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Blowing sand and surface winds in the Pisco to Chala Area, Southern Peru
ARTICLE IN PRESS Journal of Arid Environments
Journal of Arid Environments 61 (2005) 101–117 www.elsevier.com/locate/jnlabr/yjare
Blowing sand and surface winds in the Pisco to Chala Area, Southern Peru S.P. Gay Jr. Applied Geophysics, Inc., 675 South 400 East, Salt Lake City, UT 84111, USA Received 19 November 2003; received in revised form 15 June 2004; accepted 28 July 2004 Available online 7 October 2004
Abstract The coastal belt of southern Peru is noted for its extensive and varied sand dunes. A major part of the sand occurs in a 300 km long strip starting 200 km south of Lima between the towns of Pisco on the north and Chala on the south. The sand is picked up from the Pacific Ocean beaches by the nearly invariant south–southeast winds and moved inland to its resting place in huge sand masses (‘‘sand seas’’ or ‘‘ergs,’’ depending on the author), generally at higher elevations. The writer mapped the moving sands and accumulated sand masses in this interesting region in 1959–1961 with airphotos, work which is partly unpublished and largely unknown (Plate 1). Eight sand masses were mapped; a ninth (Cerro Blanco) occurs just outside the area of airphoto coverage. If an average sand depth of only 10 m is assumed for the largest sand mass at Ica in the northern part of the area, that mass contains nearly 12 billion tons of sand and covers 900 km2. Probably the average sand depth here is greater. One of the unique features of the present study was the mapping of the direction of sand movement (small arrows in Plate 1) which indicates the direction of prevailing surface winds. Offshore, these winds have a nearly constant south–southeast direction. Onshore, the winds blow in great clockwise-sweeping arcs due to frictional drag and other forces as the wind moves diagonally from the ocean over the uneven ground surface. However, there is some modification of the wind direction by topography, one good example being the obvious partitioning of the wind around both sides of Cerro Huaricangana (elev.=1725 m) in the center of the area (Plate 1b). But the most interesting wind pattern occurs in an area of subdued relief and low elevation near Ica (elev. approx. 400 m), where the sands converge on the Ica sand mass from nearly opposite directions, that is, from the northwest, west, Tel.: +1-801-328-8541; fax: +1-801-363-6243.
E-mail address: [email protected] (S.P. Gay Jr.). 0140-1963/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2004.07.012
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southwest, and southeast. The wide angle sweep (1401) of incoming sands here is believed to be unique in the world. A Landsat image was used to determine how this unusual wind pattern could develop (see Fig. 9), but the extreme local rotation of the winds is not fully understood. One idea proposed here by the author is that the surface/near-surface winds move as horizontal vortices, not in streamlines as is generally assumed, and it is the interaction of the vortices and the land surface that causes the pervasive clockwise swing in wind direction. r 2004 Elsevier Ltd. All rights reserved.
1. Introduction Along the Pacific coast of South America, the arid 3600-km long belt extending from Talara, Peru, to Santiago, Chile, is perhaps unique among desert areas of the world in the great variety and quantity of windblown sands and sand dunes exhibited (Broggi, 1952; Gay, 1962, 1999; Grolier et al., 1974; Haney and Grolier, 1991). Principal factors contributing to this situation are the strong, continuous onshore winds, the availability of large quantities of sand on the beaches derived from the recently uplifted Andes Mountains, and the extreme aridity. This dry coastal strip is sometimes called the ‘‘Atacama Desert,’’ especially the southern part in Chile. The winds coming ashore from the Pacific Ocean are nearly constant in direction yearround, particularly when strong. This is due to the ‘‘South Pacific y permanent anticyclone, or high-pressure area, around which the winds [always] move in a counter-clockwise direction under the influence of Coriolis forces’’ (Robinson, 1964, p. 233), but it has also been suggested that the constancy in direction as the winds come ashore in Peru may result in part by channeling by the imposing Andean mountain range (Haney and Grolier, 1991). It should be mentioned that weaker winds below the sand-carrying threshold are variable. The writer had the opportunity a number of years ago to gather detailed aerial photographic data on the sands of southern Peru extending from near the town of Pisco on the north (131430 S) to Chala on the south (151530 S), a distance of about 300 km, and extending inland from the coast 40–60 km. This area contains some of the largest sand masses found along the west coast of South America. Likewise, in this area the peculiar stable sand form, the barchan, or crescent dune, reaches a size and number rivaling those of barchans found anywhere in the world (Gay, 1962, 1999). A study of gross sand distribution for this area as well as for northern Peru shows the regional setting for the present detailed studies (Haney and Grolier, 1991). A detailed study of airphotos similar to the present one shows sand distribution in a 250 km stretch of the coast of northern Peru (Howard, 1985), that includes many barchan trains and a number of large accumulated sand masses. A photographic atlas by the US Geological Survey (Grolier et al., 1974) provides interesting views and comments on some of the sand dunes and sand masses mapped in both northern and southern Peru. An early paper (Broggi, 1952) discusses a number of the features mapped in the present study but contains no maps or photographs.
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2. Physiography and climate The Pisco–Chala area of southern Peru displays an indented coastline of recent and rapid emergence. There are many broad expanses of beach, but much of the coastline is a seacliff 15–45 m high. Seastacks are common. East of the coast there is a narrow shelf that widens to the north, and then the ground surface rises rapidly in a distance of only 150 km to the crest of the Andes, the continental divide between Atlantic and Pacific drainage systems. The divide is not a sharp ridge, but rather a broad, upland plain of gentle relief in many places, the ‘‘altiplano,’’ carved during late Tertiary time close to sea level and uplifted in various stages in the Pliocene and Pleistocene. Elevation of the altiplano in the area under consideration varies from 4000 to 4600 m, and mountain peaks extend up to 5500 m elevation. Into the steep western slope of the altiplano many spectacular canyons have been incised because of the rapid regional uplift and the copious summer rains which fall on the higher elevations. Rivers in these canyons carry large amounts of sand and gravel to the sea. The Pacific margin is an extremely arid region, the scant rainfall averaging less than 1 cm per year. Enigmatically, there is frequent fog and low cloud cover blown in from the Pacific Ocean across the cold northward-moving Humboldt current, which originates in the South Pacific, but derives its low temperatures from upwelling deep waters. A paradoxical fog is encountered during the winter months and is sometimes accompanied by a light drizzle (‘‘llovizna’’ or ‘‘garua’’). Lloviznas are particularly frequent where the overcast level, corresponding to a temperature inversion, impinges against the topography at 300–1000 m above sea level. Lloviznas are heavier close to the coast, and in a few localities support a thick seasonal growth of grasses and low plants, ‘‘lomas,’’ which act as barriers to the advance of windblown sand. Except for the lomas and limited areas of irrigated farmland along perennial river courses, however, the desert surface from the beaches to about the 2000 m contour is almost devoid of vegetation, allowing unimpeded sand flow from the beaches. Year-round temperatures in the coastal belt are quite uniform, varying only between about 101 and 301C. No quantitative studies of offshore winds have been made in southern Peru, to the writer’s knowledge, but presumably they are similar to those in northern Peru reported on by Schweigger (1949). He studied the direction of winds listed for several years on ship’s logs delivered to the Peruvian government’s Cia. Administradora de Guano. His chart of offshore wind directions in northern Peru (reproduced here in Fig. 1) shows a very constant southeast direction starting 240 km north of the present study area. Note the slight cyclonic (clockwise) change in direction as the winds approach the coast. Onshore, Schweigger reports that the winds rotate even more easterly, as revealed by wind measurements at airports in Lima, Trujillo, and Talara. The consistent easterly rotation onshore is also revealed in this area by the sand mapping north of Trujillo mentioned earlier (Howard, 1985). In the Pisco to Chala area offshore wind directions were recently measured in four places on Landsat photographs by the author by observing streaks in low-level cloud
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Fig. 1. Near shore prevailing (‘‘resultant’’) winds off northern Peru (from Schweigger, 1949). No such study has been done for the Pisco to Chala area of southern Peru, to my knowledge, but the winds shown here should be similar. There is a 21 gap in latitude between this map and the northern limit of Plate 1.
