian are also abundant. The second stage involved the rise of a stiff, cylindrical, essentially solid column of obsidian, which formed a viscous dome (rather than a fluid lava flow) in the floor of the explosion pit. As the obsidian continued to ascend , it eventually spilled over the rim of the explosion crater, forming a coulee with a rough , blocky steep face. The extreme ruggedness of the obsidian domes is due to the fact that they hardened at the surface whilst the interior was still molten and continued to flow. The steep faces are due to the high viscosity of the lava , which is a consequence of its relativel y high silica content. The present shape of a dome probably differs very little from the shape at the moment that it 'froze' . The Mono Craters were formed very recentl y, since the grey pumice particles that were broadcast over the area surrounding them rest on moraines formed during the last glaciation , so that at least part of the volcanic activity that formed them is post-glacial. In spite of the relatively recent eruption there is no sign of volcanic activity at Mono Craters today, though persistent tremors around Mammoth Lakes to the south suggest the rise of magma . Mono Lake lies immediately north of the craters, and a short side road left off 120 took the 1982 party down to a beach amongst tufa towers on a fossil shore line (Fig. 29B). The lake is highly saline and has not overflowed since glacial times as evaporation normally balances inflow. In recent years, however , diversion of the inflowing streams to the Los Angeles water supply system has meant that the lake level is falling rapidly owing to excess evaporation. Island bird sanctuaries are no longer cut off by open water and the bird popul ation has been decimated by coyotes ; a vigorous local environment protection lobb y has sprung up in Lee Vining and the subject of Los Angele s 'robbing' them of their water has become highly political. Mono Lake is a remnant of one of the intermontane lakes which occupied the valleys in Pleistocene times. Old shorelin es are marked by tufa left high and dryas the lake shrank , and the 1982 party enjoyed their lunch

139

amidst most unusual calcareous scenery before moving on to Yosemite National Park . Returning to US 395, both parti es cont inued to Lee Vining , where they turned west on State 120 towards Tioga Pass and Yosemite . This road follows the northern side of Lee Vining Canyon , providing panoramic views over this typical glaciated deep mountain valley, in which glacial dep ositional forms predominate in the lower reache s, whilst ero sion features dominate the higher reache s. A well-marked terminal moraine was noted in the valley, in addition to several lateral moraine ridges . Vista Point provided a marvellous view down-valley over these features . A little further on the parties passed Ellery Lake observing that it lies in an ice-scoured hollow within the cont act belt of light-coloured granites and dark-coloured metamorphosed siltstones, the latter forming part of the roof pendant; the cont act could be seen in a roadcut near the dam, and was highly complex . The groups stopped at Tioga Lake, a little further on , which clearly occupies an icc-scoured hollow, with roche moutonees being visible around its margins . Reaching the summit of Tioga Pass at 9,945 feet , the parties entered Yosemite National Park , passing onto the main granite outcrop . The eastern boundary of the Park and the crest of the Sierra Nevada roughl y coincide with the contact between the Sierra Nevad a batholith to the west and the masses of metamorphosed sedimentary and volcanic rocks that form the eastern metamorphic belt. The part ies' next stop was at Tuolumne Meadows, a large flat area on the crest of the Sierr a Nevada . This area was once buried beneath a large ice field from which tongue s of ice flowed down into Tenaya Can yon and the upper canyon of the Merced River. Tuolumne Meadows served as a gathering ground for glacial ice, and at times only the high distant peaks remained abo ve the ice. The effect of this great mass of slowly-moving ice was to scour the are a, so that when the ice sheets melted, the shallow depressions which were left became shallow lakes, subsequently filled with sand and gravel brought in by stre ams, creating the broad meadows which now exist. Reeds and grasses, also helped fill the lakes by providing organic material. Trees are presently colonising some of the meadow are as and it is likely that the meadows will eventually be transformed into forest, if the process is left to proceed unchecked. A major deb ate is currently in progress within the National Park Service as to wheth er an atte mpt should be made to retain the present status quo , which would undermine the naturalness of the area, or whe ther natural processes should be allowed to continue , with consequ ent loss of the pre sent open views. Both glacially-shaped and exfoliation domes could be well-seen from the Meadows . At Tenaya Lake the parties examined glaciallypolished granite, cut by aplite dykes and contain ing

