The two-stage concept of landform and landscape development involving etching: origin, development and implications of an idea

Earth-Science Reviews 57 Ž2002. 37–74 www.elsevier.comrlocaterearscirev

The two-stage concept of landform and landscape development involving etching: origin, development and implications of an idea C.R. Twidale ) Department of Geology and Geophysics, UniÕersity of Adelaide, G.P.O. Box 498, Adelaide SA 5005, Australia Received 5 January 2001; accepted 28 March 2001

Abstract Two-stage development of landforms has been appreciated for more than two centuries with respect to minor features, and major forms and landscapes have been viewed in similar terms for almost a hundred years. Early workers understood the significance of fractures as passages for water and, hence, as avenues of weathering, the tendency for weathering to produce rounded forms, the progression of weathering from the surface downwards, weathering contrasts between wet and dry sites, contrasted erosive susceptibility of weathered and unweathered rock, and reinforcement effects. Forms of deep and shallow derivation can be differentiated. By whatever name it is known—etch, double planation, subcutaneous, or two-stage—the concept is one of the most fruitful developed in the last century, for it bears not only on the origin of a wide range of landforms, but also on the crucial role of water and weathering, the age of landforms and landscapes, palaeogeographic reconstructions, climatic geomorphology and theories of landscape development. q 2002 Elsevier Science B.V. All rights reserved. Keywords: etching; two-stage development; double planation; regolith; weathering front; azonality

1. Introduction Over the past 50 years, it has increasingly been recognised that substantial components of the world’s landscapes were shaped not at the Earth’s surface, but at the base of the regolith. Such forms are the result of differential weathering produced by shallow groundwaters held in the weathered mantle and reacting with the local country rock. Exploitation of weaknesses in the bedrock produced a morphologi-

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Tel.: q61-8-8303-5392; fax: q61-8-8303-4347. E-mail address: [email protected] ŽC.R. Twidale..

cally differentiated surface or alternatively, a plane surface, in either instance followed by stripping of the regolith and exposure of the bedrock form. Many familiar landforms, from boulders to bornhardts, from basins to gutters and flared slopes, and from platforms to plains, originated in this way. If the geomorphological value of an hypothesis is the number of landforms and landscapes it explains, then the two-stage concept is surely one of the most significant, if not the most important, to have emerged over the past 50 years or so. How has it changed our perception of landscape, and when and by whom was this mechanism, which is referred to pro tempore as two-stage, envisaged? The recogni-

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tion of the mechanism and its naming, its derivation from, and application to, landforms at various scales and in various bedrocks, and its implications for general theories of landscape development are considered, as are some of the difficulties inherent in the concept. This review and critique is perforce based primarily on a consideration of the English-language literature. The writer’s restricted library resources and his own linguistic limitations precluded the possibility of browsing over a wide range of old reports and narratives and journals of scientific travellers. This is a weakness, for many observations and comments critical to the development of the two-stage concept were recorded incidentally, and are not captured in abstracts or keyword indices. The two-stage concept may have been realised earlier and elsewhere than is suggested here.

2. Corestones and other minor forms in granite Though other bedrock types were not ignored—as demonstrated by Sain Fond’s Ž1784. observations of basaltic corestones in Scotland and various accounts of orgues geologiques in the Chalk lands of western ´ Europe—granitic terrains received considerable at-

tention from early geologists, possibly because of associated mineral deposits, and the main thrust for various early ideas related to two-stage development arose from observations in such areas. One of the earliest suggestions of two-stage development is due to Hassenfratz Ž1791.. Late in the 18th Century and travelling in the southern Massif Central near Aumont, between St. Flour and Montpellier, he observed rounded masses of fresh granite protruding from a bank of weathered rock ŽFig. 1.. Some were only just visible, others partly exposed, and yet others detached. Hassenfratz deduced that the freestanding blocks and boulders had once been covered, and suggested that the assemblage were members of a sequential series: entre un . . . on apperc¸oit tous les intermediaires ´ bloc de granit dur contenu & enchasse´ dans la masse totale du granit friable & un bloc entiere` ment degage. ´ ´ ŽHassenfratz, 1791, p. 101.. wAll stages can be observed between a block of granite totally contained within the mass of altered rock and one wholly detached.x Hassenfratz’s comments were noted by Hutton Ž1795, II, p. 174., who recorded that such boulders originated Aby the decay of the rock around themB

Fig. 1. It was at this site, or one near and like it, at Chazeirollettes, in the southern Massif Central, that Hassenfratz in 1791 saw granitic corestones in various stages of exposure.

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rather than being expressions of structure Žvaried composition of the rock. as suggested by some Že.g., Jameson, 1820, p. 414.. The same interpretation was placed on granite boulders by several, though not all, later workers in Britain ŽTwidale, 1978a.. These early investigators were astute observers and were responsible for great advances. The significance of fractures as avenues of weathering, and the role of water in altering the bedrock were understood, as was the tendency for cubic and quadrangular blocks to be converted to rounded corestones Žor core-stones—Fig. 2., which, at various times, were known also as kernels and Ahearts of the blocksB ŽJones, 1859, p. 307., and later, core-boulders ŽScrivenor, 1931., and boulders of disintegration ŽLarsen, 1948, p. 114 et seq... Thus, MacCulloch Ž1814, p. 76. observed that granite blocks Anow rendered spherical by decomposition, have been quadrangular massesB wor as it was also expressed Žp. 74. ANature mutat quadrata rotundisB x and de la Beche Ž1839, p. 450. pointed out that: The granites are separated by divisional planes . . . into cuboidal or prismatic bodies, and the decomposition on the faces of these bodies, when the blocks are detached, and the superior facility for disintegration afforded by the corners

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would appear sufficient to produce the rounded character we often observe them to possess. At that time, de la Beche apparently considered that the shaping of the boulders took place after exposure, but later, de la Beche Ž1853. published a sketch depicting rounded corestones in situ within a weathered mantle. Others made similar observations and drew similar conclusions. It was also realised that weathering advances from the surface downwards and, thus, that initial stages of change are to be observed at the base of the mantle and that the higher in the profile, the more advanced the process: a change may always be observed to have taken place from the surface downwards to a more or less considerable depth in the stone. Sometimes even the whole mass of rock will appear to have undergone this gangrenous process at once, and to have become a bed of clay and gravel. ŽMacCulloch, 1814, p. 72.. Working on the other side of the world, Logan reached similar conclusions, commenting that: The blocks protruding from the hills or ranged along the shores of Pulo Ubin wproperly, Pulau

Fig. 2. Core-boulders, or corestones, in grus on Karimun Island, western Indonesia, with lamination at the margins of the rounded rock masses.

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Ubin in the Strait of Johor, and off the north coast of Singaporex are more solid and less decomposable masses and nuclei, of which the forms, and the directions of the sides and axes, have, in almost every instance, been determined by structural planes, and which remain after the surrounding rocks have disintegrated and been washed away. ŽLogan, 1849, p. 40.. Some early geologists, including Ansted Ž1859. working in southern China, Agassiz Ž1865. in the Rio de Janeiro area, AW.T.B Ž1896. on the Malay Peninsula and Romanes Ž1912. in Costa Rica, considered that granite boulders had been rounded during transport either by rivers or glaciers, but evidence demonstrating that they were in situ was recognised ŽFig. 3.. For example, travelling in the uplands near Canton, in southern China, Kingsmill observed corestones set in a matrix of grus and noted that: The original quartz veins of the granite, broken into small fragments by the forces which have operated on the surrounding rock, still traverse the disintegrated mass in all directions. ŽKingsmill, 1862, p. 2..

Contrasts in degree of weathering at moist and dry sites were recognised 150 years ago ŽLogan, 1851, p. 326.. Thus, once corestones are exposed as boulders, they are relatively dry and stable Žsee also Ruxton and Berry, 1957.. Plinths ŽFig. 4. are due to the protection against wetting afforded by what have become perched blocks and boulders. Core-boulders are discrete parts of the weathering front ŽMabbutt, 1961a., the abrupt transition between regolith and bedrock, examples of which were noted in Malaya and elsewhere, as were spatial variations in degree of weathering in granitic masses ŽPumpelly, 1879; and later, e.g., Scrivenor, 1931; Ingham and Bradford, 1960.. Scrivenor Ž1931, p. 137., for example, noted that though in Malaysia the regolith in places grades into fresh rock, elsewhere there is an abrupt transition, so that the granite forms what he called a Ahard platformB beneath the weathered mantle. Several early investigators noted lamination at the margins of granitic corestones ŽFig. 2.. Working on Pulau Ubin, Logan, for instance, refers to plates spalling from the granite surface, and consisting of laminae a quarter to a fifth of an inch thick ŽLogan, 1849, p. 12.. Such textures represent an early stage of weathering and are typical of the weathering front

Fig. 3. Corestones and boulders near Pine Creek, Northern Territory. The aplitic vein shows that the section is in situ.

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Fig. 4. Plinth and perched block developed in a Miocene rhyolitic tuff at the Giants of the Mimbres, southwestern New Mexico, USA. ŽJ.E. Mueller..

