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Landscape analysis: derivation and rediscovery of ideas

L’analyse des paysages : origine et redécouverte d’idées
Charles Rowland Twidale
p. 259-277

Résumés

La trilogie davisienne « Structure, processus et temps-durée » fournit encore un cadre utile pour l’analyse des paysages mais la plupart des processus et mécanismes invoqués dans le schéma originel sont abandonnés depuis longtemps. Le legs des affirmations davisiennes est d’avoir pris en compte des concepts et facteurs parfois reconnus depuis longtemps mais surestimés, alors que d’autres de conception plus récente étaient marginalisés, mais tous ont contribué à la compréhension des paysages. L’attention se porte sur les facteurs sous-estimés, comprenant les phénomènes structuraux comme l’état de contrainte et les linéaments, sur des formes variées qui se sont révélées être d’origine tectonique, sur les impacts de l’érosion profonde et du préétabli sous-jacent. Beaucoup de phénomènes familiers ont été initiés à la surface topographique mais au niveau du front d’altération par des processus actifs à cet emplacement. Beaucoup de paysages (comme ceux nombreux où domine le recul des versants) deviennent compréhensibles quand on considère la réalité de l’altération et de l’érosion différentielles, ainsi que la survie de très vieux éléments de paysage. L’azonalité et l’antiquité des formes de relief sont plus fréquentes que ce que l’on croyait auparavant.

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Notes de la rédaction

Article soumis le 25 octobre 2011, accepté le 28 octobre 2011.

Texte intégral

Introduction

1Though, “Much of Davisian geomorphology had become stale and unprofitable before its author’s death” (Leighly, 1955, p. 317), and indeed little of his conceptual framework has survived intact, the assertion that: “All the varied forms of the lands are dependent upon… three variable qualities, which may be called structure, process, and time” (Davis, 1899, p. 481) is an exception. It remains a sound basis for geomorphological analysis. As usual, however, the devil is in the detail, for all the models based in Davisian deductions have been modified, abandoned, or replaced. In their stead, various principles, factors, or mechanisms, some of relatively recent derivation, but most noted and cited, albeit incidentally, up to two centuries ago, are now seen as having a crucial role in the evolution of several familiar landscape features.

Crustal stress

Rock fabric and landforms

2Fractures have long been recognised as a pronounced influence on landform development, but they are only one component of B. Sonder’s (1948) Gefügerelief, or the correlation between landforms and rock fabric. The latter term embraces not only faults and joints but also cleavage, foliation, schistosity, and lineation (see also Turner, 1952), to which may usefully be added lineaments and ring structures. Lineaments are linear topographic features of regional scale that are expressions of crustal structure, most commonly faults (e.g., Hobbs, 1904; Krenkel, 1929; Vening Meinesz, 1947; Hills, 1956; Brock, 1957; Hills, 1961; O’Driscoll, 1980; De Kalb, 1990). Similarly, the ring structures noted by G.D. Kaminine and L.G. Richter (1956), J.M. Saul (1978), V.R. Baker et al. (1992), and others, enter into consideration for though their origin remains enigmatic, like lineaments they have determined many river, and hence topographic, patterns (O’Driscoll and Campbell, 1997).

3Many reputable geologists do not accept that lineaments exist (see e.g., Hobbs et al., 1976, p. 276, which definition was cited in R.L. Bates and J.A. Jackson, 1987, p. 380) and the concept does indeed pose problems. For example, it is difficult to reconcile the proposed global pattern of lineaments (e.g., Vening Meinesz, 1947) with the plate paradigm. But the patterns of straight lines discernible on most topographic or geological maps of almost any scale, give pause for thought, particularly as many coincide with known or suspected faults. Also and in addition to their value in explaining landscape features, many notable mineral deposits have been located using lineaments as an exploration framework (e.g., O’Driscoll, 1986; Woodall, 1994; Bourne and Twidale, 2007).

4Where open, fractures allow the transmission of water and other fluids. They accommodate ascending magmas, to produce sills and veins in geometric patterns that are congruent with local structure. They give rise to linear ridges and clefts at various scales. Dykes, on the other hand, cut across local structures so that attendant forms tend to be discordant and irregular. Conversely, as meteoric waters armed with chemical and biota penetrate into rock masses by way of partings they give rise to patterns of weathering and erosion, and hence topography, especially in rocks that are impermeable but pervious.