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K . L. D UFF & T . D . FORD

dior ite xenoliths and large orthoclase phenocrysts. Num erous perched erratics were seen resting on the polished surfaces. The final stop on the high plateau of Yosemite was at Olmstead Point , from which excellent views of Half Dome were obtained , and where it was possible to examine the classic example s of exfoliation in nearb y gra nite dome s as well as glacial striations and erra tic blocks close to the road . Driving on to their accommodation in log cabins at Curry Village, the part ies gained impressive views up the Yosemite Valley, over Bridalveil Falls and towards EI Capitan and Half Dome . Both parties stayed at Curry Village , the 1981 group for two nights and the 1982 group three. This proved to be the least comfortable accommodation of the trip, since the cabins contain no running water, no toilets and little other than a bed and a heater! Many members felt this to be something of a let-down, whilst others felt that it added to the rur al charm! However, the simple fact is that Yosemite is exceedingly popular among st Californians for their summer holidays, with the result that all the best accommodation is usually booked at least 12 months in advance and so both parties had to accept what accommodation remained . The altern ative was to stay at a hotel in Merced , some 80 miles away! There were some compensations , however , as a number of memb ers found when the y were accosted by hungry raccoons ! (m) Yosemite National Park

Yosemite Valley has evolved under the combined influences of regional uplift , erosion by runnin g water, glaciation and alteration and decay of rock through weathering processes. It is glaciation, however , which has been the prime influence on the development of the valley. Before Yosemite Valley was glaciated, it probably had the V-shaped cross-section typical of river valleys, and before that it was probably merely a broad shallow valley prior to the uplifts which gave the Sierra Nevada its present height. Successive phases of uplift throughout the Cenozoic gradually raised and tilted the Sierra block, resulting in realignment of the main strea ms and the inception of an essentially par allel drainage system flowing westwards perpendicular to the axis of the Sierr a block . Matthes' (1930) interpretation of the evolution of the Yosemit e valley from preglacial times onwards is still bro adly accept ed today and is shown diagrammatic ally in Fig. 32. Figure 32A represent s the broad-valley stage when the Merced River flowed in a bro ad valley formed in response to the original gentle tilt of the Sierra block . The gradient was sufficient to allow the Merced River and its tributaries to erode a wide valley, above which the irregular uplands arose perh aps 500 to 1,000 feet above the valley floor. The tribut ary strea ms that joined the Merced River did so without passing over wat erfall s or steep cascades. The

A

B

Fig. 32. Simplified geomo rphological diagram showing the evolut ion of the Yosemit e Valley (ba sed on a diagram by F. E. Matthes). ec - EI Capitan; g - Glacier Point; hd - Half Dome ; mr - Merced River; sr - Sentinel Rock; LY - Lake Yosemite.

broad- valley stage ended when the Sie rra block was uplifted again, ushering in the moun tain-valley stage (Fig. 32B) pro bably in the Pliocene or early Pleistocen e. Th is caused the river to cut down deep ening the main valley as well as tho se of the tributa ry streams. Many of the tributar ies now entered the main valley down steep cascades, left hanging as the main valley was cut deeper. Later, the re was addition al uplifting of the Sierr a block , causing the Merced River to erode its valley still deeper , achieving canyon-like proportions, with a depth of 2,000 to 3,000 feet , and reaching the canyon stage (Fig. 32C). The canyon, prior to invasion by glacial ice, was somewhat V-shaped in overall cross-section, and the river had carved a narrow inner gorge perhaps as deep as 1,500 feet in places. The narrowness of the canyon was accentuated by the numerous irregular spurs which projected into it , and the topographic relief became more pron oun ced through rapid downcutt ing of the main stream, and makin g the hanging valleys even more prominent. The sequence of events which transformed the can yon stage into the present situation (Fig. 320) was controlled by the glacial history of Yosemite . The whole of the Yosemite Valley area was filled by ice gener ated on the high Sierra to the east , and fed down the valleys of the Merced River and Tenaya Creek, to coalesce just west of Half Dome. The effect of the ice was to grind away the projecting spurs, plane off the sloping canyon walls to leave near -vertical cliffs, and to gouge a dee p basin