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formations over the ocean. The first measurement was made 65 km west of the town of Paracas on the north boundary of the study area. The direction, S281E (20 November 1978) is almost identical to the direction of wind in northern Peru south of Huacho in Fig. 1. The second measurement was located over the ocean 120 km southwest of Chala south of the present study area and yielded a wind vector of S 461E (30 January 1979). Closer inland, 60 km southwest of Chala, the vector was S 411E; and 50 km west of the port of San Juan, the wind vector was S 391E (24 December 1973). These limited measurements are consistent with a slight clockwise rotation of the offshore winds from south to north and from far offshore to nearshore. 3. Regional distribution, movement and sources of sands In the coastal desert of Peru, one occasionally sees evidence that suggests a pattern to sand movement and distribution, such as the narrow, elongate sand masses streaming inland from a beach or, viewed from an airplane, a long line of barchans marching majestically for miles across the pampa (see Figs. 2 and 3). It was to unravel any such regular pattern of regional sand distribution that the present studies were undertaken.
Fig. 2. Dune belt streaming off Yanyarina Beach 20 km south of the town of San Juan. This sand migrates only 20–30 km before it blows back onto the beaches in San Juan and San Nicolas Bays. (Photo by the author.)
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Fig. 3. A long ribbon of active sand dunes advancing across the arid plain near San Juan Bay. The origin of this sand is the beach near the town of Lomas, 45 km ‘‘upstream.’’ (Photo taken by the author from the Marcona-San Nicolas iron ore conveyor line.)
Aerial photographs covered most of the sand accumulations around Puerto San Juan, the base of operations, and those farther north in the Ica area (Plate 1). The photography, of good quality, was flown in 1957 by the Servicio Aerofotogra´fico Naciona´l del Peru´ at a scale of approx. 1:30,000. Information taken from the aerial photographs was transferred to a principal point plot prepared from an uncontrolled stapled index mosaic. The transfer was made with a Ryker model L-l vertical sketchmaster at a scale of 1:1. Using the same apparatus, size reduction and location control was made in two steps to a scale of 1:200,000 for final plotting on a composite base map prepared from Peruvian national topographic maps (Plate 1). Small arrows represent the direction of sand movement interpreted from the photographs. Lines represent lineation or discoloration observed on the photos where light colored sand moved over darker terrain. Hachures represent barchans and other sand masses, and the shaded areas are huge, uninterrupted sand masses of unknown depth lying, in the main, upon topographic highs. One of the first features noted was that the movement of sand, traced upwind to its source (see arrows in Plate 1) begins in every case on the beach (Fig. 2). This demonstrates definitively that the beaches are the major sand source of the region, and not the river valleys or the desert floor itself as was thought by early authors. Another proof that the beaches are the source of the sand is seen at Tanaca where a large sand mass is located adjacent to the beach (see Plate 1b). Tremendous
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Plate 1. (a) Sand Dunes and Blowing Sands, Pisco to Chala area, Peru. West Half. (b) Sand Dunes and Blowing Sands, Pisco to Chala area, Peru. East Half.
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Plate 1. (Continued)
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quantities of loose, blowing sand moving inland can be seen (and felt) here on windy days, and sandblasting of paint from south-moving cars is common on the stretch of Pan American Highway that parallels the beach between Lomas and Tanaca. One unusual factor contributing to the quantity of wind-available sand in this region is the rapid emergence of the shoreline due to subduction of the Nazca Plate and the consequent uplift of the adjacent Andean land mass. This repeated emergence has periodically exposed new widths of flat sandy beach to the wind. In San Juan Bay, a 1-m uplift was reported after a severe earthquake in 1940 (Broggi, 1946). At the same locality an unrivaled series of over 30 stacked marine terraces extending to 900 m elevation is evidence of the recent, rapid uplift. Along many of the present-day beaches the width of exposed sand from the strand line to the base of the seacliff exceeds 150 m, and in one place south of the village of Lomas, the measured width of upraised sandy beach is 800 m. On leaving the beaches the sand typically forms large irregular dunes and mounds streaming in the direction of the prevailing wind. Where the topography is relatively smooth, barchans form from these mounds. Gay (1999), discusses some interesting deductions about the movement of barchans that resulted from the airphoto studies. Inland from the beaches the sand migrates in wide or narrow ‘‘avenues’’ (see Fig. 3), sometimes as barchans, sometimes as irregular masses, sometimes as broad sheets (‘‘streamers,’’ as per Lettau and Lettau, 1978) moving along the surface of the ground leaving little or no evidence of their passing except for the lineations visible on aerial photographs. In traversing wetter vegetation-covered ‘‘loma’’ areas, migration is arrested, and in a few localities the sand is completely stabilized (Broggi, 1952). By the time the wind rises to elevations of 500–2000 m above sea level it has lost the major part of its energy, the velocity drops below the sand-carrying threshold, and the sands accumulate in huge deposits covering hundreds of square kilometers. Eight such massive sand deposits (‘‘ergs’’ or ‘‘sand seas’’) are illustrated in Plate 1. They are named, from north to south: (1) Ica, (2) Sta Cruz, (3) Coyungo, (4) Huaricangana, (5) Cerro Copara, (6) Pico Blanco, (7) Acari, and (8) Tanaca. The number and variety of sand forms in these massive deposits is remarkable, and the quantity of sand contained therein is enormous (see, for example, Figs. 4 and 5). The largest of the eight sand masses mapped is the Ica deposit, lying just west of the city of Ica (see Plate 1). This immense accumulation of sand covers approximately 900 km2. If the average depth of sand is only 10 m, then this deposit contains over 12 billion metric tons. Probably the average depth is greater. The sand standing above the resort village of Huacachina on the east side of the sand sea, for example, reaches a height exceeding 50 m (see Fig. 6). Haney and Grolier (1991) call this the ‘‘Ica erg complex’’ and name four of the lobes as separate ‘‘ergs’’. The presence of this large sand mass and the others shown here have not been well known outside Peru, leading to a statement by Lancaster (1995, p. 474) that in ‘‘y South America there are no large sand seas, and dunes cover less than 1% of the arid zone,’’ which the present study belies. One well-known sand mass we did not map (no airphotos) is Cerro Blanco lying above and to the southeast of the town of Nazca between the drainages of the
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Fig. 4. Part of Cerro Acarı´ sand mass deposited on steep slopes of the Andean foothills 30 km inland from the Pacific coast. (Photo courtesy of H. Bartlett.)