Instability

5The impacts of earthquakes and volcanic events, and the landforms associated with them, are well documented. The entire Earth’s surface is unstable, though some areas are more susceptible to frequent and intense disturbance than others. Even the shields and cratons, however, at one time considered so stable that they could be used as natural tide gauges (e.g., Steers, 1937; Teichert, 1947), are constantly shaken causing widespread slight dislocation or joggling. Faulting produces scarps, horsts, and graben of regional and local extent but also at the millimetre scale, like those resulting from the tectonic joggling of granite platforms in northwestern Western Australia (Vidal Romani and Twidale, 1998, p. 359; Clark and Bodorkos, 2004; Twidale and Vidal Romani, 2005, p. 260). Such contemporary tectonic landforms and any that (arbitrarily) post-date the Miocene are termed “neotectonic” (Obruchev, 1948), though such features also have been - and still are - described as “recent” and “contemporary” (e.g., Lees, 1955; Meshcheryakov, 1959).

New tectonic and neotectonic forms

6In addition to hitherto neglected microtectonic features, some landforms that were previously explained in other terms are now recognised as being of tectonic origin.

7G.K. Gilbert (1904), but see also T.N. Dale (1923, p. 29), famously suggested pressure release or erosional (lithostatic) offloading in explanation of the massive slabs known variously as off-loading joints or sheet structures. Even before Gilbert, however, others had considered sheet fractures as planes of dislocation - faults - caused by shearing and torsion, and thus to be tectonic (Harris, 1888; Merrill, 1897). Like other partings they find expression in near-surface zones of low confining lithostatic stress. Some display striations (slickensides, comparable to those formed by bedding plane slippage in folded strata) and steps, polishing and recrystallisation, imbrication, dislocation of strata, split crystals (especially phenocrysts), and, in conglomerates, split cobbles (e.g., Twidale et al., 1996; Bourne and Twidale, 2003; Twidale and Bourne, 2003).

8Sheet fractures are well developed in granitic rocks but also occur in sandstone and dacite (e.g., Bradley, 1963; Twidale, 1978; Campbell and Twidale, 1991). Calculations have suggested that the effects of erosional unloading, though real (e.g., Chapman, 1956; Bowling and Woodward, 1979), are inadequate to explain the compressive stress and rupture implied by sheet fractures and other features (Coates, 1964; Adams, 1982). Sheet fractures that were at one time regarded as conclusive proof of pressure release (e.g., Lewis, 1954; Gage, 1966) are now interpreted as due to stress trajectories becoming re-aligned in parallel to youthful erosional surfaces of least principal stress (Müller, 1964). The tectonic construction also finds support in field evidence, and monitoring of sites. For instance, sheet fractures were formed during a low energy earthquake that affected Minnipa Hill, northwestern Eyre Peninsula, on 19 January 1999 (Twidale and Bourne, 2000a). Laboratory experiments indicate a mode of formation of sheet fractures that is compatible with the favoured two-stage hypothesis of bornhardt development discussed below (e.g., Holzhausen, 1989; Twidale et al., 1996).

9Dislocation along adjacent sheeting planes of different radii of curvature has caused the development of triangular wedges where the fractures intersect the surface of hill slopes (lateral wedges). The vertical wedges of gentle slopes and platforms are caused by direct compression and shearing applied during earthquakes. Most take the form of thin splinters, though a large arched wedge located on the slopes of Mt Wudinna, Eyre Peninsula, is of this type (Twidale and Sved, 1978).

10Many early workers resorted to insolation or pressure release in explanation of landforms ranging in size from surficial flakes to sheet structures, and including A-tents or pop-ups (Scott, 1897, p. 223; Jutson, 1914; Peterson, 1975). H.P. Cushing et al. (1910), however, entertained the possibility that buckled rocks were caused by crustal compression (fig. 1), and examples have been noted associated with the horizontal stresses instantaneously generated during earthquakes (e.g., Rutty and Cruden, 1993; Wallach et al., 1993; Twidale and Bourne, 2000a, 2009). A tectonic origin is suggested also by the common orientation of the fractured crests of A-tents in a given region (e.g., Ericson and Olvmo, 2004). As with sheet fractures and lateral wedges, stress was applied and strain patterns established at depth. They probably were not manifested as fractures defining sheets, wedges or buckles until brought by erosion near or to the surface, where confining lithostatic stress was diminished and overcome. Alternatively, and as demonstrated in specific observed instances, a later input of stress has overcome lithostatic constraints.