Že.g., Larsen, 1948, p. 115; Hutton et al., 1977; Twidale, 1986.. That the implied differences in rates of weathering induced feedback or reinforcement mechanisms was also appreciated. Thus, describing the results of wave attack on the shores of Pulau Ubin washing away the weathered granite, Logan Ž1851, p. 326. states that this left: the more resistant masses to emerge from the soil and stand out above the influence of decomposition. wfollowed, in a footnote by:x When an exposed rock is attacked, the decomposing portion is washed or falls off, and the decomposition is arrested for the time. Under-ground decomposition tends to spread unchecked on all sides. On Pulau Ubin, Logan Ž1849, 1851. observed that many granite boulders are scored by grooves Ž Rillen, Granitrillen, Silikatrillen. and noted Ž1849, p. 6. that some, Aif not mostB, continue beneath the weathered granite in situ, which extended to considerable depths beneath the slopes around the blocks and boulders. This observation has not been replicated for steeply inclined surfaces, but on more gentle slopes, gutters have been observed to extend below soil level at

several sites ŽTwidale and Bourne, 1975a.. Logan Ž1851, p. 328. considered that the grooves corresponded with zones of weakness in the rock and, though such a structural interpretation does not find support in the field, he put the argument for subsurface initiation thus: If . . . we conceive the external layer of the island, when it first became exposed to decomposition, to have resembled in character the zone that has been laid open for our inspection . . . it is easy to comprehend how the wasting away of the more decomposable parts might at last leave exposed masses, including bands of the less stubborn material already partially softened or disintegrated under ground, and that the action of the atmosphere and rain-torrents would gradually excavate the more yielding portions, until the solid remnants exhibited their present shapes. The rapidity with which rocks were decayed in the humid tropics, and the capriciousness of nature, also impressed early investigators. Scrivenor Ž1931, pp. 136–137., for instance, noted a small boulder of granite split by blasting and exposed in a road cutting in the uplands NNE of Kuala Lumpur. Fresh

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when first exposed, it was rotted through within a decade Žcf. Caillere ` and Henin, 1950.. Elsewhere, however, fresh and rotted rocks, stand side by side.

3. Two-stage development of minor forms in other lithologies Two-stage boulders are also well and widely developed in other lithological environments. Well developed and moderately spaced Žideally 1–3 m or so. orthogonal fracture systems, low permeability and a susceptibility to chemical weathering, are conducive to their formation. They occur in norite in the Mann Ranges of central Australia and elsewhere Že.g., Hutton et al., 1977., and in many other plutonic rocks. Sain Fond had in 1784 noted basaltic corestones with marginal lamination in Scotland and similar basaltic core-boulders are well exposed in many parts of the Drakensberg of southern Africa ŽFig. 5a.. Corestones have been reported in sandstone in the southern Flinders Ranges of South Australia and in the Appalachians of Tennessee, and in gritstone in the English Pennines ŽPalmer and Radley, 1961; Linton, 1964.. Many other minor sandstone forms—flared slopes, platforms, basins, gutters—are also of twostage type Žsee Twidale, 1962, 1980; Young and Young, 1992; Fig. 5b.. Though the subsurface initiation of granitic forms attracted early attention, carbonates are more readily soluble than most other rocks and two-stage forms are well represented in karst terrains. Congeners of corestones, for example, are well developed in limestone. Most are irregular and blocky rather than spheroidal in shape and after exposure are known as Karrenblocke and Karrensteine. Moreover, mor¨ phology varies in detail according to whether a surface is exposed or covered, Rundkarren, for example, developing under a soil cover, but Rinnenkarren on bare surfaces. Indeed, karst assemblages have been differentiated on the basis of devel-

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opment beneath a cover or after exposure Že.g., Sweeting, 1972, pp. 252 et seq., especially p. 298., for example, bare karst Ž nackter karst., karst couÕert ŽCorbel, 1947., covered karst Ž bedeckter karst or limestone surfaces overlain by allochthonous materials, and cryptokarst ŽNicod, 1976: cited in Jennings, 1985, p. 80. or limestone covered by a soil or regolith derived from weathering of the country rock. According to Corbel Ž1947., covered karst differs from bare Ž nackter . karst in several respects, but principally in the prime direction of solution: vertical on bare surfaces, lateral on covered. Such distinctions, however, are not in accord with all the field evidence. The earliest covered karst forms described in the literature were the pipes of the Chalk lands of western Europe and variously known as swallow holes, gulls, puits naturels, orgues geologiques, and ´ geologische Orgeln ŽCuvier and Brogniart, 1822; Buckland, 1839; Lyell, 1840; Prestwich, 1855; Van den Broek, 1881.. They are clearly due to vertical solution and infilling, as are the prongs found in crystalline limestone, typically beneath a terra rossa soil cover ŽFig. 5c.. Ingham and Bradford Ž1960, p. 30., for example, report prongs and pinnacles, many of them with flared or concave sidewalls, developed in limestone beneath 30 m of regolith in the Ipoh district of West Malaysia. Some of the limestone surface, formerly the weathering front, carries a coating of siderite, or ferruginous carbonate comparable to the ferruginous weathering front concentrations found in granitic terrains Žsee Twidale, 1986..

4. Major forms: bornhardts and inselberg landscapes 4.1. Early reports By the beginning of the 20th Century, the subsurface origin of several minor forms had been noted and, in some instances, as with corestones, widely

Fig. 5. Ža. Corestones in basalt, with peripheral lamination, southern Drakensberg, Eastern Cape Province, South Africa. Žb. Gutters related to decantation flows on a recently exposed gentle sandstone slope, in the southern Drakensberg. Žc. Limestone prongs exposed beneath terra rossa soil in Galong Quarry, central New South Wales. Note also the rough surface, the equivalent of pitting in granitic rocks, caused by the etching of calcite crystals, possibly in the zone of fluctuating water table.

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accepted. Thus, the intellectual climate had been prepared for the consideration of the two-stage mechanism in the context of larger landforms and of landscapes. Explorers of the Australian interior had encountered astonishingly flat plains interrupted by scattered, isolated, steep-sided hills they likened to rocky islands and which they called island mounts Že.g., Eyre, 1845, I, p. 203.. Such features were encountered by German travellers in southern Africa and they used the same simile: . . . there rose above the wide plain innumerable, peculiarly shaped hills, resembling islands, steep and rocky and several hundred, yes in certain cases more than 500 meters high . . . ŽBornhardt, 1900, pp. 127–128: translated by Willis, 1934, p. 123.. They referred to them as Inselberge; and it is this name that is now universally accepted in the scientific literature. Domical granitic hills, now widely known as bornhardts, after the German explorer of that name ŽWillis, 1934., were described by several explorers and scientific travellers during the 19th Century. Several explanations were offered for them and for the landscapes in which they stood. Bornhardt Ž1900. briefly considered the possibility of their being literal island mounts, but soon came to the view that repeated phases of fluvial planation were involved. Passarge Ž1895, 1904. interpreted the inselberg landscapes of Adamaua ŽWest Africa. as of four types, some due to wind abrasion, some to a combination of aeolian and fluvial action. Holmes Ž1918, p. 93. advocated scarp recession under the influence of water and wind, in some instances, resulting in the exposure of intrusive stocks ŽHolmes and Wray, 1912.. The abrupt transition from cohesive to rotten rock Žwhich was known to extend several tens of metres below the surface. was used to explain the morros of the Rio de Janeiro area ŽBranner, 1896.; and, indeed, the answer to the problem of the origin of the residuals lay not at, but beneath, the surface. 4.2. Pumpelly, Falconer, and subsurface initiation Pumpelly was a widely travelled geologist and explorer. His observations and reading led him to the

conclusion that weathering in places extended to great depths ŽPumpelly, 1879.. He also had the wit to consider what would be found if the mantle of weathered rock were removed, and wrote: As different rocks are affected in very different degrees . . . the plane marking the boundary between disintegrated rock and still hard rock wthe weathering frontx must be an exceedingly irregular one. If we could imagine the loose altered rock removed where this process has been active in depth, the surface exposed would present a remarkable topography . . . ŽPumpelly, 1879, pp. 136–137. He clearly contemplated two stages of development, with weathering followed by the evacuation of the products of disintegration and alteration, and had in mind ŽA . . . a remarkable topography . . . B . landform assemblages or landscapes, rather than individual forms. A two-stage concept was used to explain the spectacular fields of inselbergs found in many parts of Africa; though without reference to Pumpelly’s speculations. Basing his synthesis on his studies of Nigerian landscapes, Falconer Ž1911. suggested that the landscapes are due to subsurface moisture attack exploiting bedrock contrasts and that the salient features of inselberg landscapes are exposed weathering fronts. He noted that within the walls of Kano, the flat-topped dioritic hill known as Kogan Dutsi included corestones and he asserted that had erosion continued to evacuate all weathered material, it would have become a hill of loose boulders resting on a smooth rounded base. ŽFalconer referred to the form as a AkopjeB Žor ‘koppie’., Afrikaans for a small hill, but it would today be called a nubbin or knoll, the humid tropical development of the bornhardt; see Twidale, 1981.. He was able to imagine future events and envisage their results. In a masterly and succinct outline of a two-stage theory of inselberg landscape development, he stated: A plane surface of granite and gneiss subjected to long-continued weathering at base level would be decomposed to unequal depths, mainly according to the composition and texture of the various