Guanillo and Tambo Quemado canyons. Photographs of this interesting ‘‘perched’’ sand mass were published long ago by Rich (1942, Fig. 258) and later by Grolier et al. (1974, Fig. 91). The noteworthy photo by Rich, taken from a commercial airplane window in 1939, is reproduced here in Fig. 7. Our airphotos covering the sand migration routes leading to Cerro Blanco do not indicate from where the sand could have come. It would have had to cross wide expanses of what are now agricultural lands. It is evidently a fossil sand mass dating back to pre-Inca times when there was no agriculture in the valleys to arrest sand migration, although Haney and Grolier (1991) believe the Cerro Blanco sand mass accumulated even earlier, before the latest phases of Andean uplift occurred and the beaches were farther inland. In this same category they also place the Cerro Copara, Pico Blanco, and Acari sand masses, which may be true in part, but those masses are clearly receiving sand at the present time. A curious feature observed occasionally within the massive sand deposits are broad, relatively level surfaces of hard packed sand of elevated heavy mineral content. The most interesting of these ‘‘sand aprons’’ is located in the southwestern quadrant of the Pico Blanco sand group at an elevation of 1340 m (see Fig. 8). This flat sand plateau, 6.5 km long by 4 km wide, is so smooth and hardpacked that a vehicle may be safely driven anywhere over its broad undulating surface at high speed (H. Bartlett, personal communication, 1961).
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Fig. 5. Another photo of the Cerro Acarı´ sand mass showing the huge area of sand deposited on steep slopes of the Andean foothills. Survey crew in foreground. (Photo courtesy of H. Bartlett.)
Other impressive sand features in this region are the long, constant slopes of sand spilling down from the large sand masses into deep river canyons (see Fig. 5). West of the town of Acarı´ the slope distance of sand falling into the Acarı´ canyon measures over 1.5 km. The sand drops 800 m in elevation from the plateau down to the valley bottom at a nearly constant 301—almost the angle of repose. Other such spectacular sand slopes occur on the sides of the Cerro Copara, Pico Blanco, and Tanaca sand groups.
4. Prevailing surface winds as revealed by sand movement An important result of the present study, in addition to the revelation of the source and distribution of sand, was the map of prevailing surface winds as shown by the arrows of Plate 1. That these arrows do represent the true prevailing surface winds merits consideration. First, as mentioned earlier, winds along the Peruvian littoral are almost invariant in direction when strong enough to lift and move sand grains. For example, the prevailing wind direction at San Juan Bay (from the Port Authority for San Juan), S451E, is the same as indicated by the sand arrows in Fig. 1. Second, lineations on aerial photographs due to light colored sand moving over a darker background have sharp boundaries. This would not occur under variable
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Fig. 6. The culmination of the Ica sand mass lies just behind the resort village of Huacachina (shown here) where the sand reaches a maximum depth of 50–60 m. (Photo courtesy of H. Bartlett.)
wind conditions. Also, direction of the lineation in certain areas covered by overlapping aerial photography taken in 1943, 1952, and 1957 was observed to have remained constant; and many sand streaks can be seen to be identical on all three groups of photographs. Additionally, it has been found that barchan dunes, which are common throughout the area, are unstable except under essentially unidirectional wind conditions. Because of this, Lettau and Lettau (1978, p. 14) stated that one may ‘‘consider the barchan dunes as natural anemometers,’’ referring to dunes 300 km southeast in the Arequipa area. It is therefore safe to conclude that the arrows of Plate 1 do represent the true prevailing surface winds of the region. Howard (1985), reached a similar conclusion for northern Peru. This being the case, the definition of the term ‘‘prevailing wind,’’ comes into question. Many people believe (as the author did) that a prevailing wind is one which prevails through great expanses of time and space. But it can be seen by reference to the figures that the winds in the Pisco–Chala area, while prevailing through time, certainly do not prevail through space. In several instances they make turns of 901 or more in traveling only a few tens of kilometers. Part of this results from the influence of topography on the wind currents. The wind divides around Cerro Huaricangana, it avoids the crests of the Marcona Plateau and Cerro Pongo, and it sweeps sand around topographic highs in the Acarı´ sand group (Plate 1b).
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Fig. 7. Airphoto taken from a commercial airliner in 1939 by well-known American geologist John Rich (1942) in his famous ‘‘Aerial Traverse of South America,’’ shortly after commercial flights were first initiated.
But, more impressive are the many great clockwise sweeping arcs of the wind as it moves inland from the sea. These arcs can be observed throughout the whole of Plate 1, from Pisco on the north to Tanaca on the south. Hastenrath (1978, p. 74) commenting on dunes in the Arequipa area, states ‘‘yregions that we inspected from air photographs showed a gradual shift of dune direction toward the right, y[clockwise]y over downwind distances of 30 km.’’ In northern Peru, Howard’s (1985) (Figs. 2 and 3) thorough and detailed aerial photographic study of 250 km of the coastline between Trujillo and Huarmey likewise shows clockwise-turning paths of sand throughout almost his entire area. This sand comes to rest at higher elevations in large sand masses in that region similar to the nine sand masses I show in this study. Perhaps the most interesting revelation of the present study is the extreme clockwise rotation of the wind just south of the town of Paracas in the northern part of Plate 1. The wind nearly doubles back on itself, adding sand to the Ica sand mass from a northwesterly direction between Paracas and Ica. The Ica sand mass thus receives sand from the northwest, west, southwest, and southeast—an angular sweep of 1401. (The northwest winds are rotated nearly 1601 from the incoming offshore wind direction.) Cars traveling south on the PanAmerican highway have a tailwind north of Ica and a headwind south of Ica—this, is an area devoid of tall mountains to deflect the air currents!
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Fig. 8. A ‘‘sand apron’’ lying within the elevated Pico Blanco sand mass 40 km from the Pacific coast and about 60 km downwind from the beach ‘‘as the sand blows.’’ (Photo courtesy of H. Bartlett.)