11By contrast with the instant effect of the horizontal stresses generated by earthquakes, compressional forces may be applied gradually, over time. A blister or arched slab in the Condering Hills, in the southwest of Western Australia, an area known to be slowly arching (Wellman and Tracey, 1987), has over the last hundred years or so not only slowly increased in amplitude but also has developed a crestal fracture that will in due time convert it into an A-tent (Twidale and Bourne, 2003, 2009).

12Applied stress finds expression even at the miniature scale. Laminae were produced or made evident along planes of dislocation during the Minnipa Hill 1999 earthquake. Differential fracture-controlled weathering of rocks like granite, norite, and basalt has resulted in corestones set in a matrix of weathered rock, with laminae adjacent to the spheres of fresh rock. In the eastern Mt Lofty Ranges, however, adjacent to the Palmer–Milendella Fault Zone that defines the upland on its eastern side, some granite corestones within joint blocks are barrell-shaped with corners of fresh rock shaped like tetrahedra, and construed as the result of shearing in cohesive, brittle rock.

13Crustal stresses are continually renewed. Even the shields and cratons are unstable and in particular tend to rise either as an isostatic reaction to long-continued erosion, or as a result of regional uplift caused by regional compression, or by rising convection currents in the upper mantle (Burke, 1996; Partridge, 1998). Whatever the causation, runoff drains from these ancient continental nuclei. Because the granitic rocks that are prominent components of the shields and cratons are relatively stable when dry (e.g., Logan, 1851), but also because granite is brittle, tectonic landforms are especially well preserved and represented on these supposedly stable shield lands (Twidale, 2011a).

Fig. 1 – A-tent in granite developed on lower slope of Kokerbin Hill, Yilgarn Craton, southwestern Western Australia
Fig. 1 – Une forme de tente triangulaire développée en bas de pente de la colline de kokerbin, Craton du Yilgarn, sud-ouest de l’Australie occidentale

Fig. 1 – A-tent in granite developed on lower slope of Kokerbin Hill, Yilgarn Craton, southwestern Western Australia Fig. 1 – Une forme de tente triangulaire développée en bas de pente de la colline de kokerbin, Craton du Yilgarn, sud-ouest de l’Australie occidentale

Photo: C.R. Twidale.
Photo : C.R. Twidale.

Underestimated factors

14Structural influences on landscape, whether of the tectonic active or passive Gefügerelief type have long been recognised and are still essential aspects of landscape analysis, but some structural factors have remained unrecognised or underrated.

Strain

15In places, and depending on their intensity and duration and on the rheology of the rock, applied stresses have produced zones of strain rather than fracture. The crystal lattices of rocks in strain are distorted, rendering them more susceptible to weathering and hence erosion (Russell, 1935; Nabarro, 1967, p. 4): hence the common development of linear zones of preferential weathering and discontinuous fractures. Thus, roughly linear clefts in granite have been formed in parallel with adjacent Kluftkarren (or fracture-controlled clefts). The former display few or no fractures, but by contrast with the random orientation of crystals in the rock mass as a whole, the feldspars within a few centimetres of the suspected strain zone are aligned in parallel with it. Also, whereas many lineaments are coincident with fault zones, in other instances such fractures are discontinuous or absent. Such lineaments may be based on strain zones that have been exploited by weathering and erosion to produce linear topographic features (Twidale and Bourne, 2007).

Deep erosion

16In orogenic or fold mountain belts, structural patterns change in depth and deep erosion has exposed rocks differing in stress and rheological characteristics from those at and near the land surface. As indicated in the discussion of sheet fractures and A-tents lithostatic loading is a relevant factor. In addition, the tensional upper zones of anticlinal structures give way at depth to compressional environments, and vice versa in synforms (e.g., Price, 1966). Thus, in sedimentary sequences structural/topographic inversions are known as Jura-type relief (e.g., Derruau, 1965, p. 320; see also Bourne and Twidale, 2011).