US A FIELD MEETINGS 1981 & 1982

in the valley floor; geophysical evidence indicates that the bedrock floor of the valley generally lies more than 1,000 feet below the alluvial deposits which form the present valley floor. Massive end-moraines in the vicinity of Bridalveil Fall and El Capitan are believed to have acted temporarily as a dam , behind which a lake (Lake Yosemite) was impounded, and which was subsequently filled with sediment during the later glacial stages. The large rock domes , such as Half Dome, are perhaps the most distinctive features of Yosemite. These result from the behaviour of massive granitic rocks , such as the Half Dome quartz monzonite, which spontaneously undergo fracture so as to create huge curving exfoliation sheets. The fractures may form as the granite mass, which formed at great depth, expands after erosion has removed overlying material that was once several miles thick . The curving fractures allow shells of granite to separate and fall away, locally leaving dome-like features. Whilst some granitic domes were covered with ice, others (including Half Dome) were not. The general form and the steep face of Half Dome were determined by a set of nearly vertical joints trending NE-SW , and which divided the mass into steeply-dipping sheets of rock which were readily plucked away by the Tenaya Glacier , which at one time reached within 500 feet of the top of Half Dome. Vertical slabs continue to fall away today . It is probable that the principal means by which Half Dome formed were through the processes of exfoliation, coupled with the local joint system in the granite. It is not generally believed that a former, more or less symmetrical, exfoliation dome existed, and was cut in half by glaciers moving down the valley; glaciation was probably only of secondary importance. The presence of Tenaya Canyon immediately below the steep face of Half Dome has, however , no doubt been influential by providing a route through which exfoliation debris from Half Dome's steep face could be carried away. During their stay in Yosemite, both parties undertook a wide range of collective and individual activities , according to their particular tastes. They rapidly discovered, however, that Yosemite's waterfalls are not at their best during the summer months and the evocatively-named Mirror Lake is also notable for its absence! Excursions undertaken included walks to Vernal Falls, with good views of Nevada Falls, the Liberty Dome roche moutonee and the backslope of Half Dome; the Nevada Falls , with similar good views; and to the summit of Half Dome, a precipitous journey occupying a full day, and only undertaken by four members of the 1981 party and two in 1982. The final 700 feet of the ascent resembled Angel 's Landing in Zion , in that cable ladders and chains were needed, but if anything the Half Dome route is more exposed; those who followed this trail returned with some spectacular pictures!

141

The most popular visit, however , undertaken by most members , was the visit to Glacier Point, standing over 3,000 feet above the floor of the valley and providing marvellous views across and up the valleys of Tenaya Creek and the Merced River (Fig. 33A). The route passes along the valley floor, with views of El Capitan showing the 3,000 feet sheer cliff in granite, cut by diorite inclusions and dykes ; one of these dark masses is known as the 'Map of North America' due to its vague resemblance to the overall shape of the continent. The 1981 group drove straight to Glacier Point, but the 1982 party stopped en route at Sentinel Dome (Fig. 33B). Contorted granite-gneiss was observed on the walk up to the dome but the summit itself was seen to be a porphyritic granodiorite with biotite-rich pegmatite veins, aplites and scattered basic xenoliths. Cracks contained deeply rotted kaolinized granite, evidence that whilst the canyon below had been occupied by an active glacier in the late Pleistocene, the dome had at most a sluggish thin cover of ice. Sheet jointing was evident on this and other domes and was thought to be due more to release of confining stresses and expansion as the cover was eroded off, than to thermal expansion and contraction. The leader here gave a peroration to both the G.A. and a fair number of other visitors and this was followed by a lunch-time discussion on the question of the setting of this mass of plutonic rocks in relation to plate tectonic theory . Yosemite's granites are but a part of the Sierra Nevada granite 'batholith', some 400 miles long and up to 100 miles wide. The leader noted that , at first sight , the solid geology of Yosemite appears to be all granite , but that a study of the topographic model in the Visitor Center with geological colours and accompanying specimens shows that there is a wide range from diorites, monzonites and granodiorites to true granites, some porphyritic, some not. At least seven major intrusions can be seen in the 3,000 foot high walls of Yosemite Canyon and Merced Valley and others form domes in more distant parts of the National Park . Cross-cutting relationships can be seen in the canyon walls, as in El Capitan, with the addition of dark diorite dykes intersecting the granite at all angles. In the higher parts of the park the granites (s.l.) form a plexus of domes such as Half Dome, separated by shallow canyons, some containing roof pendants or septa of country rocks into which the plutons were intruded. The country rock is highly metamorphosed sediments, generally of greywacke aspect and of uncertain age , Jurassic and older. The difficult concept of all this granite being intruded as one single molten mass is disproved when one realises that it is really a large series of separate plutons, each at most a few miles across, intruded over a period of somewhat over 100 million years, from Late Triassic to the Late Cretaceous. Individual