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rocks. When elevation and erosion ensued, the weathered crust would be removed, and an irregular surface would be produced from which the more resistant rocks would project. Those rocks which had offered the greatest resistance to chemical weathering beneath the surface would upon exposure naturally assume that configuration of surface which afforded the least scope for the activity of the agents of denudation. In this way would arise the characteristic domes and turtlebacks . . . ŽFalconer, 1911, p. 246.. Though bornhardts develop for various reasons, a wide range of evidence and argument suggests that the two-stage theory accounts for most. Compositional differences of various origins have been exploited, in some instances Že.g., Holmes and Wray, 1912; du Toit, 1937, e.g., p. 61; Herrmann, 1954; Thorp, 1969; Selby, 1977; Hagedorn, 1980, p. 833., but contrasts in fracture density have most frequently led to differential weathering. Le Conte Ž1873, p. 324. and Mennell Ž1904, p. 74. had earlier pointed to this factor in explanation for domes being upstanding, but had not envisaged exploitation prior to exposure. That was Falconer’s great contribution. Evidence and argument supportive of the twostage hypothesis are many and varied ŽTwidale, 1982a, pp. 139–149, 1982b; Vidal Romani and Twidale, 1998.. For instance, the relationship between the residuals and old planation surfaces is

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suggestive, for the former commonly stand lower in the landscape than the latter, and King Ž1949, pp. 84 and 85, referring to the work of Obst, 1923. cites this relationship between planation surface and residual as characteristic of bornhardts Žsee also Handley, 1952; Lister, 1976, 1987; Whitlow, 1978–1979.. That bornhardts occur in multicyclic landscapes can be construed as indicating first, that there has been time for differential subsurface weathering, and second, that there has been deep erosion of the land mass to expose compressive zones of antiformal structures, the convex-upward sets of sheet structures which are typical of bornhardts Žsee, e.g., Dale, 1923; Lamego, 1938; King, 1949; Birot, 1958; Twidale et al., 1996.. Perhaps the most telling evidence, however, is the occurrence of incipient bornhardts, already shaped in the subsurface ŽFig. 6., and exposed in quarries and road cuttings ŽBoye´ and Fritsch, 1973; Twidale, 1982a, pp. 142–144, 1982b.. Residuals already shaped at the weathering front have been detected also in limestone terrains. In the Kuala Lumpur district of West Malaysia, for example, geophysical surveys to investigate foundation conditions for the Sepang international airport disclosed steep-sided limestone projections up to 50 m high and reaching to within 20 m of the land surface in one instance, but more commonly 40 m ŽFig. 7.. They are bordered by deep bedrock depressions occupied by weathered limestone, and the whole se-

Fig. 6. Large-radius dome exposed at Elkington Quarry near Minnipa, northwestern Eyre Peninsula, South Australia.

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Fig. 7. Profile showing morphology and distribution of Palaeozoic limestone bedrock, weathering mantle and alluvium at Sepang, near Kuala Lumpur, West Malaysia, based on a seismic refraction survey. Vertical exaggeration approximately= 4. Žafter Ho, 1993..

quence is buried beneath 40–60 m of alluvium Že.g., Ho, 1993.. Within the city limits of Kuala Lumpur, the proposed site of the Twin Towers had to be moved in order to place the load centre on a limestone, rather than a deep soil, foundation. Nascent domes or towers have not so far been exposed in artificial excavations in sandstone or conglomeratic strata, but many steep-sided remnants in arenaceous and rudaceous rocks stand below palaeosurface remnants, and other evidence and argument similar to that derived from the consideration of granitic forms is germane. For example, the flared flanks of the Meteora towers, located in central Greece, and eroded in Miocene conglomerate, argue subsurface weathering and exploitation of a massif Žsee Twidale, 1962.. Again, the sandstone towers of the Bungle Bungle and George Gill ranges, in northwestern and central Australia respectively, not only stand below prominent planation surfaces, but they display mineral concentrations ŽYoung, 1986. compatible with weathering front accumulation. They are two-stage forms. 4.3. Jutson and the Western Australian inselberg landscape

rocks ŽFig. 8.. Weathering and planation produced the duricrusted Old Plateau. Following uplift and river rejuvenation, the regolith was extensively stripped to expose as the New Plateau the erstwhile weathering front related to the formation of the Old. Mesa remnants of the Old Plateau, as well as numerous granitic domes, stand on the New Plateau or high plain in an extensive inselberg landscape. Later workers have built on this foundation and have detailed the extent and relationship of the surfaces Že.g., Woolnough, 1918; Mabbutt, 1961b; Finkl and Churchward, 1973; Mulcahy, 1973.. Also, Walther Ž1915. realised that if the age of the lateritised surface could be determined, the rates at which subsequent valley incision and extension through scarp retreat could be established. Examination of valley deposits suggests that the Old Plateau predates the Eocene and that the headward recession of valleys to form the New Plateau can be plotted from Eocene in and near antecedent and inherited valleys, through Miocene and Pliocene in the middle and headward reaches ŽCommander, 1989; Clarke, 1994; Waterhouse et al., 1995; Salama, 1997; Twidale and Bourne, 1998.. 4.4. Willis, Wayland and etching

Three years after the appearance of Falconer’s seminal hypothesis, Jutson Ž1914. published an explanatory description and critique of the landscapes of Western Australia. Inter alia, he proposed a twostage explanation for the Old and New plateaux of the southern Yilgarn Craton, developed mainly on granitic rocks and ‘greenstones’, or basic igneous

Thanks to the work of early investigators, the two-stage origin of some landforms was appreciated almost a century ago. The concept and, in particular, the process or processes involved did not, however, have a name. Two-stage development was clearly implied by early workers investigating boulders and

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Fig. 8. Jutson’s diagrams showing the development and relationship of the Old and New plateaux, Western Australia. ŽJutson, 1914, p. 143..

corestones and the term was widely used informally. The first formal naming of the concept, however, is due to Willis and Wayland, working in East Africa in the late twenties and early thirties of the last century. To ‘etch’ is to engrave by eating away by acids or, more generally to corrode a surface by means of

aggressive reagents. Bearing in mind the attack of bedrock by shallow groundwaters armed with chemicals and biota, it is an apt and evocative term for the process involved in the development of the various two-stage bedrock features described above. But it was not a term that sprang readily to mind and its history is rather involved.

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Both the name ‘etch’ or ‘etched’ plain, and by implication the associated concept, are usually attributed to Wayland Žsee, e.g., Thornbury, 1954, p. 193; Thomas, 1994, p. 287., but apart from the mechanism having been earlier recognised, Wayland himself gives a different account of the genesis of the name. Though he did not publish it until 1936, Willis Ž1936, p. 124. had evidently earlier used the term ‘etch’ in its geomorphological sense of forms initiated at the weathering front and later exposed. According to Wayland, writing in the Uganda Geological Survey Annual Report and Bulletin for the year ending 31 March 1934, it was coined Athe other day by Bailey Willis who aptly names this erosive operation etching B ŽWayland, 1934, p. 79.. Willis worked in the field on the Rift Valley in 1929, suggesting that, unless Wayland applied geological scales of time to everyday events, Athe other dayB most likely refers to correspondence. Willis Ž1936. may have shown his colleague a draft of what eventually became his monograph on East African rifts and plateaux. Nevertheless, Wayland Ž1934, pp. 77–79. clearly understood the twostage mechanism. Indeed, he claimed he had endeavoured to explain it 14 years previously ŽWayland, 1921, p. 40, para. 165., but it is fair to comment that his statement on the point at that time was brief and vague, rather than explicit and unmistakeable, and is more relevant to the question of unequal activity than to etching Žsee below, Section 5.6.. Wayland properly and generously acknowledged the provenance of the term ‘etch’, and it is worthwhile examining Willis’ 1936 monograph on that account alone. Willis Ž1936, pp. 118–121. first presented evidence for deep weathering in East Africa and elsewhere, but he was also aware of the propensity of masses of resistant rock to persist and to increase in relief through successive phases of weathering and lowering of the adjacent plains. He discussed etching and its consequences, without defining or mentioning the term until p. 124, when he offered it as an alternative to the then conventional wisdom: The fact that East Africa has been raised is demonstrable on the evidence of the plateau landscapes, whether they be interpreted as peneplains

or as etched plains with inselbergs, . . . ŽWillis, 1936, p. 124.. Then, however, Willis Ž1936, p. 134. categorically stated that particular exposed bedrock surfaces Arepresent the former contact between saprolite, the product of decay, and the fresh undecayed rockB and on p. 135, remarked that A . . . the rock floor of the Tanganyika Plateau may be described as the etched surface produced by the penetration of decay to groundwater level below an older plainB. Thus, though not as succinct as Falconer Žwhose work was not cited., Willis clearly interpreted some of the East African land surfaces as of subsurface derivation, and of two-stage type. Willis presented his evidence and also suggested AetchingB as an appropriate and evocative term for the process; and indeed it is not difficult to imagine shallow groundwaters charged with chemicals and biota eating into the bedrock with which they are in contact, eventually to produce, and to utilise MacCulloch’s Ž1814. wonderful metaphor, a AgangrenousB mass. 4.5. Linton, tors and inselbergs The two-stage or etch hypothesis was well understood by the 1940s, not only through the writings of the authors mentioned, but also through discussion of the ideas of Jutson and Wayland, for example, in some British and Australian universities and at scientific meetings Že.g., Hills, 1962.. Yet it was not everywhere appreciated. Many texts of the early and mid 20th Century referred to multicyclic landscape development, but few to two-stage development of features of regional extent, either tacitly or by name, and if the idea were noted, it was underplayed. Thornbury Ž1954, p. 193., for instance, cited Wayland’s etch concept, but concluded that it was of local importance only: A . . . it is difficult to visualize the process operating widely enough to produce etchplains of regional extentB. Today, the concept is well known and widely utilised in landscape analysis. The revival and present wide acceptance of the two-stage concept are due largely to Linton and Budel. ¨ Linton’s contributions to this state of affairs flow from his analysis of tors. The word ‘tor’ Ž torr, twr, turris . means a tower and, in Britain, has long been