17In granitic rocks, bornhardt inselbergs most commonly display convex-upward sheet fracture sets. Some, like those described from the Rio de Janeiro area of southeastern Brazil (Lamego, 1938), are based on the deep compressional cores of antiforms. Other bornhardts, however, are developed in granite subdivided by concave-upward fracture sets (Twidale et al., 1996, see also Branner, 1896). Such inversions are a result of the deep differential weathering and erosion of the granite masses. They provide further demonstration that sheet fractures and forms are not necessarily associated with the erosional relief of pressure. They also show that the rounding of bornhardts though enhanced by the common presence of convex-upward sheet fractures is, like that of corestone boulders, most plausibly attributed to differential weathering.

18With slope, fractures are major influences in the development of drainage patterns and hence topography (e.g., Zernitz, 1931). Some stream sectors, however, defy geological controls and cut across the structural grain. Stream deviation, antecedence and superimposition have all reasonably been invoked in particular instances (Twidale, 2004) but deep erosion combined with the persistence of rivers due to reinforcement effects, better accounts for several otherwise anomalous stream sectors (Meyerhoff and Olmstead, 1936; Strahler, 1945; Oberlander, 1965; Twidale, 1966). Also, fault-line streams have been maintained even when deep erosion has caused, say, a dipping fault plane that initially was exploited by the stream, to have migrated laterally while the stream course persists in its original location (e.g., Bourne and Twidale, 2011).

Underprinting

19The effects of tectonic activity in basement rocks, i.e. older formations, usually igneous or metamorphic, occurring beneath sedimentary sequences have been imposed on the superincumbent strata in a process known as underprinting or upward generation (Saul, 1978), and at scales ranging from the regional, to the local, to the site.

20At the regional scale, H. Wopfner (1960) suggested that the gentle folding of silcreted Cretaceous strata in southwestern Queensland and adjacent areas had been imposed from below from compression in the deep basement. At about the same time, E.S. Hills (1961; the William Smith Lecture to the Geological Society of London published 1960) noted the remarkably straight course of the Darling-Culgoa River, which is some 600 km long, and which flows mainly in Quaternary alluvia. He attributed the pattern to underprinting, with joggling on the Darling Lineament in the underlying basement rocks having been transmitted upwards through the unconsolidated cover to produce ephemeral dirt scarps and fault sags that guided the course of the river.

21At the local scale, aligned hilltop dolines developed in the Pleistocene dune calcarenite of western Eyre Peninsula have been attributed to a different type of underprinting. C.R. Twidale and J.A. Bourne (2000b) suggested that the anomalous sinkholes were the result of the concentration of groundwaters along basement fractures and consequent volume decrease, compaction and localised collapse and surface lowering of the surficial calcarenite. At site scale, linear sags and breaks in turf overlying fresh granite at Yarwondutta Rock, northwestern Eyre Peninsula, have developed as result of collapse of the cover into linear voids resulting from slabs in the underlying fresh granite having been pulled apart, possibly during an earthquake (Twidale and Bourne, 2000a).

Pseudostructural forms

22In addition to the recognition of new tectonic forms, features that look like tectonic forms but are caused by gravity, extraterrestrial forces, and surficial processes also have been described. They include knee folds, pseudoanticlines, impact craters, pingos, and tepees (Price, 1925; Harrison and Falcon, 1936; Hollingworth et al., 1944; Müller, 1959; Jennings and Sweeting, 1961; MacCallien et al., 1964; Mackay and Stager, 1966; Brown, 1969; Milton and Sutter, 1987; Williams, 1994; Gurney, 1998; Paull et al., 2007).