As the aerial photographic coverage of the Ica sand mass was limited to the immediate area of the dunes in our original mapping, we could not follow the progress of the sand/wind from the beaches with airphotos. Therefore, a Landsat image (21398-14105-6, 20 November 1978) was recently studied to help map the paths of sands between the beaches and the airphoto coverage. The results are shown in Fig. 9, the long arrows marking the wind paths so mapped. Note the approximate right angle turns the wind makes just south of Paracas. The topographic maps were studied to see if these short-radius turns in wind/sand direction are related to local topographic prominences, as happens farther south, but this cannot be the case, as the area is relatively flat. One possibility is that the northwest winds here are related to the easterly swing in direction of the coastline north of Bahia de la Independencia (Fig. 9). Another is that frictional drag on the wind as it moves inland for such a great distance gives rise to the short radius turns. The distance between the coast and the foot of the Andes is wider here than at any other place in southern Peru, allowing more room for rotation. However, these two effects would only explain part of the angular turn of the winds, the writer believes. Haney and Grolier (1991) incorrectly ascribe the northwesterly winds that converge on the Ica sand mass to air being ‘‘drawn in’’ from Pisco Bay due to the Ica Platform being ‘‘protected from cold marine air’’ by the coastal cordillera. Fig. 9 shows that the low coastal cordillera does not protect the Ica area from marine air,
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Fig. 9. Sand dune location map showing wind/sand directions (long arrows) between the Peruvian coast and the area of Ica air photo coverage as determined by study of Landsat image E-21398-14105-6.
as the winds blow directly across it, and the wind in the northwesterlies does not come from Pisco Bay but from the open Pacific Ocean south of Paracas. This is also shown by Haney and Grolier’s Fig. 5. Another explanation for the anomalous winds
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in the Ica area was given by Robinson (1964, p. 241) who believed that heating over the broad Ica Platform resulted in a localized ‘‘thermal depression,’’ or low pressure zone, thus drawing in air from the surrounding region. However, this is unlikely, as the wind is apparently equally as strong on cloudy days and at night. An anonymous reviewer also suggested that the sand mass itself is responsible for upwelling wind currents here due to absorption of heat by sand during the day and its release at night, thus creating updrafts, but the wind many times blows day and night. In eight of the nine sand masses in this region the deposition of sand may be ascribed to a loss of energy, and hence velocity, as the air moves to higher elevations. However, in the Ica sand mass the localization of the sand apparently comes from the loss of energy and velocity when the converging winds collide. The wind that results then apparently moves vertically and easterly at a much reduced velocity. The chief pilot for the Marcona Mining Company DC-3 aircraft that made daily flights over the Ica area for many years in traveling from Lima to San Juan told the author: ‘‘There are always updrafts over Ica’’ (M. Freeman, personal communication, 1960). Offshore northern Peru (Fig. 1) the winds curve slightly in a clockwise direction even before reaching the coast (Schweigger, 1949). Warren (1976) discussed many areas in both the northern and southern hemispheres around the world where a swing in wind direction is evident in the patterns of sand dunes and stated that this was a meteorological phenomenon known as the ‘‘Ekman spiral.’’ He said ‘‘yas the [surface] roughness increases, the deflection becomes further to the left in the Northern Hemisphere and further to the right in the Southern Hemisphere’’. However, he does not mention Coriolis force, the key driving force behind the Ekman spiral as known and explained in both oceanography and meteorology texts. Surface ‘‘roughness’’ creates frictional drag on the winds, which results only in a deceleration of the airmass. It could be that this deceleration under the influence of Coriolis force causes the extreme wind deflections we observe south of Paracas. Another possibility, which I favor, is that surface wind, in general, moves not in streamline fashion as most assume, but in a series of horizontal vortices or vortical tubes pointing downwind. It would be the interaction of these horizontal vortices and the rough surface of the land that causes the swing in wind direction. The differences between the northern and southern hemisphere winds would be due to vortices rotating one way in one hemisphere and the other way in the other. Whatever the answer, I must leave the details for a future generation of scientists to deduce, as the present-day meteorological literature (that I have examined) does not explain this phenomenon, and no conversations with professional meteorologists have shed any light on the problem. For those who might wish to continue, or augment, the present work, the aerial photographs used for this study are believed to be still available at the well-equipped Servicio Aerofotogra´fico Naciona´l (SAN) laboratories in Lima, Peru. Newer photography may be available now, and acquisition of additional photography may be undertaken at any time for organizations willing to foot the bill. Landsat and other satellite photography is also available, of course, but does not have the same resolution for dune mapping as aerial photographs.
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Acknowledgments The writer is grateful to the former Marcona Mining Company and its parent company, Utah Construction and Mining, for providing the opportunity of making the foregoing studies, and particularly to the late Dr. John Jesse Hayes, Chief Engineer, who encouraged their publication. To geologist and colleague Howard Bartlett he is indebted for many first hand reports and descriptions of the features herein discussed as well as for the remarkable photographs shown in Figs. 4–6, and 8. To Peter Ottiker, an engineering student at the time, he is most grateful for the long hours spent in diligently transferring data from the aerial photographs to map sheets. And to the staff of Applied Geophysics, Inc., Ben Opfermann, Diann Pap, Rob Andrus, and Anna Mariea Gay for locating obscure references, typing many successive manuscripts, and performing state of the art computer magic on ancient illustrations and photos, he is also grateful. References Broggi, J.A., 1946. Las terrazas marinas de la Bahia de San Juan en Ica. Sociedad Geologica del Peru´ 19, 21–33. Broggi, J.A., 1952. Migracio´n de arenas a lo largo de la Costa Peruana. Sociedad Geologica del Peru´ 24, 1–25. Gay, S.P., 1962. Origen, distribucio´n y movimiento de las arenas eo´licas en el a´rea de Yauca a Palpa. Boletin de la Sociedad Geologica del Peru´ 37, 37–58. Gay, S.P., 1999. Observations regarding the movement of barchan sand dunes in the Nazca to Tanaca area of southern Peru´. Geomorphology 27, 279–293. Grolier, M.J., Ericksen, G.E., McCauley, J.F., Morris, E.C., 1974. The desert land forms of Peru´: a preliminary photographic atlas. USGS Open File Report 74-1044, Washington, DC, 146p. Hastenrath, S., 1978. Mapping and surveying—dune shape and multiannual displacement. In: Lettau, H.H., Lettau, K. (Eds.), Exploring the World’s Driest Climate. University of Wisconsin, Madison. Haney, E.M., Grolier, M.J., 1991. Geologic map of major Quaternary eolian features, northern, and central coastal Peru. USGS Misc. Investigations, v. I-2162. Howard, A.D., 1985. Interaction of sand transport with topography and local winds in the northern Peruvian coastal desert. In: Barndorff-Nielsen, O.E., Moller, J.T., Rasmussen, K.R., Willets, B.B. (Eds.), Proceedings of International Workshop on the Physics of Blown Sand, vol. 3. University of Aarhus, Aarhus, Denmark, pp. 511–544. Lancaster, N., 1995. Geomorphology of Desert Dunes. Routledge, London 290p. Lettau, H.H., Lettau, K., 1978. Exploring the world’s driest climate, Madison, Wisconsin, Center for Climatic Research, University of Wisconsin, Madison, 264p. Rich, J.L., 1942. The face of South America, an aerial traverse. American Geographical Society, Special Publication No. 26, 299p. Robinson, D.A., 1964. Peru in Four Dimensions. American Studies Press, Lima 424p. Schweigger, E., 1949. Vientos marinos y su influencia en el continente. Boletin de la Sociedad Geologica del Peru´ 25 (Part II), 1–21. Warren, A., 1976. Dune trend and the Ekman Spiral. Nature 259, 653–654.