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used of isolated, steep-sided, bare blocky hills Aabout the size of a houseB ŽLinton, 1952.; though whether a cottage or a ducal mansion is not clear ŽFig. 9.. Linton categorically dismissed the possibility of the granite outcrops of Dartmoor having been shaped by weathering at the surface. Instead, he appreciated the effectiveness of subsurface moisture attack, and attributed the tors of Dartmoor to Aa two-stage processB ŽLinton, 1955, p. 472.. His papers ŽLinton, 1952, 1955. on the tors of Dartmoor and other parts of Britain and western Europe, formalised and provided the essential scientific underpinning for the two-stage ideas that were in wide circulation at the time, though without referring to the pioneers of the concept. They did much to establish the two-stage concept as it applied to tors and boulders in granite, though not in all quarters. For example, in his well-known text, Holmes Ž1965, pp. 609–617. accepted deep weathering and the two-stage mechanism as they applied to ‘tors’, but he outlined an epigene and multicyclic explanation for the much larger African inselbergs with which he had long been familiar. Citing Dixey’s Ž1942. and King’s Ž1950, e.g.. papers on planation surfaces he stated: With each major uplift a new cycle of erosion started at the coast, encroaching on the pediment

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of an earlier cycle . . . advancing up the rivers and their tributaries, wearing back the escarpments, but leaving massive bastions and projections behind to be slowly worn into groups of inselbergs and eventually into isolated peaks . . . ŽHolmes, 1965, pp. 612–613.. Like King Ž1949. before him, and in spite of citing Falconer’s account of Nigerian inselbergs, Holmes Ž1965, p. 610. could not accept a similar mechanism for the much larger African residuals. He was not alone in this, for as astute, innovative and open-minded a geomorphologist as Lewis Ž1955, p. 483., commenting on Linton’s thesis that tors are due to differential deep weathering stated: AI was inclined to think he wLintonx was overdoing the deep weathering process . . . B, but he was convinced on seeing the field evidence. 4.6. Budel ¨ and A double planationB Linton’s expositions on deep weathering referred mainly to granitic terrains Žbut see Linton, 1964. and to residuals. With the concept of Doppelten EinebŽ1957, nungsflachen or ‘double planation’, Budel ¨ ¨ 1977. not only contributed much corroborative evidence for, and a valuable refinement of, the basic two-stage hypothesis, but he argued its applicability

Fig. 9. Haytor, a typical tor on Dartmoor, southwestern England.

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to the analysis of landscapes; to such good effect that for many the concept is now inextricably linked with his name Že.g., Jones, 1999, pp. 17–18.. Budel ¨ envisaged that given tectonic stability, weathering and river action would together produce a planation surface. A more-or-less thick regolith develops beneath this surface. In many places, the base of the regolith is marked by an abrupt transition into relatively fresh rock: the weathering front, Tiefenfront or Verwitterungs-Basisflache. The front is irregular, if only ¨ because the progress of weathering varies according to the character of the country rock, but two surfaces are simultaneously developed, one at ground level, and the other at the base of the regolith shaped by moisture held in the weathered mantle ŽFig. 10; Abb. 5 in Budel, 1957; see also Ollier, 1960.. ¨ Budel emphasised the effectiveness of regolithic ¨ moisture in reducing rocks and producing planate surfaces. His model accounts for plains transecting rocks of varied resistance in terms of intensive weathering and at various scales Žsee also Mabbutt, 1966.. Minor levels within the weathering front at various scales and due either to weak bedrock, baselevel lowering, climatic fluctuations or, in the deserts of Western Australia, to localised weathering and planation by migrating lakes ŽJutson, 1914, p. 156, 1934, pp. 235 et seq., also Fig. 98, at p. 239. are readily accommodated within the double planation scheme Žsee, e.g., Busche, 1980.. So are secular

variations in the locus of weathering due either to climatic or tectonic changes, which result in variations in the level of the water table, and leading to stepped relief at various scales ŽJessen, 1936; Crickmay, 1974.. In her review of Budel’s double planation, Bre¨ mer Ž1993. analysed how the regolith developed beneath the Obere Einebnungsflache may be evacu¨ ated Žsecond stage of the two-stage mechanism.. Backwearing and downwearing are both theoretically feasible, depending on the structural setting, and both types of development are evidenced in the field. Evidence implying lowering of the regolith in the Gawler Ranges has been deduced ŽCampbell and Twidale, 1991; Twidale, 1994, q.v. below.. On the other hand, isolated plateau remnants capped by ferruginous or siliceous carapaces persist on Jutson’s Ž1914. New Plateau. In such structural situations, scarp retreat is inherently probable, both in terms of general theory Že.g., Tricart, 1957. and given the consistent inclinations of slopes bordering remnants of varied dimensions Že.g., King, 1942; Fair, 1947.. Bremer Ž1993, p. 190. also broached the question of the origin of inselbergs in the context of Budel’s ¨ model, suggesting that AInselbergs are not explained by slope retreat or as erosional remnants due to hard rockB. Rock resistance reflects not only composition and physical hardness, but also, and especially, fracture density and accessibility to circulating ground-

Ž1957. diagram showing double planation: Ži. initial surface, Žii. development of regolith and weathering front, and Žiii. Fig. 10. Budel’s ¨ lowering of plain and water table, and development of lower weathering front.

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waters. In Budel’s own diagram ŽFig. 10., the weath¨ ering front is depicted as irregular, with a topography already developed, as it is in reality in some areas. To cite a specific example, the Early Cretaceous etch landscape of the Gawler Ranges, in the arid interior of South Australia, is one of domes and

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fracture-controlled valleys developed on a mass of Mesoproterozoic silicic effusive volcanics. Though banks of columnar joints are prominent throughout, weathering patterns and the resultant bornhardt landscape are due to the exploitation of orthogonal systems of steeply inclined fractures ŽFig. 11a. and

Fig. 11. Ža. Diagram of fracture-defined bornhardts in the central Gawler Ranges, South Australia, and Žb. view of bornhardts near Mt Nott, southern Gawler Ranges.

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associated sets of sheet fractures. Many of the domical hills are bevelled and the massif is dominated by the Nott Surface, a summit surface of etch type ŽCampbell and Twidale, 1991; Fig. 11b.. The regolith was stripped during Neocomian–Aptian times and the resultant debris, including dacitic corestones, was deposited in the Eromanga Basin, where it is now known as the Mt. Anna Sandstone ŽWopfner, 1969, p. 152.. The contained corestones are smaller low in the sequence than at higher levels. This is the reverse of the distribution found in a standard weathering profile and suggests that the regolith was eroded in layers, i.e., the surface was lowered, rather than being reduced by scarp retreat, which would have resulted in deposits containing corestones of mixed size. 5. Some difficulties and implications 5.1. What is A subsurfaceB? The two-stage mechanism has found wide acceptance in respect of granitic forms and is also applicable in other lithological environments. But consideration of the depth at which etching occurs, and of possible origins of residuals, in particular, introduces conceptual and terminological complications. The question can be considered by examining first some

minor forms and then certain major landscape features. ‘Pitting’ is a term used to describe surfaces that are rough as a result of differential weathering at the crystal scale ŽTwidale and Bourne, 1976; Fig. 12.. It is demonstrably a two-stage form for it can be revealed by clearing the regolith and is developed on recently exposed surfaces. It represents the first stage in the breakdown of the country rock. In granite, for instance, mica and feldspars are attacked by moisture at the base of the soil cover and are converted to clay, while the quartz and some feldspar phenocrysts resist weathering and remain upstanding. It is well developed in crystalline rocks, which include minerals of contrasted susceptibility to water attack, but is also found on limestone where crystal boundaries and cleavages are preferentially weathered ŽFig. 5c.. Several early workers Že.g., Scrivenor, 1931, p. 137; Roe, 1951. recorded such surfaces, some of them quite spectacular, with feldspars projecting more than a centimetre. They did not, however, use the term pitting to describe it and they did not attribute it to subsurface weathering. In some instances, pitting develops only a few centimetres below the ground surface, yet it is indubitably of etch and two-stage character. Core-boulders are also etch forms and the resultant boulders develop in two stages. The perimeters

Fig. 12. Pitting on granite boulders at Mt Bundey near Darwin, Northern Territory, Australia.