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K. L. DUFF & T. D. FORD

Fig. 33A.

Fig. 338.

USA FIELD MEETINGS 1981 & 1982

plutons vary greatly in size, their outcrop areas ranging from less than a square mile to more than 500 square miles. If the Sierra Nevada batholith is considered as a whole, it appears to consist of a few large plutons and a great many smaller ones which tend to be grouped between them. Radiometric dates of various plutons in the central part of the Sierra Nevada batholith suggest that there are three widely separated epochs of plutonism, the first occurring roughly 190 to 210 million years ago (during the Late Triassic) the second about 126 million years ago (Late Jurassic and Early Cretaceous) , and the last about 80 to 90 million years ago (early Late Cretaceous). The younger intrusions are generally the more easterly. Whether the intrusion processes were continuous or intermittent is difficult to determine. A reasonable explanation suggests that the leading edge of the North American plate lay above a subducting Pacific Ocean margin consistently at one position throughout that period, and that subducted masses of ocean sediments, themselves derived from the over-riding plate , melted at intervals and rose as diapiric plutons through the cover (Fig. 34). It seems that the granitic magmas were generated by melting of siliceous and aluminum-bearing components in preexisting rocks in deeper regions at high temperatures. Laboratory experiments on the melting of granitic rock materials suggest that under pre ssure and in the presence of sufficient water , melting begins at temperatures between 600° and 700°C, and if other substances such as fluorine or chlorine are present, the temperature at which melting begin s is even lower. The depth at which rocks of appropriate composition can be expected to melt to yield a granitic magma is difficult to specify because of inadequate knowledge about the generation and distribution of heat in the crust. In stable parts of the crust where heat is carried to the surface by conduction, a temperature of 600° to 700° may be attained at depths of 20 to 30 miles. However, the melting could have occurred at greater depth; under the corresponding higher confining pressures , higher temperatures would be required. The exposure of plutons at the earth's surface implies remov al through erosion of great thicknesses of overlying rock. There is no simple way of estimating the thicknes ses of eroded material , but perhaps it was in the order of 10 miles thick in the Sierra Nevada. A non-geological side excursion was taken by members of both parties to the south end of Yosemite

Fig. 33A. Yosemite National Park . View eastwards from Glacier Point to Half Dome and Ten aya Canyon, the northern tributary of the Merced River. The summit of Half Dome towers some 4,700 feet above the valley floor. Fig. 33B. Sent inel Dome , Yosemite Nation al Park . One of the many domes in the Sierra Nevada granite batholith.

w

143 E

Fig. 34. Diagrammatic sketch section from the Pacific Ocean to the Basin and Range Province showing the general relationsh ips of the major structural units. PreCPrecambrian ; Pal - Palaeozoic; Mes - Mesozoic; Ter Tertiary sediments and volcanics. In the centr al part of the sketch late granitic plutons are portrayed as being derived from the partial melting of deeply buried and metamorphosed Mesozoic sediments of an accretionary wedge and rising into earlier granitic plutons.