Journal of Arid Environments 61 (2005) 101–117 www.elsevier.com/locate/jnlabr/yjare
Blowing sand and surface winds in the Pisco to Chala Area, Southern Peru S.P. Gay Jr. Applied Geophysics, Inc., 675 South 400 East, Salt Lake City, UT 84111, USA Received 19 November 2003; received in revised form 15 June 2004; accepted 28 July 2004 Available online 7 October 2004
Abstract The coastal belt of southern Peru is noted for its extensive and varied sand dunes. A major part of the sand occurs in a 300 km long strip starting 200 km south of Lima between the towns of Pisco on the north and Chala on the south. The sand is picked up from the Pacific Ocean beaches by the nearly invariant south–southeast winds and moved inland to its resting place in huge sand masses (‘‘sand seas’’ or ‘‘ergs,’’ depending on the author), generally at higher elevations. The writer mapped the moving sands and accumulated sand masses in this interesting region in 1959–1961 with airphotos, work which is partly unpublished and largely unknown (Plate 1). Eight sand masses were mapped; a ninth (Cerro Blanco) occurs just outside the area of airphoto coverage. If an average sand depth of only 10 m is assumed for the largest sand mass at Ica in the northern part of the area, that mass contains nearly 12 billion tons of sand and covers 900 km2. Probably the average sand depth here is greater. One of the unique features of the present study was the mapping of the direction of sand movement (small arrows in Plate 1) which indicates the direction of prevailing surface winds. Offshore, these winds have a nearly constant south–southeast direction. Onshore, the winds blow in great clockwise-sweeping arcs due to frictional drag and other forces as the wind moves diagonally from the ocean over the uneven ground surface. However, there is some modification of the wind direction by topography, one good example being the obvious partitioning of the wind around both sides of Cerro Huaricangana (elev.=1725 m) in the center of the area (Plate 1b). But the most interesting wind pattern occurs in an area of subdued relief and low elevation near Ica (elev. approx. 400 m), where the sands converge on the Ica sand mass from nearly opposite directions, that is, from the northwest, west, Tel.: +1-801-328-8541; fax: +1-801-363-6243.
E-mail address: [email protected] (S.P. Gay Jr.). 0140-1963/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2004.07.012
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southwest, and southeast. The wide angle sweep (1401) of incoming sands here is believed to be unique in the world. A Landsat image was used to determine how this unusual wind pattern could develop (see Fig. 9), but the extreme local rotation of the winds is not fully understood. One idea proposed here by the author is that the surface/near-surface winds move as horizontal vortices, not in streamlines as is generally assumed, and it is the interaction of the vortices and the land surface that causes the pervasive clockwise swing in wind direction. r 2004 Elsevier Ltd. All rights reserved.
1. Introduction Along the Pacific coast of South America, the arid 3600-km long belt extending from Talara, Peru, to Santiago, Chile, is perhaps unique among desert areas of the world in the great variety and quantity of windblown sands and sand dunes exhibited (Broggi, 1952; Gay, 1962, 1999; Grolier et al., 1974; Haney and Grolier, 1991). Principal factors contributing to this situation are the strong, continuous onshore winds, the availability of large quantities of sand on the beaches derived from the recently uplifted Andes Mountains, and the extreme aridity. This dry coastal strip is sometimes called the ‘‘Atacama Desert,’’ especially the southern part in Chile. The winds coming ashore from the Pacific Ocean are nearly constant in direction yearround, particularly when strong. This is due to the ‘‘South Pacific y permanent anticyclone, or high-pressure area, around which the winds [always] move in a counter-clockwise direction under the influence of Coriolis forces’’ (Robinson, 1964, p. 233), but it has also been suggested that the constancy in direction as the winds come ashore in Peru may result in part by channeling by the imposing Andean mountain range (Haney and Grolier, 1991). It should be mentioned that weaker winds below the sand-carrying threshold are variable. The writer had the opportunity a number of years ago to gather detailed aerial photographic data on the sands of southern Peru extending from near the town of Pisco on the north (131430 S) to Chala on the south (151530 S), a distance of about 300 km, and extending inland from the coast 40–60 km. This area contains some of the largest sand masses found along the west coast of South America. Likewise, in this area the peculiar stable sand form, the barchan, or crescent dune, reaches a size and number rivaling those of barchans found anywhere in the world (Gay, 1962, 1999). A study of gross sand distribution for this area as well as for northern Peru shows the regional setting for the present detailed studies (Haney and Grolier, 1991). A detailed study of airphotos similar to the present one shows sand distribution in a 250 km stretch of the coast of northern Peru (Howard, 1985), that includes many barchan trains and a number of large accumulated sand masses. A photographic atlas by the US Geological Survey (Grolier et al., 1974) provides interesting views and comments on some of the sand dunes and sand masses mapped in both northern and southern Peru. An early paper (Broggi, 1952) discusses a number of the features mapped in the present study but contains no maps or photographs.
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2. Physiography and climate The Pisco–Chala area of southern Peru displays an indented coastline of recent and rapid emergence. There are many broad expanses of beach, but much of the coastline is a seacliff 15–45 m high. Seastacks are common. East of the coast there is a narrow shelf that widens to the north, and then the ground surface rises rapidly in a distance of only 150 km to the crest of the Andes, the continental divide between Atlantic and Pacific drainage systems. The divide is not a sharp ridge, but rather a broad, upland plain of gentle relief in many places, the ‘‘altiplano,’’ carved during late Tertiary time close to sea level and uplifted in various stages in the Pliocene and Pleistocene. Elevation of the altiplano in the area under consideration varies from 4000 to 4600 m, and mountain peaks extend up to 5500 m elevation. Into the steep western slope of the altiplano many spectacular canyons have been incised because of the rapid regional uplift and the copious summer rains which fall on the higher elevations. Rivers in these canyons carry large amounts of sand and gravel to the sea. The Pacific margin is an extremely arid region, the scant rainfall averaging less than 1 cm per year. Enigmatically, there is frequent fog and low cloud cover blown in from the Pacific Ocean across the cold northward-moving Humboldt current, which originates in the South Pacific, but derives its low temperatures from upwelling deep waters. A paradoxical fog is encountered during the winter months and is sometimes accompanied by a light drizzle (‘‘llovizna’’ or ‘‘garua’’). Lloviznas are particularly frequent where the overcast level, corresponding to a temperature inversion, impinges against the topography at 300–1000 m above sea level. Lloviznas are heavier close to the coast, and in a few localities support a thick seasonal growth of grasses and low plants, ‘‘lomas,’’ which act as barriers to the advance of windblown sand. Except for the lomas and limited areas of irrigated farmland along perennial river courses, however, the desert surface from the beaches to about the 2000 m contour is almost devoid of vegetation, allowing unimpeded sand flow from the beaches. Year-round temperatures in the coastal belt are quite uniform, varying only between about 101 and 301C. No quantitative studies of offshore winds have been made in southern Peru, to the writer’s knowledge, but presumably they are similar to those in northern Peru reported on by Schweigger (1949). He studied the direction of winds listed for several years on ship’s logs delivered to the Peruvian government’s Cia. Administradora de Guano. His chart of offshore wind directions in northern Peru (reproduced here in Fig. 1) shows a very constant southeast direction starting 240 km north of the present study area. Note the slight cyclonic (clockwise) change in direction as the winds approach the coast. Onshore, Schweigger reports that the winds rotate even more easterly, as revealed by wind measurements at airports in Lima, Trujillo, and Talara. The consistent easterly rotation onshore is also revealed in this area by the sand mapping north of Trujillo mentioned earlier (Howard, 1985). In the Pisco to Chala area offshore wind directions were recently measured in four places on Landsat photographs by the author by observing streaks in low-level cloud
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Fig. 1. Near shore prevailing (‘‘resultant’’) winds off northern Peru (from Schweigger, 1949). No such study has been done for the Pisco to Chala area of southern Peru, to my knowledge, but the winds shown here should be similar. There is a 21 gap in latitude between this map and the northern limit of Plate 1.