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of slightly rounded blocks ŽFig. 13a. are as much part of the weathering front as those of a perfectly rounded corestone. The breakdown of sheet structure after exposure to produce a blocky veneer and the formation of a nubbin ŽFig. 13b and c. involves a complex weathering front, but the development took place in the subsurface, albeit at shallow depth at the base of a thin regolith. Waters penetrating along steeply inclined joints in granite have produced acicular forms and towers in granitic terrains, for instance, in the Sierra Nevada of California, around Mt. Whitney and, at a smaller scale, in the Cathedral Rocks of the Yosemite Valley; in the Crow Tors of Wyoming and Silent City of Rocks, Idaho; in the Organ Mountains of New Mexico; and in The Needles, South Dakota Že.g., Bateman and Wahrhaftig, 1966; Cunningham, 1969, 1971; Seager, 1981.. Similar columns or towers are also found in other lithological settings, in sandstone or conglomerate plateaux, as in Monument Valley, Utah, the Parana valley of southern Brazil and the Roper River basin of the Northern Territory, and in conglomerate in the Meteora district of central Greece, in parts of the Pyrenees Že.g., Barrere, ` 1968. and in the Olgas massif of central Australia ŽTwidale and Bourne, 1978.. Such fracture-defined columns and towers, whether exposed in escarpments and valley sides or in massifs, represent extensions of a weathering front. They evolved just below the land surface, and not necessarily at the base of a thick regolith, for later erosion has in many places outpaced weathering, yet they are, surely, etch and two-stage forms. Etch features evolve just below the surface as well as at depth, and beneath massifs as well as plains. The extent and context of the problems arising from forms originating at different locations in the landscape can be illustrated by a consideration of towerkarst ŽTurmkarst, karst a` tourelles . in limestone terrains. Limestone towers are convergent forms for they evolve in various ways. First, and as cited earlier Žq.v.., in some areas limestone towers and domes originate deep below the land surface as gigantic prongs, resulting from differential, presumably fracture-controlled, subsurface moisture attack. They are classic etch forms. Second, and by contrast with features of deep-seated origin, but like their counterparts in sandstone and conglomerate, some

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towers are due to the exploitation of major steeply inclined fractures ŽRichardson, 1947; Brook and Ford, 1978; Twidale and Centeno, 1993; Fig. 14a.. That such exploitation took place below the surface of massifs or plateaux is strongly suggested by the occurrence of incipient towers beneath palaeosurface remnants. The vertical zonation of cave systems preserved within residual towers suggests a phased development Že.g., Sweeting, 1950; Lehmann, 1954; Jennings, 1963; Drogue and Bidaux, 1992.. Whether domical hills or towers emerge from such exploitation is a function of the spacing of open steeply inclined fractures, and this in turn varies with structure and stress, plus duration of attack. Most of the karst massifs and towers of Perlis, West Malaysia, for example, are developed on synclinal troughs in Permian limestone, zones in which the fractures are tight and virtually impenetrable to meteoric waters ŽJones, 1978.. The limestones of the Merapoh area occur in regional synclines and are, in addition, metamorphosed to marble ŽRichardson, 1950.. Open fractures are quite widely spaced and rounding of the upper corners of fracture-defined blocks initially produced squat domes Ž Cupolakarst . rather than comparatively tall towers. Subsequent basal attack, undermining and collapse have transformed some domes into towers ŽFig. 14b and d.. Evidence of such scarp-foot solution includes the cliff-foot caves, basal notches or swamp slots ŽFig. 14c., the bedrock limits of which demonstrably extend several metres below plain level and are found around much of the basal perimeter ŽLehmann, 1954; Sweeting, 1958; Corbel, 1959a; Wilford and Wall, 1965; Jennings, 1976; Twidale, 1987a; but see also MacDonald, 1976.. Stages in the conversion of domical hills to residuals with some or all flanks undermined, collapsed and precipitous can be seen in many areas Žsee Fig. 14b.. Such a mechanism accounts not only for the conversion of limestone domes to towers, but also for the local distribution of the two types ŽVerstappen, 1960; Sweeting, 1990.. This is the third mode of origin, involving basal subsurface attack on exposed masses. 5.2. Contrasts in rates of weathering on exposed and coÕered surfaces The morphology of a few, but unusual, minor forms has been attributed to the contrast in rates of

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weathering on exposed and relatively dry rock surfaces beneath a Žmoist. soil cover. Many rock basins, for example, are initiated as saucer-shaped depressions on platforms ŽFig. 15a. and on the flattish upper surfaces of fracture-defined blocks. Near the edges of such blocks the regolith falls away so that bare rock is exposed. The central areas retain the soil and the contained moisture. Weathering proceeds more rapidly in the latter areas than at the margins, which thus come to form annular rims enclosing depressions: what have been termed rock doughnuts ŽBlank, 1951; Fig. 15b.. Rock levees ŽFig. 15c. also develop in this manner ŽTwidale, 1988; Vidal Romani and Twidale, 1998, pp. 285–288, 300–301.. Contrasts in rates of weathering have been invoked also in partial explanation of some landform assemblages, such as flared or concave slopes ŽTwidale, 1962. and the stepped granitic topography of parts of the southern Sierra Nevada of California ŽWahrhaftig, 1965., though variations in fracture density and reinforcement or positive feedback mechanisms have also played their part, here as elsewhere. Rock doughnuts and ‘fonts’, or benitiers, in sandstone on the west coast of Eyre Peninsula ŽSouth Australia. also have been interpreted as ‘part-free’ forms ŽTwidale and Campbell, 1998; Fig. 15d.. Flared slopes and rock basins are well developed in rhyolitic tuff in the Grant County City of Rocks, New Mexico ŽMueller and Twidale, 1988.. Vertical pipes analogous to the orgues geologiques of chalk ´ terrains are well represented in siliceous strata, from the high rainfall tropics ŽUrbani, 1986. to arid lands ŽMilnes and Twidale, 1983., etc.—many minor forms are of two-stage origin and have a considerable lithological and climatic range Žsee, e.g., Campbell and Twidale, 1995.. 5.3. Contrasts between deep and shallow weathering Subsurface scarp-foot weathering is especially pronounced in readily soluble carbonate terrains, but is not restricted to such environments. Frequent ac-

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cessions of meteoric waters and the relative abundance of biota and organic remains at and just beneath the surface also ensure that shallow subsurface groundwaters are, however, the most aggressive and effective in chemical weathering. Paton Ž1964; see also MacDonald, 1967., for instance, has interpreted the karst towers of West Malaysia as monadnocks de position located on divides, and preserved because meteoric waters are less effective in dissolving the country rock than are river and swamp waters with their acquired biota and chemicals. Thus, as they travel over and just beneath the surface, soil moisture, shallow groundwaters and streams become more aggressive and more rapidly dissolve the carbonate country rock so that the headwater zones are preserved while those downstream are reduced. It might well be asked whether location on divides is cause or effect, but the suggestion highlights the perceived significance of water quality in weathering. Aggressive subsurface moisture attack is commonly responsible for the piedmont angle, the sharp break of slope between hillslope and piedmont ŽTwidale, 1967.. Around some granite inselbergs Že.g., Kokerbin Hill in the southern Yilgarn Craton of Western Australia., undermining and collapse of hillslopes has resulted in the slippage of sheet structures, thus lowering the slope and reducing the size of the dome, without, however, modifying its shape. The collapse and steepening of limestone slopes has radically changed the profiles of the residuals. Collapse following undermining due to subsurface scarp-foot weathering and erosion is also evidenced at the base of the arkosic Ayers Rock in central Australia. The bevelled summit of Ayers Rock ŽFig. 16a. is an etch surface of latest Cretaceous age ŽHarris and Twidale, 1991.. The steep sides of the inselberg have developed and been exposed in at least two phases of scarp-foot subsurface weathering during the Cainozoic: the earlier resulting in a row of gaping-mouth caves and breaks of slope 35–60 m above present plain level, the later in fretted and flared basal slopes

Fig. 13. Ža. Blocks of dolerite, little rounded by weathering, exposed in a road cutting near Umtata, Eastern Cape Province, South Africa. Žb. Slope on Enchanted Rock, central Texas, showing sheet structure broken down into angular blocks. Žc. Nubbin, or block- and boulder-strewn inselberg, Naraku, north of Cloncurry, northwestern Queensland.

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Fig. 14. Ža. Incipient dolomite towers exposed in the flanks of the Arroyo de Valdecabras, near Cuenca, central Spain. Note the flared sides of some of the columns. Žb. Towerkarst near Ipoh, West Malaysia. Note steepened sidewall of tower in central middle distance. ŽJ.N. Jennings.. Žc. Swamp slot near Ipoh, West Malaysia. Žd. Diagram illustrating conversion of cone- to tower-karst through basal sapping. At a and b, the development of swamp slots at and just below the surface causes undermining, followed by collapse and steepening of slopes ŽTwidale, 1987a..