National Park , to the Mariposa Grove of Giant Redwood trees . Some of the giants have probably been growing through much of the span of human recorded history . The small cones of the giant trees contain spores which appear to require the stimulus of a forest fire clearing the ground for germin ation , a fact only recently appreciated by the fire-conscious Park Service! (n) Yosemite to San Francisco Both parties spent an easy day traversing the western flanks of the Sierra Nevada across the San Joaquim Valley and into the coastal ranges near San Francisco . Leaving Yosemite , soon after crossing the granite contact at El Portal , the form of the valley changes into a fluvial incision in the floor of a wider U'-shape , evidence that an early advance of ice extended further , but that the later ice provided melt-water to cut the incision. Road-cuts give good sections in outwash deposits and the U-shaped old glacial trough is soon lost as the Merced River winds in a normal fluvial valley. The river exposes complex plunging folds in Palaeozoic chert and phyllites, and road- sections show slates alternating with massive metavolcanics. A stop in the town of Mariposa gave the 1982 party time for a visit to an excellent Historical Museum, with many relics of the gold rushes , from its discovery in 1848 up to modern times . Further section s in slates and metavolcanics followed , this time of Jurassic age, and as the mountains were left a final road-cut gave fine sections in agmatite, a complex igneous breccia with more xenoliths than granitic matrix! Nearby the yellow-green chromium-mica, maripo saite , was obtained from some loose blocks . The long drive over the flat floor of the San Joaquim valley gave no geological stops , but the parties were reminded that the Qu aternary alluvium here concealed a deep basin filled with Tertiary rocks. From

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K . L. DUFF & T . D . FORD

Eocene to Pliocene in age , these represent the ero sion debris from the Sierra Nevada , with intercalations of marine sediments , and occasional volcanic showers . Once thought to be an entirely flat-lying succession, recent geoph ysical studies, followed up by drilling , have shown that the lower Tertiaries are comple xly folded and faulted , with branches of the San Andreas fault system shea ring off structures laterally. One such fault has moved recently at Coal inga with disastrous results. Oil has been found in a number of structures, precipitat ing a burst of intensive study of this basin. The road rises from the western margin of the valley onto gently undulating hills of older Tertiaries and volcanics of the Coast Ranges, which continue until the Bay Bridge into San Francisco is crossed . The 1981 party spent two night at the Manx Hotel on Powell Stre et , with the picturesque and famous cable-cars passing the door. The 1982 group spent two nights at the nearby Hotel California, on Taylor and O'Farrell Streets. During their stay in San Francisco the parties comb ined a mixture of sightseeing and geology. Flight-tim ings meant that the 1981 group were more restricted , and so all tastes were satisfied by taking a

half-day conducted tour around the city, variously takin g in Twin Peaks , Golden Gate Park and the Pacific Coast , the Golden Gate Bridge and Fisherrnans Wharf. During this, Keith Duff contrived to lose his second jacket of the trip ! The afternoo n was left free for members to use as they wished , and Enid Evans took the opportunity to demonstrate her organisational skills by arra nging a mini-bus hire to tak e fifteen members of the part y out to the Earthqu ake Park near Point Re yes (see below for deta ils). Other memb ers took the opportunity to go shopping or to sample the highly-efficient Bay Area Rapid Transit System, an upgrad ed version of the London Und erground ! The 1982 party decided that , since San Francisco lies astride the San Andreas Fault , they should see someth ing of the fault's effect. Indeed , two of the city's water supply reservoirs are built in the valley along which the fault trace is a prom inent feature . However , the structur al situation is mor e easily seen north of the City across the Golden Gate Bridge (Fig. 35), and the party studied this under the able leadership of Prof. John Harbaugh of Stanford University, author of the 'Geological Guide to North ern California' which had been so useful on the

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USA FIELD MEETINGS 1981 & 1982

Fig. 36A. Intensely-folded greywackes of the Mesozoic Franciscan Complex north of the Golden Gate Bridge, San Francisco.

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Fig. 36B. The trace of the San Andreas Fault in Earthquake Park, north of San Francisco, showing the 16 feet offset of the fence caused by the 1906 earthquake. Professor John Harbaugh is on the right.