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formations over the ocean. The first measurement was made 65 km west of the town of Paracas on the north boundary of the study area. The direction, S281E (20 November 1978) is almost identical to the direction of wind in northern Peru south of Huacho in Fig. 1. The second measurement was located over the ocean 120 km southwest of Chala south of the present study area and yielded a wind vector of S 461E (30 January 1979). Closer inland, 60 km southwest of Chala, the vector was S 411E; and 50 km west of the port of San Juan, the wind vector was S 391E (24 December 1973). These limited measurements are consistent with a slight clockwise rotation of the offshore winds from south to north and from far offshore to nearshore. 3. Regional distribution, movement and sources of sands In the coastal desert of Peru, one occasionally sees evidence that suggests a pattern to sand movement and distribution, such as the narrow, elongate sand masses streaming inland from a beach or, viewed from an airplane, a long line of barchans marching majestically for miles across the pampa (see Figs. 2 and 3). It was to unravel any such regular pattern of regional sand distribution that the present studies were undertaken.
Fig. 2. Dune belt streaming off Yanyarina Beach 20 km south of the town of San Juan. This sand migrates only 20–30 km before it blows back onto the beaches in San Juan and San Nicolas Bays. (Photo by the author.)
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Fig. 3. A long ribbon of active sand dunes advancing across the arid plain near San Juan Bay. The origin of this sand is the beach near the town of Lomas, 45 km ‘‘upstream.’’ (Photo taken by the author from the Marcona-San Nicolas iron ore conveyor line.)
Aerial photographs covered most of the sand accumulations around Puerto San Juan, the base of operations, and those farther north in the Ica area (Plate 1). The photography, of good quality, was flown in 1957 by the Servicio Aerofotogra´fico Naciona´l del Peru´ at a scale of approx. 1:30,000. Information taken from the aerial photographs was transferred to a principal point plot prepared from an uncontrolled stapled index mosaic. The transfer was made with a Ryker model L-l vertical sketchmaster at a scale of 1:1. Using the same apparatus, size reduction and location control was made in two steps to a scale of 1:200,000 for final plotting on a composite base map prepared from Peruvian national topographic maps (Plate 1). Small arrows represent the direction of sand movement interpreted from the photographs. Lines represent lineation or discoloration observed on the photos where light colored sand moved over darker terrain. Hachures represent barchans and other sand masses, and the shaded areas are huge, uninterrupted sand masses of unknown depth lying, in the main, upon topographic highs. One of the first features noted was that the movement of sand, traced upwind to its source (see arrows in Plate 1) begins in every case on the beach (Fig. 2). This demonstrates definitively that the beaches are the major sand source of the region, and not the river valleys or the desert floor itself as was thought by early authors. Another proof that the beaches are the source of the sand is seen at Tanaca where a large sand mass is located adjacent to the beach (see Plate 1b). Tremendous
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Plate 1. (a) Sand Dunes and Blowing Sands, Pisco to Chala area, Peru. West Half. (b) Sand Dunes and Blowing Sands, Pisco to Chala area, Peru. East Half.
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Plate 1. (Continued)
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quantities of loose, blowing sand moving inland can be seen (and felt) here on windy days, and sandblasting of paint from south-moving cars is common on the stretch of Pan American Highway that parallels the beach between Lomas and Tanaca. One unusual factor contributing to the quantity of wind-available sand in this region is the rapid emergence of the shoreline due to subduction of the Nazca Plate and the consequent uplift of the adjacent Andean land mass. This repeated emergence has periodically exposed new widths of flat sandy beach to the wind. In San Juan Bay, a 1-m uplift was reported after a severe earthquake in 1940 (Broggi, 1946). At the same locality an unrivaled series of over 30 stacked marine terraces extending to 900 m elevation is evidence of the recent, rapid uplift. Along many of the present-day beaches the width of exposed sand from the strand line to the base of the seacliff exceeds 150 m, and in one place south of the village of Lomas, the measured width of upraised sandy beach is 800 m. On leaving the beaches the sand typically forms large irregular dunes and mounds streaming in the direction of the prevailing wind. Where the topography is relatively smooth, barchans form from these mounds. Gay (1999), discusses some interesting deductions about the movement of barchans that resulted from the airphoto studies. Inland from the beaches the sand migrates in wide or narrow ‘‘avenues’’ (see Fig. 3), sometimes as barchans, sometimes as irregular masses, sometimes as broad sheets (‘‘streamers,’’ as per Lettau and Lettau, 1978) moving along the surface of the ground leaving little or no evidence of their passing except for the lineations visible on aerial photographs. In traversing wetter vegetation-covered ‘‘loma’’ areas, migration is arrested, and in a few localities the sand is completely stabilized (Broggi, 1952). By the time the wind rises to elevations of 500–2000 m above sea level it has lost the major part of its energy, the velocity drops below the sand-carrying threshold, and the sands accumulate in huge deposits covering hundreds of square kilometers. Eight such massive sand deposits (‘‘ergs’’ or ‘‘sand seas’’) are illustrated in Plate 1. They are named, from north to south: (1) Ica, (2) Sta Cruz, (3) Coyungo, (4) Huaricangana, (5) Cerro Copara, (6) Pico Blanco, (7) Acari, and (8) Tanaca. The number and variety of sand forms in these massive deposits is remarkable, and the quantity of sand contained therein is enormous (see, for example, Figs. 4 and 5). The largest of the eight sand masses mapped is the Ica deposit, lying just west of the city of Ica (see Plate 1). This immense accumulation of sand covers approximately 900 km2. If the average depth of sand is only 10 m, then this deposit contains over 12 billion metric tons. Probably the average depth is greater. The sand standing above the resort village of Huacachina on the east side of the sand sea, for example, reaches a height exceeding 50 m (see Fig. 6). Haney and Grolier (1991) call this the ‘‘Ica erg complex’’ and name four of the lobes as separate ‘‘ergs’’. The presence of this large sand mass and the others shown here have not been well known outside Peru, leading to a statement by Lancaster (1995, p. 474) that in ‘‘y South America there are no large sand seas, and dunes cover less than 1% of the arid zone,’’ which the present study belies. One well-known sand mass we did not map (no airphotos) is Cerro Blanco lying above and to the southeast of the town of Nazca between the drainages of the
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Fig. 4. Part of Cerro Acarı´ sand mass deposited on steep slopes of the Andean foothills 30 km inland from the Pacific coast. (Photo courtesy of H. Bartlett.)