4–5 m high ŽTwidale, 1978b; Fig. 16b–d.. At each stage, the exposed form was modified by subsurface scarp-foot weathering, which caused undermining and collapse of slopes. The contrasted effectiveness of deep and shallow regolithic weathering also finds expression in the morphology of granitic residuals. The bornhardt is the basic form and rapid deep stripping of the regolith results in such domical hills being exposed, but nubbins and koppies reflect the difference between subsurface weathering in warm humid environments and weathering of a partly buried, partly exposed, mass ŽTwidale, 1981.. On the other hand, the deep and prolonged weathering that is an intrinsic feature of much two-stage development carries different implications for landscape development. The regolith holds water, the most important single weathering agency. Soluble

products of weathering are translocated within the regolith by circulating shallow Žvadose. groundwaters. Where the soluble products of weathering are evacuated out of the system, volume decrease may, according to Trendall Ž1962. lead to the settling, compaction and general lowering of the surface. Thus, there is an argument for considering shallow, as well as deep, long-term as well as ephemeral, two-stage development. It may also be useful to distinguish between etching beneath plains, as in Falconer’s and Budel’s models, and similar pro¨ cesses operating in massifs and uplands where, because of lower water tables and through-drainage, descending meteoric waters are only briefly in contact with bedrock. Certainly the potency of shallow groundwaters calls for particular consideration. At depth, weathering may be long continued, and stripping of the regolith long delayed, frequently

58 C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74 Fig. 15. Ža. Saucer-shaped depressions on a newly exposed granite platform, Kwaterski Rocks, north of Minnipa, northwestern Eyre Peninsula, South Australia. Žb. Rock doughnut in granite, high on the slope of Enchanted Rock in the Llano, central Texas. The basin is crucial to its development. Water from the weathered granite or grus drained into the basin. The bedrock beneath this comparatively dry grus was not weathered as rapidly as was that a few centimetres farther from the basin. Thus, an annular rim of rock developed around the depression. Žc. Rock levees bordering a gutter on the slopes of Domboshawa, a granitic bornhardt near Harare, Zimbabwe. Žd. Doughnuts and fonts in sandstone, Talia, west coast of Eyre Peninsula, South Australia. Like the doughnut illustrated in Fig. 15b each of the forms originated around a rock basin into which drained water from the adjacent regolith or, as here, beach sand. As contrasted weathering continued, the rims and basins were left higher and higher in the local relief, converting doughnuts into fonts, arbitrarily when the height becomes greater than the maximum diameter.

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with spectacular results in the form of plains of extraordinary flatness and considerable extent. For example, the Meekatharra Plain ŽFig. 17a., an inselberg landscape in the central Yilgarn Craton of Western Australia, cuts across granite and gneiss, and is an exposed weathering front formed beneath a regolith of Triassic–Cretaceous age, lateritic remnants of which remain ŽFig. 17b.. Similarly, the Bushmanland Surface, which extends from northern Namaqualand ŽWestern Cape Province. into central Namibia, South Africa, is cut across granite, gneiss, sandstone and schist ŽFig. 17c.. It too is of etch type for duricrusted regolithic remnants are preserved in places, e.g., near Platbakkies ŽPartridge and Maud, 1987., and is of probable Cretaceous age ŽFig. 17d.. The etch plain, which occupies much of Finland, is similarly devoid of relief Že.g., Soderman, 1985.. ¨ Susceptibility to chemical attack can, however, compensate for duration of weathering. The flatness of the Nullarbor Plain ŽFig. 18. has long puzzled investigators ŽJennings, 1963.. It is not a structural feature, ‘a single exposed bedding plane’, but what degradational process could produce such a feature? The surface, some 200,000 km2 in extent, is eroded in flat-lying Miocene limestone, but at least 60 m of section have been removed near the southern or coastal margin of the plain ŽLowry, 1970; Lowry and Jennings, 1974.. Weathering Ždissolution. by moisture retained in a thin regolith, of which remnants survive, may be responsible ŽTwidale, 1990a.. But karst regions are also, and especially, subject to double planation. In addition to the extreme shallow planation exemplified in the Nullarbor Plain, deeper water table weathering and erosion at various levels Že.g., Sweeting, 1950; Jennings, 1963. simultaneously induce solution and collapse to that level or levels, eventually producing a limestone plain or a towerkarst landscape like those found in many parts of the Antilles and Southeast Asia. Such karstic double planation is surely in train in carbonatedominated sequences, such as that of the western Murray Basin in South Australia. 5.4. Alternations of weathering and erosion, or steady-state deÕelopment? Two-stage development implies periods when weathering was dominant followed by phases of

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erosion, and several workers have investigated possible reasons for such alternations. Willis was aware that changes due to earth movements, climatic variations and river development Žextension and incision. had impacted on the landscape. Fairbridge and Finkl Ž1978. examined the evolution of the Yilgarn Craton of Western Australia in terms of its tectonic and climatic history, traced drainage adjustments to these events and identified several alternations of weathering and erosion. This idea was developed into what is termed a cratonic regime ŽFairbridge and Finkl, 1980., which takes account of tectonic, eustatic and climatic changes, and is characterised by alternations of thalassocratic–biostatic phases of deep chemical weathering and epeirocratic–rhexistatic periods during which the regolith is eroded in part or in whole. Working in West Africa, Thomas and his colleagues have linked etch planation to known episodic environmental changes of the Quaternary Že.g., Thomas, 1989a; Thomas and Thorp, 1985., but in many areas, the reasons for alternations of weathering and erosion remain obscure. The environmental settings of the many flared slopes developed, exposed and preserved on the inselbergs of southern Australia, for example, remain conjectural. Could erosion become dominant not through any environmental change, but as a result of the progress of weathering such that the alterites become susceptible to transport when a certain texture or grain size is attained, or could erosion be triggered by a catastrophic event, such as a local flood, the effects of which extend in time throughout the catchment? However, the many forms that are manifestly of etch origin have involved weathering and then erosion. The most obvious two-stage forms are those in which, for various possible reasons, erosion most recently has outpaced weathering, but where there are enough remnants of the former cover to demonstrate the likely mechanism responsible for the assemblage. But as Lewis Ž1955. astutely appreciated, near the surface, the two processes may be active yet result in no morphological change. Where the two proceed at equal rates, however, the surface is in dynamic equilibrium and bedrock forms are produced with little or no evidence of a regolith or of a weathering front: some etch forms may be difficult to identify as such. Fortunately, local variations in rates of weathering and erosion may have caused

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Fig. 16. Ža. Bevelled summit surface of Ayers Rock, central Australia. Note also the steep flanks with break of slope about 35 m above the plain and, in the foreground, a rock platform some 800 m from the base of the inselberg. Žb. Break of slope and associated gaping-mouth caves or tafoni at the southeastern corner of Ayers Rock. Žc. Fretted base of Rock. Note bevelled crest, tafoni, and blocks and boulders fallen from slope, and Žd. flared southern base of Ayers Rock.

patches of regolith and other evidence Žsuch as pitting. of a former regolith to be preserved, thus allowing reconstruction of the sequence of events and of the origin of the forms. Thus, the etch origin of many rock pediments or platforms, such as those so graphically recorded by McGee Ž1897., is indicated by the patches of thin regolith preserved alongside the bedrock surface and, in granitic terrains, by small Žcore. boulders. 5.5. Zonation, climatic and lithological? Two-stage forms are well evidenced in the humid tropics Že.g., Thomas, 1989a,b; Thomas and Thorp, 1985., where regoliths in the order of 60–200 m are not uncommon Že.g., Branner, 1896; Scrivenor, 1931; Ingham and Bradford, 1960; Ollier, 1965; Thomas, 1966.. Particular forms and assemblages, such as the towers and domes of karst areas and the nubbins of granitic terrains, are well represented there. Though the development of karst towers in the Yukon ŽBrook and Ford, 1978. has focused attention on structural