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tour. By turning west off the freeway immediately north of the Bridge onto an old fort road good sections of the complexly-folded greywackes of the Franciscan Series were seen at the roadside (Fig. 36A). These sediments form part of a melange of continental margin detritus, principally of Jurassic and Cretaceous age although few fossils have been found, extending for several hundred miles along the Coast Ranges. Here the outcrops are on the east side of the San Andreas Fault. Taking Route 1 through Stinson Beach to Bolinas a series of road-cuts expose other parts of the Franciscan Complex. Keratophyre lavas are apparently interlayered with the greywackes. Serpentinite, appear to be part of an ophiolite complex close to the San Andreas Fault. At Bolinas Lagoon, Franciscan metamorphics lie on one side (east) whilst Pliocene sandstones outcrop on the other side of a salt marsh. Nearby the latter rest on Miocene conglomerates. The highway follows the fault trace to Earthquake Park, where a barn was torn in half by the 1906 earthquake, and a fence can still be seen with its 16 ft. dextral offset (Fig. 36B). A signboard marks the boundary between the American and Pacific plates, and several members were photographed astride the suture lines risking bisection if the fault moved again! The road then turns west to Point Reyes. Some 12 miles west of the fault line, the Point Reyes headland by the lighthouse has fine sections in massive coarse conglomerates, of Miocene age, resting on granite. Polymict in character, these contain halfmetre diameter boulders of granite, and many other igneous rocks. The parent rocks of some of these boulders lie on the other side of the San Andreas Fault near Los Angeles some 400 miles to the South, illustrating the cumulative effect of the wrench fault movement since Miocene times. These features and some sea-lions were seen as cold fog banks swirled in from the Pacific Ocean, giving the party a reminder of the British weather to come two days later! A few miles to the south of the headland the party visited Drake's Bay, reputed to be where Drake careened his ships in Elizabethan times. The cliffs are in the Monterey Shales, (actually radiolarian marls) of Mio-Pliocene age. On the return journey to San Francisco, road sections in sheared pillow lavas of the Franciscan series were examined near Nicasio. On this final evening a celebration dinner was held in the Rosebud restaurant and the party was joined by their leader for the day, John Harbaugh.

The 1981 party also spent their last night at a farewell dinner in the Rosebud restaurant on Geary, close to their hotel, and noted (quite coincidentally) for its supposedly English decor and cuisine. This influence extended as far as the pseudo-mediaeval costumes of the staff, who seemed somewhat bemused by a mass influx of authentic British! The evening provided a fitting and enjoyable climax to a major new venture by the GA, and the Director took pains to thank members for their forebearance during their high-speed tour of the west. He stressed that the following year's party would be better prepared, as a result of the lessons which had been learnt on this trip, especially the need to avoid the backseat of a Greyhound bus, where temperatures can attain near-volcanic intensity! As a momenta of his style of leadership, Keith Duff was presented with a whistle-shaped bar of soap, to commemorate his rallying calls and his apparent lack of spare shirts! A year later, at the equivalent end-of-tour dinner, Trevor Ford was completely surprised to receive magnificent samples of Kanab 'Wonderstone'-Navajo Sandstone with leisegang banding simulating landscapes. The final day of the 1982 party's tour was occupied by shopping, tourism, sampling the BART underground railway and a visit to the California Division of Mines Mineral Museum. Both excursions turned out to be highly successful, and provided those fortunate enough to attend with a wealth of memories which they will not forget easily! Deserts, canyons, glaciated mountains, oilflelds, volcanoes, great faults-all helped towards a fresh outlook on global geology, far beyond the confines of the areas most normally frequented by GA excursions. ACKNOWLEDGEMENTS The Directors wish to thank Tony King for the efforts which he undertook as excursion secretary for the 1982 meeting, and also for the assistance which he provided to the leader in 1981; John Dennis for his 1981 welcome to Los Angeles and his guidance to places of local geological interest; John Harbaugh for leading the 1982 party in San Francisco and at Point Reyes; and to Don Baars for supplying photographs and diagrams used in this report. The production of most of the diagrams in the report was funded by a grant from the Association, and the authors would like to record their thanks to Council.

References BAARS, D. L. 1983. The Colorado Plateau-a geologic history. Univ. New Mexico Press, Albuquerque. 279 pp. - - 1983. The geology in and near Canyonlands and Arches National Parks, Utah. pp. 75-92 In Utah Geological and ASH, S. R. & D. D. MAY, 1969. Petrified Forest-the story Mineralogical Survey Special Studies No. 60, Guidebook behind the scenery. Petrified Forest Museum Association, Holbrook. Part 2. GSA Rocky Mountain Meeting. Books and Periodicals

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& G . M . STEVENSON . 1982. Sub tle stratigraphic traps in Paleozoic rocks of Paradox Basin . In (eds: M. Halbouty & Crook) Deliberate Search for the Subtle Trap. Amer. Assoc . Petro!. Geo!' Memoir 32, 131-58. BAILEY, E. H . (ed.) 1966. Geology of northern California . Calif. Division of Mines & Geol., Bull. 190. _ , W. P. IRWIN & D . L. JONES . 1964. Franciscan and related rocks , and their significance in the geology of western California. Calif. Division of Mines & Geol., Bull.