Guanillo and Tambo Quemado canyons. Photographs of this interesting ‘‘perched’’ sand mass were published long ago by Rich (1942, Fig. 258) and later by Grolier et al. (1974, Fig. 91). The noteworthy photo by Rich, taken from a commercial airplane window in 1939, is reproduced here in Fig. 7. Our airphotos covering the sand migration routes leading to Cerro Blanco do not indicate from where the sand could have come. It would have had to cross wide expanses of what are now agricultural lands. It is evidently a fossil sand mass dating back to pre-Inca times when there was no agriculture in the valleys to arrest sand migration, although Haney and Grolier (1991) believe the Cerro Blanco sand mass accumulated even earlier, before the latest phases of Andean uplift occurred and the beaches were farther inland. In this same category they also place the Cerro Copara, Pico Blanco, and Acari sand masses, which may be true in part, but those masses are clearly receiving sand at the present time. A curious feature observed occasionally within the massive sand deposits are broad, relatively level surfaces of hard packed sand of elevated heavy mineral content. The most interesting of these ‘‘sand aprons’’ is located in the southwestern quadrant of the Pico Blanco sand group at an elevation of 1340 m (see Fig. 8). This flat sand plateau, 6.5 km long by 4 km wide, is so smooth and hardpacked that a vehicle may be safely driven anywhere over its broad undulating surface at high speed (H. Bartlett, personal communication, 1961).
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Fig. 5. Another photo of the Cerro Acarı´ sand mass showing the huge area of sand deposited on steep slopes of the Andean foothills. Survey crew in foreground. (Photo courtesy of H. Bartlett.)
Other impressive sand features in this region are the long, constant slopes of sand spilling down from the large sand masses into deep river canyons (see Fig. 5). West of the town of Acarı´ the slope distance of sand falling into the Acarı´ canyon measures over 1.5 km. The sand drops 800 m in elevation from the plateau down to the valley bottom at a nearly constant 301—almost the angle of repose. Other such spectacular sand slopes occur on the sides of the Cerro Copara, Pico Blanco, and Tanaca sand groups.
4. Prevailing surface winds as revealed by sand movement An important result of the present study, in addition to the revelation of the source and distribution of sand, was the map of prevailing surface winds as shown by the arrows of Plate 1. That these arrows do represent the true prevailing surface winds merits consideration. First, as mentioned earlier, winds along the Peruvian littoral are almost invariant in direction when strong enough to lift and move sand grains. For example, the prevailing wind direction at San Juan Bay (from the Port Authority for San Juan), S451E, is the same as indicated by the sand arrows in Fig. 1. Second, lineations on aerial photographs due to light colored sand moving over a darker background have sharp boundaries. This would not occur under variable
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Fig. 6. The culmination of the Ica sand mass lies just behind the resort village of Huacachina (shown here) where the sand reaches a maximum depth of 50–60 m. (Photo courtesy of H. Bartlett.)
wind conditions. Also, direction of the lineation in certain areas covered by overlapping aerial photography taken in 1943, 1952, and 1957 was observed to have remained constant; and many sand streaks can be seen to be identical on all three groups of photographs. Additionally, it has been found that barchan dunes, which are common throughout the area, are unstable except under essentially unidirectional wind conditions. Because of this, Lettau and Lettau (1978, p. 14) stated that one may ‘‘consider the barchan dunes as natural anemometers,’’ referring to dunes 300 km southeast in the Arequipa area. It is therefore safe to conclude that the arrows of Plate 1 do represent the true prevailing surface winds of the region. Howard (1985), reached a similar conclusion for northern Peru. This being the case, the definition of the term ‘‘prevailing wind,’’ comes into question. Many people believe (as the author did) that a prevailing wind is one which prevails through great expanses of time and space. But it can be seen by reference to the figures that the winds in the Pisco–Chala area, while prevailing through time, certainly do not prevail through space. In several instances they make turns of 901 or more in traveling only a few tens of kilometers. Part of this results from the influence of topography on the wind currents. The wind divides around Cerro Huaricangana, it avoids the crests of the Marcona Plateau and Cerro Pongo, and it sweeps sand around topographic highs in the Acarı´ sand group (Plate 1b).
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Fig. 7. Airphoto taken from a commercial airliner in 1939 by well-known American geologist John Rich (1942) in his famous ‘‘Aerial Traverse of South America,’’ shortly after commercial flights were first initiated.
But, more impressive are the many great clockwise sweeping arcs of the wind as it moves inland from the sea. These arcs can be observed throughout the whole of Plate 1, from Pisco on the north to Tanaca on the south. Hastenrath (1978, p. 74) commenting on dunes in the Arequipa area, states ‘‘yregions that we inspected from air photographs showed a gradual shift of dune direction toward the right, y[clockwise]y over downwind distances of 30 km.’’ In northern Peru, Howard’s (1985) (Figs. 2 and 3) thorough and detailed aerial photographic study of 250 km of the coastline between Trujillo and Huarmey likewise shows clockwise-turning paths of sand throughout almost his entire area. This sand comes to rest at higher elevations in large sand masses in that region similar to the nine sand masses I show in this study. Perhaps the most interesting revelation of the present study is the extreme clockwise rotation of the wind just south of the town of Paracas in the northern part of Plate 1. The wind nearly doubles back on itself, adding sand to the Ica sand mass from a northwesterly direction between Paracas and Ica. The Ica sand mass thus receives sand from the northwest, west, southwest, and southeast—an angular sweep of 1401. (The northwest winds are rotated nearly 1601 from the incoming offshore wind direction.) Cars traveling south on the PanAmerican highway have a tailwind north of Ica and a headwind south of Ica—this, is an area devoid of tall mountains to deflect the air currents!
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Fig. 8. A ‘‘sand apron’’ lying within the elevated Pico Blanco sand mass 40 km from the Pacific coast and about 60 km downwind from the beach ‘‘as the sand blows.’’ (Photo courtesy of H. Bartlett.)