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factors, it cannot be denied that towerkarst is best developed and most widely preserved in the humid tropics; so much so that extratropical occurrences, like those of southern Switzerland and southern Poland Že.g., Gilewska, 1964., are explained in terms of exhumation or climatic change, just as are granitic nubbins found in contemporary arid zones ŽOberlander, 1972.. Again, inselberg landscapes comprise not only extensive plains, but also inselberg remnants, and there has evolved a suggestion that the latter, and by implication the whole assemblage, is most commonly associated with the tropical savannas Že.g., Krebs, 1942: cited in Hovermann, 1978.. In these ¨ terms, the occurrence of two-stage inselberg landscapes in, say, arid or temperate or cold lands implies climatic change Že.g., Linton, 1955; Budel, ¨ 1978.. Yet the assumption that any two-stage form initiated at the weathering front is climatically zonal must surely be questioned. Etch forms are due to chemical weathering by shallow groundwaters armed with chemicals and biota. Such groundwaters are ubiquitous. Concentrated in the 800 m or so below the land surface, though detected at depths of up to 10 km ŽKozlovsky, 1987., groundwaters extend beneath each and every part of every continent in either liquid or solid form. Even in the midlatitude deserts, they occur at depth. The hydrosphere of which the groundwater zone is part forms a continuous, though in places much attenuated, shell. Regoliths are widely developed and, depending in considerable measure on the nature of the country rock, clearly defined weathering fronts also are well represented. It is topographically differentiated, initially according to the structure of the country rock, though later with positive feedback mechanisms taking effect. Thus, fractures and fracture zones are first exploited to produce, say, the bedrock projections revealed in subsurface investigations at Sepang ŽFig. 7., but once formed, shallow groundwaters flow along the bedrock surface and into depressions where consequently basal slopes are weathered and steepened. Discrete sectors of the front are also ubiquitous and, as Linton Ž1955, p. 472. pointed out, ACore-stones are not . . . confined to the tropicsB. Different weathering complexes are dominant in different climaticrbiotic environments as well as in

62 C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74 Fig. 17. Ža. Part of the Meekatharra Plain cut across granite and gneiss, central Yilgarn Block, Western Australia. Žb. A lateritic remnant of the Old Plateau standing above the New, near Cue, Western Australia. Žc. The Bushmanland Surface in northern Namaqualand ŽWestern Cape Province., South Africa is cut across granite but to the north, transects gneiss, schist and sandstone. Žd. Silcrete-capped mesa, near Platbakkies, northern Namaqualand ŽWestern Cape Province.. The siliceous capping has been undermined and the underlying white kaolinised zone eroded to expose the weathering front in gneiss as an etch plain ŽJ.A. Van Zyl..

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Fig. 18. View over the Nullarbor Plain, underlain by Miocene Nullarbor Limestone which overlies the Eocene Wilson Bluff Limestone. ŽAdvertiser Newspapers, Adelaide..

different lithological settings. Rates of development vary Že.g., Corbel, 1959b; Judson and Ritter, 1964.. Regoliths have been evacuated mostly by rivers, but also in places and at times by glaciers and waves and, though more rarely and locally, by the wind Že.g., Peel, 1966.. Some forms, such as bornhardts, karst towers and pediments, are widely and well developed in some climatic regions, but, though scarce and poorly developed, are also found in others: absolute zonality is inherently unlikely. Forms originating at the weathering front—etch forms and surfaces—whether pitting, rock basins, corestones or clefts, scarp-foot embayments or fracture-defined residual masses, are ubiquitous. The occurrence of two-stage forms in the present humid and arid tropics and in midlatitude lands is well documented, but in view of the many remnants of preglacial regoliths known to have survived passage of ice sheets Že.g., Gauthier, 1980; Bouchard, 1985; Fogelberg, 1985; Grant, 1989., it is not unreasonable to interpret many of the bedrock plains and associated minor forms of the high-latitude shields in North America and Scandinavia as etchplains. They are integral components of all oldlands ŽWilson, 1903; Twidale, 1990a, 1999..

Forms resulting from the etch or double planation mechanism are developed in a wide range of rock types ŽCampbell and Twidale, 1995., though they are most commonly preserved in rocks which are, for whatever reason, inherently resistant when dry. Thus, the extensive, if fragmentary, etch surface of central Australia ŽMabbutt, 1965. is preserved on sandstone or quartzitic ridge crests in such fold uplands as the Macdonnell and Davenport ranges. Granite, sandstone and limestone are common hosts to etch forms. Those developed in weak materials, such as argillites, are only preserved in particular circumstances. In the Flinders Ranges of South Australia, for example, etch plains in shale, mudstone, etc., are preserved Ža. standing close to local baselevel ŽFig. 19a., Žb. where covered and protected by a lag of gravel or gibber, Žc. where buttressed by resistant strata, and Žd. in favourable structural settings, e.g., in the cores of deeply eroded anticlines ŽTwidale and Bourne, 1996.; or where more than one of these conditions obtains. Two-stage surfaces are developed in coastal as well as interior locations, for waves and other marine processes are capable of stripping regoliths and exposing weathering fronts. Thus, many shore plat-

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Fig. 19. Ža. Etch plain in dipping shale and thin limestone near Rawnsley Bluff, central Flinders Ranges, South Australia. X indicates exposure of kaolinitic regolith in old piedmont remnant. Žb. Diagrammatic section through Point Drummond, on the west coast of Eyre Peninsula showing the regolith capped by a small remnant of dune calcarenite and with the weathering front exposed as a rocky shore platform.

forms in granitic rocks found on the west coast of Eyre Peninsula are of etch type ŽMolina Ballesteros et al., 1995; Fig. 19b., as are some of the platforms developed in mudstone on the California coast Že.g., Fig. 4 in Bradley and Griggs, 1976.. 5.6. Landscape deÕelopment Appreciation and awareness of the two-stage development influences how landscapes are interpreted. First, as signalled by Willis Ž1936, p. 124., the two-stage model offers an alternative to the multicyclic interpretation of the stepped landscapes that

dominate many parts of the world Že.g., King, 1962; Crickmay, 1974.. Thus, the landscape of northwest Queensland has been interpreted in conventional cyclic terms, albeit with significant exhumed elements ŽTwidale, 1956a.. A lateritic surface of mid Tertiary age has been dissected and on the divide between the exoreic and endoreic drainage the resultant Late Cainozoic fluvial plain is extending at the expense of the duricrusted Kynuna Plateau. The weathering front preserved beneath the latter is coincident with that of the adjacent plain, so that in the vicinity of the Kynuna Plateau the younger plain could be of etch type.

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Second, two-stage forms are well named for they have two ages, one referring to the period of subsurface weathering and the preparation of the weathering front, the other to the period of exposure of the front as part of the land surface ŽTwidale, 1987b.. This is of some significance in palaeogeographical reconstructions ŽTwidale, 2000.. Thus, that the Nott Surface of the Gawler Ranges ŽFig. 11b. is demonstrably an etch surface, and an untere Einebnungsflache, proves the former existence of an obere ¨ Einebnungsflache beneath which it evolved by dif¨ ferential subsurface weathering. There is no known sign of this Beck Surface in the present landscape ŽCampbell and Twidale, 1991., yet its former existence is undeniable ŽTwidale, 2000.. The term ‘two-stage’, though apt, is, in many instances, a simplified description of the mechanism, for the origin of many such forms can be traced back much further in time, in some instances, to the formation of the bedrock in which they are shaped. Thus, the bornhardts of the Gawler Ranges were produced by etching in the Triassic and Jurassic Žstage 1., and exposed as landforms in the Early Cretaceous Žstage 2.. The etching exploited orthogonal fracture systems formed by regional stress soon after the volcanic rocks had cooled and consolidated almost 1600 Ma. This bornhardt landscape is a multistage rather than simply a two-stage feature ŽTwidale and Vidal Romani, 1994., though it is the etching and subsequent stripping of the regolith that in an immediate sense are responsible for the contemporary landscape. Third, the recognition of surfaces as of two-stage and etch origin has confirmed the suggestion that some have been stable since exposure while all around has been weathered and eroded. For instance, surfaces dimpled by rock basins and scored by gutters are typical etch forms. The crest of Yarwondutta Rock, on northwestern Eyre Peninsula, is of this type ŽFig. 20a. and it shows that the surface has remained essentially unchanged since exposure, while successive alternations of subsurface weathering and erosion have produced a stepped inselberg ŽFig. 20b–d., lower and lower plains, and increasingly greater local relief amplitude ŽTwidale, 1982c; Twidale and Bourne, 1975b.. Fourth, and as Wayland Ž1921., Knopf Ž1924., Crickmay Ž1932, 1976., Willis Ž1936. and Horton

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Ž1945. appreciated, examples of etched and multiphase stepped residuals, such as Yarwondutta Rock, demonstrate that destructive geomorphological activities are unevenly distributed over the land surface. Wayland Ž1921, p. 40., for example, observed that: the excavation of the lower ground may accelerate upon that of the hills . . . thus may the turtle-backs of the past become the inselberg hills of the present. In similar vein, Knopf Ž1924, p. 637. pointed out that in the Appalachians some divides are Aout of reach of erosionB and Amore-or-less immune from destructional agencies of the present cycleB Žp. 642.. Clearly, increases in relief amplitude that are independent of tectonism have developed, assisting in the preservation and persistence of very old palaeoforms ŽTwidale, 1976, 1991, 1994.. Fifth, the concept of double planation impinges on the interpretation of anomalous drainage patterns. As Budel ¨ pointed out ŽBremer, 1993, p. 190. untere Einebnungsflachen are independent of surface drai¨ nage Žthough a subterranean pattern may well evolve on the topographically differentiated weathering front or lower surface.. Erosional lowering may result in the drainage pattern of the original surface being let down on to the weathering front, where it may well be transverse to structure: inherited drainage ŽCotton, 1948, p. 56.. Several anomalous rivers, but notably the Finke, which drain the Macdonnell Ranges and adjacent areas of central Australia, flow toward Lake Eyre across a formerly duricrusted surface, now partly eroded to form an etch plain ŽMabbutt, 1965. and across folds, faults and a multitude of rock types. The transverse courses of such rivers can be explained partly as inherited from a regolithic surface, partly as due to AautosuperpositionB, or stream persistence and valley impression ŽOberlander, 1965; Twidale, 1972.. 5.7. Questions of terminology Forms and surfaces, which originate at the weathering front and later have been exposed as a result of the stripping of the regolithic cover, are widely distributed in the Earth’s landscapes. The mechanism was recognised early, at least in the context of the development of Geomorphology, and is increasingly

66 C.R. Twidaler Earth-Science ReÕiews 57 (2002) 37–74 Fig. 20. Ža. Dimpled granite platform on the summit of Yarwondutta Rock, near Minnipa, Eyre Peninsula, South Australia. ŽC. Wahrhaftig.. Žb. The residual viewed from the north. The reservoir at the northern margin is seen, in close up, Žc. wherein flared slopes are naturally exposed Žbackground., and revealed in the excavation. Žd. The stepped northwestern slope of Yarwondutta Rock, due to exposure of the residual by episodic lowering of the adjacent plains.