183. BATEMAN , P. C. & J . P. EATON . 1967. Sierra Nevada Batholith. Science, 158, 1407-17. _ - & C . WAHRHAFTIG. 1966. Geology of the Sierra Nevada. Calif. Div. Mines & Geol. , Bull., 190, 107-72 . BEAL, M . D . 1975. Grand Canyon-the story behind the scenery. KC Publications, Las Vegas . BEUS , S. S. & R. R. RAWSON. (eds .) 1979. Carboniferous stratigraphy in the Grand Canyon , northern Arizona and south ern Nevada. Guidebook for Internal. Congr. Carbo Strat. & Geo!. pub!. Amer. Geo!. Inst. BEZY, J. 1981. Bryce Canyon - the story behind the scenery. KC Publications , Las Vegas. BREED, W . J. & E . C. ROAT (eds.) 1978. Geology of the Grand Canyon. Museum of Northern Arizona , Flagstaff. CLARK, W. D . 1981. Death Valley-the story behind the scenery. KC Publications, Las Vegas . CONEY , P. J ., D . L. JONES & J . W. M. MONGER. 1980. Cordilleran suspect terrains. Nature. 288, 329-33 . CURRY , R . R . 1966. Glaciation approximately 3,000,000 B.P. in the Sierra Nevada , Cal ifornia. Science, 154, 770-1. DENNY, C. S. 1965. Alluvial fans in the Death Valley Region , California and Nevada . U.S. Geol. Surv. Prof. Paper 466. DICKINSON, W. R . 1978. Plate tectonic evolution of North Pacific Rim . J. Phys. Earth. , 26, Supp . 1-19. EARDLEY, A. J . & J . S. SCHAAK. 1976. Zion-the story behind the scenery. KC Publications, Las Vegas. GREGORY , H . E. 1950. Geology and geography of the Zion Park region , Utah and Arizona . U.S . Geol. Survey, Prof. Paper 220. HARBAUGH , J . W. 1975. Field guide to Northern California. Kendall & Hunt , Dubuque , Iowa . HARRIS , A. G. 1977. Geology of National Parks. Kendall & Hunt, Dubuque, Iowa. HARRIS , D . V. 1978. The geologic story of the National Parks and Monuments. Colorado State Univ . Press. HIGGINS, C. G. 1961. San Andreas Fault north of San Francisco , California. Bull. Geol. Soc. America, 72, 51-68. HILDRETH, W. 1976. Death Valley Geology . Death Valley Natural History Assoc. HILL, M . 1975. Geology of the Sierra Nevada. Univ . California Press , Berkeley. HILL, M. L. & T. W. DIBBLEE. 1953. San Andreas, Garlock and Big Pine Faults, California . Bull. Geol. Soc. America , 64, 443-58. HINTZE, L. F. 1979. Geologic History of Utah. Brigham Young Uni v. Geology Studies No . 20. (3). HOWELL, D . G ., J . K. CROUCH , H. G. GREENE , D . S. McCULLOCH, & J. G . VEDDER. 1980. Basin development along the late Me sozoic and Cainozoic Cali fornia marg in: a plate tectonic margin of subduction, oblique subduction and transform tectonics . In (eds P. F. Ballance & H . G . Reading) Sedimentation in Oblique slip mobile zones. Spec. Publn. Int . Assoc. Sedim ent. 4, 43-62. HUNT, C. B. 1956. Cenozoic geology of the Colorado