As the aerial photographic coverage of the Ica sand mass was limited to the immediate area of the dunes in our original mapping, we could not follow the progress of the sand/wind from the beaches with airphotos. Therefore, a Landsat image (21398-14105-6, 20 November 1978) was recently studied to help map the paths of sands between the beaches and the airphoto coverage. The results are shown in Fig. 9, the long arrows marking the wind paths so mapped. Note the approximate right angle turns the wind makes just south of Paracas. The topographic maps were studied to see if these short-radius turns in wind/sand direction are related to local topographic prominences, as happens farther south, but this cannot be the case, as the area is relatively flat. One possibility is that the northwest winds here are related to the easterly swing in direction of the coastline north of Bahia de la Independencia (Fig. 9). Another is that frictional drag on the wind as it moves inland for such a great distance gives rise to the short radius turns. The distance between the coast and the foot of the Andes is wider here than at any other place in southern Peru, allowing more room for rotation. However, these two effects would only explain part of the angular turn of the winds, the writer believes. Haney and Grolier (1991) incorrectly ascribe the northwesterly winds that converge on the Ica sand mass to air being ‘‘drawn in’’ from Pisco Bay due to the Ica Platform being ‘‘protected from cold marine air’’ by the coastal cordillera. Fig. 9 shows that the low coastal cordillera does not protect the Ica area from marine air,
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Fig. 9. Sand dune location map showing wind/sand directions (long arrows) between the Peruvian coast and the area of Ica air photo coverage as determined by study of Landsat image E-21398-14105-6.
as the winds blow directly across it, and the wind in the northwesterlies does not come from Pisco Bay but from the open Pacific Ocean south of Paracas. This is also shown by Haney and Grolier’s Fig. 5. Another explanation for the anomalous winds
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in the Ica area was given by Robinson (1964, p. 241) who believed that heating over the broad Ica Platform resulted in a localized ‘‘thermal depression,’’ or low pressure zone, thus drawing in air from the surrounding region. However, this is unlikely, as the wind is apparently equally as strong on cloudy days and at night. An anonymous reviewer also suggested that the sand mass itself is responsible for upwelling wind currents here due to absorption of heat by sand during the day and its release at night, thus creating updrafts, but the wind many times blows day and night. In eight of the nine sand masses in this region the deposition of sand may be ascribed to a loss of energy, and hence velocity, as the air moves to higher elevations. However, in the Ica sand mass the localization of the sand apparently comes from the loss of energy and velocity when the converging winds collide. The wind that results then apparently moves vertically and easterly at a much reduced velocity. The chief pilot for the Marcona Mining Company DC-3 aircraft that made daily flights over the Ica area for many years in traveling from Lima to San Juan told the author: ‘‘There are always updrafts over Ica’’ (M. Freeman, personal communication, 1960). Offshore northern Peru (Fig. 1) the winds curve slightly in a clockwise direction even before reaching the coast (Schweigger, 1949). Warren (1976) discussed many areas in both the northern and southern hemispheres around the world where a swing in wind direction is evident in the patterns of sand dunes and stated that this was a meteorological phenomenon known as the ‘‘Ekman spiral.’’ He said ‘‘yas the [surface] roughness increases, the deflection becomes further to the left in the Northern Hemisphere and further to the right in the Southern Hemisphere’’. However, he does not mention Coriolis force, the key driving force behind the Ekman spiral as known and explained in both oceanography and meteorology texts. Surface ‘‘roughness’’ creates frictional drag on the winds, which results only in a deceleration of the airmass. It could be that this deceleration under the influence of Coriolis force causes the extreme wind deflections we observe south of Paracas. Another possibility, which I favor, is that surface wind, in general, moves not in streamline fashion as most assume, but in a series of horizontal vortices or vortical tubes pointing downwind. It would be the interaction of these horizontal vortices and the rough surface of the land that causes the swing in wind direction. The differences between the northern and southern hemisphere winds would be due to vortices rotating one way in one hemisphere and the other way in the other. Whatever the answer, I must leave the details for a future generation of scientists to deduce, as the present-day meteorological literature (that I have examined) does not explain this phenomenon, and no conversations with professional meteorologists have shed any light on the problem. For those who might wish to continue, or augment, the present work, the aerial photographs used for this study are believed to be still available at the well-equipped Servicio Aerofotogra´fico Naciona´l (SAN) laboratories in Lima, Peru. Newer photography may be available now, and acquisition of additional photography may be undertaken at any time for organizations willing to foot the bill. Landsat and other satellite photography is also available, of course, but does not have the same resolution for dune mapping as aerial photographs.
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Acknowledgments The writer is grateful to the former Marcona Mining Company and its parent company, Utah Construction and Mining, for providing the opportunity of making the foregoing studies, and particularly to the late Dr. John Jesse Hayes, Chief Engineer, who encouraged their publication. To geologist and colleague Howard Bartlett he is indebted for many first hand reports and descriptions of the features herein discussed as well as for the remarkable photographs shown in Figs. 4–6, and 8. To Peter Ottiker, an engineering student at the time, he is most grateful for the long hours spent in diligently transferring data from the aerial photographs to map sheets. And to the staff of Applied Geophysics, Inc., Ben Opfermann, Diann Pap, Rob Andrus, and Anna Mariea Gay for locating obscure references, typing many successive manuscripts, and performing state of the art computer magic on ancient illustrations and photos, he is also grateful. References Broggi, J.A., 1946. Las terrazas marinas de la Bahia de San Juan en Ica. Sociedad Geologica del Peru´ 19, 21–33. Broggi, J.A., 1952. Migracio´n de arenas a lo largo de la Costa Peruana. Sociedad Geologica del Peru´ 24, 1–25. Gay, S.P., 1962. Origen, distribucio´n y movimiento de las arenas eo´licas en el a´rea de Yauca a Palpa. Boletin de la Sociedad Geologica del Peru´ 37, 37–58. Gay, S.P., 1999. Observations regarding the movement of barchan sand dunes in the Nazca to Tanaca area of southern Peru´. Geomorphology 27, 279–293. Grolier, M.J., Ericksen, G.E., McCauley, J.F., Morris, E.C., 1974. The desert land forms of Peru´: a preliminary photographic atlas. USGS Open File Report 74-1044, Washington, DC, 146p. Hastenrath, S., 1978. Mapping and surveying—dune shape and multiannual displacement. In: Lettau, H.H., Lettau, K. (Eds.), Exploring the World’s Driest Climate. University of Wisconsin, Madison. Haney, E.M., Grolier, M.J., 1991. Geologic map of major Quaternary eolian features, northern, and central coastal Peru. USGS Misc. Investigations, v. I-2162. Howard, A.D., 1985. Interaction of sand transport with topography and local winds in the northern Peruvian coastal desert. In: Barndorff-Nielsen, O.E., Moller, J.T., Rasmussen, K.R., Willets, B.B. (Eds.), Proceedings of International Workshop on the Physics of Blown Sand, vol. 3. University of Aarhus, Aarhus, Denmark, pp. 511–544. Lancaster, N., 1995. Geomorphology of Desert Dunes. Routledge, London 290p. Lettau, H.H., Lettau, K., 1978. Exploring the world’s driest climate, Madison, Wisconsin, Center for Climatic Research, University of Wisconsin, Madison, 264p. Rich, J.L., 1942. The face of South America, an aerial traverse. American Geographical Society, Special Publication No. 26, 299p. Robinson, D.A., 1964. Peru in Four Dimensions. American Studies Press, Lima 424p. Schweigger, E., 1949. Vientos marinos y su influencia en el continente. Boletin de la Sociedad Geologica del Peru´ 25 (Part II), 1–21. Warren, A., 1976. Dune trend and the Ekman Spiral. Nature 259, 653–654.