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accepted as providing the key to understanding significant spatial and temporal aspects of landscape and landform development. Yet there is no agreement as to what the concept ought to be called. The term Doppelten Einebnungsflachen is disad¨ vantageous as a general description of the mecha-

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nism because of the connotations attaching to the term ‘planation’. In a geomorphological sense, it suggests both an event, a phase of lowering, and its result, the formation of a plain or plane surface, i.e., a surface of low relief. In nature, however, the weathering front may not be flat so that many two-

Fig. 21. Ža. Part of the group of granite pillars known as Murphy Haystacks, western Eyre Peninsula, South Australia. Note the concave-inward, or flared, sidewalls. Žb. Domical residuals in sandstone, western George Gill Range, central Australia. The platforms at midslope are erosional and may be due to subsurface weathering associated with past water table fluctuations.

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stage etched landscapes are topographically differentiated. Some etch surfaces are flat, but many wellknown forms of this type are not planate, so that Budel’s concept most directly refers to admittedly ¨ notable features, which were, however, developed in particular circumstances. Otherwise, ‘double-planation’ is an admirably evocative term describing the mechanism responsible for some notable examples of the genre: but only some. ‘Two-stage’ is equally meritorious and evokes the mechanism involved, and though many forms are demonstrably multistage in origin, the two most critical stages in landform development are those involving subsurface initiation and subsequent exposure. The word ‘etch’ suggests the character of the processes involved in their formation, as does A la corrosion chimiqueB ŽTwidale, 1990b., though the latter is tautological. Would the French verb graÕer —to engrave, fulfil a similar purpose? All these terms have the advantage of applying to forms at any scale. The prefix ‘crypto’ and the adjective ‘covered’ have been suggested in the karst context while Zwittkovits Ž1966., possibly taking his cue from Miller Ž1953., referred to covered Žkarst. forms as subkutan, a term which has the advantages of being readily translated into French Ž sous-cutane´. and English Žsubcutaneous., and of allowing forms which originate within the regolith, forms, such as corestones, logically to be termed AintracutaneousB ŽTwidale, 1987b.. All are useful terms, particularly those which distinguish between process and mechanism. The term ‘tor’ has also given rise to some confusion. Most writers have used the word to imply a small steep-sided hill. Several, though not all workers Že.g., Williams, 1936; Mabbutt, 1952; Thomas, 1965., like their colleagues earlier working in southwestern England, applied the term ‘tor’ to what are clearly boulders as sedimentologically defined, i.e., detached and rounded rock masses with a diameter of at least 256 mm Žabout 10 in.. and no formal upper limit ŽLane et al., 1947.. Linton Ž1955. expressed a preference for Acore-stoneB over Scrivenor’s Acore-boulderB Žq.v.. because, he argued, ‘stone’ carries no size limitation, whereas ‘boulder’ does. In fact, and as stated, no upper limit is implied by the term ‘boulder’. Some authors varied in their usage implying a boulder in some

papers, but a steep-sided hill elsewhere. Another difficulty is that the castellated remnants called ‘tors’ in southwestern England and in New Zealand Že.g., Cotton, 1917, p. 288; Raeside, 1949. are in other, and especially African, settings referred to as castle koppies. But the critical difference between a boulder and a tor or tower is not size. It is that the former is detached and separated from the main rock mass by a zone of weathered rock, whereas the latter remains in physical continuity with it, with the rock extending without break from the substrate into the residual or separated only by a fracture. Linton’s diagram depicted a tor, for the rock mass of the residual is in essential continuity with the underlying rock, but the residual was shown as comprising blocks some of which are rounded. Contiguity is a significant characteristic distinguishing boulders from small domes, pillars and tors or koppies. In foliated gneisses, tabular residuals known variously as penitent rocks, monkstones, ŽAckermann, 1962., and tombstones, Bussersteine ¨ larger residuals Žwhat some have called Gefugerelief ¨ —Turner, 1952., are shaped in the shallow subsurface, but, unlike corestones, they remain in physical continuity with the underlying bedrock. The larger rather squat pillars in granite ŽFig. 21a. constitute a genetic link between small projections and bornhardts Že.g., Twidale and Campbell, 1984; but see also Brajnikov, 1953.. Similarly, beehive forms or ŽTwidale, 1956b., found in monsoonal Bienenkorbe ¨ north Queensland have their larger scale counterparts both in the George Gill Range ŽFig. 21b. of central Australia and the Bungle Bungle Ranges of the eastern Kimberleys of Western Australia ŽYoung, 1986.. On the other hand, a large isolated rounded mass like The Leviathan, an ellipsoidal mass measuring 33 m long by 21 m wide by 12 m high, and standing in the Mt. Buffalo massif of southeastern Victoria ŽDunn, 1908., is a boulder because it is detached.

6. Conclusions Many of the investigations, which led to the interpretation of landscapes in terms of two-stage development, were carried out in the tropics, but the

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concept is of much wider, and indeed global, application. Many, perhaps most, destructional forms originate in the subsurface in greater or lesser measure. The recognition and understanding of minor etch forms were readily achieved, for evidence is widely exposed. The application of the concept to major features, however, called for imagination and has been complicated by considerations of scale Ždepth of weathering and consequent scarcity of exposure, plus a failure to link shallow and deep developments., time Žboth absolute, and the relative rates of weathering and erosion. and the mechanisms involved Žthe effects of weathering in time, the mode of regolith removal., and space Žconfusion of absolute zonality as opposed to prevalence of occurrence.. Nevertheless, the two-stage concept is now widely accepted and is an invaluable tool in the analysis and interpretation of landscapes in their spatial and temporal settings. The two-stage concept has a lengthy history, with published expositions relating to minor forms dating from more than 200 years ago and to major features and landscapes from more than a century. In terms of landscape evolution it is properly associated with the names of Wayland, Linton and Budel. But others ¨ prepared the way, with Hassenfratz, MacCulloch, Logan, Kingsmill, and Pumpelly in the van and Falconer and Willis contributing crucial insights concerning the sequence of events implied and, the processes involved, in two-stage evolution. Hassenfratz and Falconer, in particular, provided succinct accounts of the essential mechanism. The etch or two-stage concept did not, as is commonly implied, originate with Wayland. Analysis of how the two-stage concept evolved confirms the suggestion that in science the credit frequently—and perhaps deservedly—goes to the person who assembles evidence and argument wherewith to sustain the idea, to demonstrate its viability pro tempore, and not to the one who first thought of it; ingenuity in devising tests for ideas is almost as important as the ideas themselves. While praising those who convinced the world, however, sight surely ought not to be lost of those who had the imagination and courage—for people are afraid of unfamilar ideas and the unconventional can attract obloquy—to interpret familiar forms in unaccustomed terms: it is salutary to bear in mind that

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AWhat is now proved was . . . once only imaginedB. Whitehead is said to have warned that a science, which hesitates to forget its founders is lost, and though the justification for such a statement is obvious, credit for fertile ideas ought surely to be directed to those to whom it is due. This is especially so of ideas like the two-stage and etch concepts, which have changed geomorphological perceptions of the world. For instead of seeking explanations for the varied forms of the land surface only in epigene processes, we must perforce also look beneath the surface and examine and consider the processes operating at the base of, and within, the regolith.

Acknowledgements The writer thanks various friends and colleagues, particularly, Professor Dr. Hanna Bremer of Wilhelmsfeld, Germany, and Dr. Jennie Bourne, Adelaide, for a critical reading of the paper in draft form and for helpful suggestions; Professor Brian Skinner of Yale University for directing me to Aaron Waters’ informative and inspiring obituary of Bailey Willis; and Professor Philip Withers ŽPerth., Professor Charles Hutchison, and Dr. H.D. Tjia ŽKuala Lumpur. for invaluable assistance in tracing certain references. Professor Ian Douglas and an anonymous referee offered constructive suggestions, which are much appreciated. The views expressed are, however, his own. Much of the Australian field work on which this review is based was supported over the years by various grants from the Australian Research Committee.

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