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Plateau. U.S. Geol. Surv, Prof. Paper 279. - -. 1975. Death Valley: Geology, Ecology , Ar chaeology . Univ . Californi a Press . - - & D. R. MABEY. 1966. Strati graphy and structure , Death Valley, California. U.S . Geol. Sur v, Prof. Paper 494A . JAHNS, R . H . (ed .) 1954. Geology of southern California . Calif. Division of Mines & Geol., Bull. 170. JONES , W. R . 1971. Yosemite-the story behind the scenery. KC Publications, Las Ve gas. . LINDQUIST, R. C. 1977. The geology of Bryce Canyon National Park. BCNHA. MATTHES , F. E. 1930. Geologic history of the Yosemite Valle y. U.S. Geol. Sur v. Prof. Paper 160. McKEE, E . D. 1982. The Supai Group of the Grand Canyon . U.S. Geol. Surv. Prof. Paper. 1173, 504 pp. NOBLE, L. F. 1954. The San Andreas Fault Zone from Soledad Pass to Cajon Pass, Californi a. In Geology of south ern California. Chapter IV, Structural Features, 37-48 . Calif. Division of Mines & Geol., Bull . 170. - - & L. A. WRIGHT. 1954. Geology of central and southern Death Valley Region, California . In Geology of Southern California. Chapter II, Geology of the Natural Prov inces , 143-160. Calif Division of Mines & Geol. , Bull. 170. OAKESHOTT, G . B. (ed .) 1962. Geol ogic guide to the Merced Canyon and the Yosemite Valley , California. Calif. Division of Mines & Geol., Bull., 182. PERKINS , J . W. 1981. Utah and Northern Arizona: a field guidebook . Univer sity College, Cardiff. PEWE , T . L. & R . G . UPDIKE. 1976. San Francisco Peaks: a guidebook to the geology. Museum of Northern Arizona, Flagstaff. RIGBY , J. K. 1976. Geology field guide to the northern Colorado Plateau. Kendall & Hunt, Dubuque, Iowa . RINEHART , C. D. & W. C. SMITH. 1982. Earthquak es and yo ung volcanoes along the eastern Sierra Nevada. William Kaufmann Inc . Los Altos, California . 62 pp . ROWE , R . C . 1974. Geology of our western national parks and monuments. Binfords & Mort, Portland, Oregon . SANFORD , R . F. 1982. Preliminary model of regional Mesoz oic groundwater flow and uranium deposition in the Colorado Plate au . Geology, 10 (7) , 348-52 . SHARP, R . P. 1972. Geology field guide to southern California. W. C. Brown, Dubuque, Iowa. WILSON, E . D . 1962. A resume of the geology of Arizona. Arizona Bureau of Mines, Bull. 171. WRIGHT, L. A. & B. M. TROXEL. 1954. Geol ogic guide No.1 , Western Mojave Desert and Death Valley Region. In Geology of southern California. Calif. Division of Mines & Geol. , Bull. 170. YERKES , R. F., T . H . McCULLOCH , J . E . SCHOELLHAMER & J . G. VEDDER. 1965. Geology of the Los Angeles Basin , California-an introduction . U.S. Geol. Surv . Prof. Paper 420A.

Charts GRATER, R . K. & W. L. HAMILTON. 1978. Zion . Unicorn Associates , Denver, Colorado . HIRSCHMANN, F. 1980. A natural history guide to Bryce Canyon . BCHNA. BREED , W. J . 1975. Geologic cross-section of the Cedar Breaks - Zion - Grand Canyon region. (Grand Can yon to Verd e Valle y Section on revers e) . ZNHA.

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Maps These are listed alphabetically under the name of the publishers.

AMERICAN ASSOCIATION OF PETROLEUM GEOLOGISTS. Geological Highway Map No.2, Southern Rocky Mountains Region (Utah-Colorado-ArizonaNew Mexico). AMERICAN ASSOCIATION OF PETROLEUM GEOLOGISTS. Geological Highway Map No.3. Pacific Southwest Region (California - Nevada).

ARIZONA GEOLOGICAL SOCIETY. Arizona Highway Geologic Map. BRIGHAM YOUNG UNIVERSITY. Utah Geologic Highway Map. Geological Studies, Special Publication No.3. GRAND CANYON NATURAL HISTORY ASSOCIATION. Geologic map of the eastern part of the Grand Canyon National Park. UNITED STATES GEOLOGICAL SURVEY. Death Valley National Monument & Vicinity.l" to 1 mile. ZION NATURAL HISTORY ASSOCIATION. Topographic map of Zion National Park. 1: 31,680. ZION NATURAL HISTORY ASSOCIATION. Geological map of Zion National Park. I: 31,680.