Processes at work

Morphogenetic view

23For obvious reasons, landscapes traditionally have been explained in terms of the various processes visibly active at the land surface. Landform assemblages were related to and classified in terms of climatically induced and controlled surface processes, predominantly those active at present, but with reference also to the recent past (e.g., Davis, 1905, 1906). Thus, the effects of Pleistocene glaciations featured prominently in accounts of many midlatitude landscapes where rivers are dominant today, as were the dunefields associated with arid climatic phases and evidenced in presently semiarid parts of Africa and Australia (e.g., Crocker, 1946; Grove, 1958). The suites of landforms associated with climatic extremes are undeniably distinctive, but other less well-defined climatic or morphogenetic regions were postulated (e.g., Peltier, 1950; Tricart and Cailleux, 1958, and subsequent regional accounts; Büdel, 1977). Despite errors such as the underestimation of the work of water in deserts (e.g., Barton, 1916; Bosworth, 1922; Peel, 1941), and in glaciated terrains (e.g., Derbyshire, 1962), many textbooks were organised in terms of climatic control. Also, the concept was applied to climatic influences in specific lithological settings (e.g., Wilhelmy, 1958), as well as in the coastal context (Davies, 1964).

24But associations between climate and the processes responsible for the shaping of landforms are well known. Taking weathering as an example, frost action or gelifraction is an obvious example, while haloclasty is active in arid lands and in the coastal context. In broader vein, most chemical processes are favoured by high temperatures and the ready availability of water so that the weathered mantles or regoliths of the humid tropics are thicker and more intensely altered that those developed elsewhere. This has led to the common occurrence of mass movements particularly in earthquake-prone areas, where there has been human interference with slopes, and following heavy rains.

25Early explorers of the monsoon lands, reported a curious weathered material that was weak and malleable when wet, but which dried to an irreversible hardness. Because it was used to make bricks they called it laterite (from the Latin later = brick), or brickstone (Buchanan, 1807; Babington, 1821; Maignien, 1966). It is a zonal feature comprising a ferruginous horizon and clayey (commonly kaolinitic) mottled or pallid zones that is developed only in the tropical monsoon lands. When dry, the ferruginous pisolites or fragments develop a crystalline continuity (Alexander and Cady, 1962), so that it is resistant and still forms a protective carapace capping the land surface in many low- and mid-latitude lands. It is a duricrust, one of several that are differentiated on the basis of composition, as in calcrete, ferricrete, silcrete and gypcrete, and the aluminous bauxite (Goudie, 1973).

26Most duricrusts are associated with particular climatic regimes, for, like laterite, bauxite is formed in the humid tropics, and silcrete, though in some instances associated with riverine flood plains and though preserved in aridity, appears to have formed in humid tropical environments (Wopfner et al., 1974; Benbow et al., 1995). By contrast, gypcrete and calcrete are of arid and semiarid provenance. However, travertine as a calcareous spring or stream deposit is found in a range of climates. Duricrusts form cappings on plateaus or, where folded, cuestas and other ridge forms. Like lava flows, where deposited in valleys either initially or following transport and deposition, they have changed local drainage patterns and induced topographic inversions (e.g., Miller, 1937; Twidale et al., 1985).

Zonality and azonality

27Although it was recognised that tectonic and structural forms are azonal, and also that typical fluvial landforms transgress climatic boundaries (e.g., Twidale and Lageat, 1994) climate was widely accepted as a basis for landscape classification and understanding. However, early investigators had already signalled the development of another assemblage of forms that is widely distributed and that further undermines the concept of zonality. They had recognised that some boulders are shaped not at the surface but below ground, where their development was not directly affected by climatically-induced processes and events.

Two-stage development and etching

28Forms initiated at the base of the regolith, at the weathering front (Mabbutt, 1961), have been shaped largely by hydro-weathering processes - solution, hydration, hydrolysis - and are known as two-stage, or etch, forms. Most regoliths are friable and readily stripped, resulting in the widespread exposure of the weathering front. ‘To etch’ is to attack chemically, and later workers (Wayland, 1934; Willis, 1936) used the word as an evocative shorthand term for the two-stage combination of subsurface weathering and subsequent erosion. D.W. Carnegie, an early explorer of the interior of Western Australia, likened the etching of the rock basins he referred to as “namma-holes” - or gnammas - to the “gradual hollow formed by the decay in a tooth” (Carnegie, 1898, p. 82); and the comparison is apt.

29Even earlier, J.-H. Hassenfratz (1791) - whose comments on landforms observed on the southern Massif Central were noted by J. Hutton (1795, II, p. 174) - and several others realised that the corestone boulders they had noted in the field had been shaped in two stages, namely fracture-controlled weathering in the shallow subsurface, followed by the exposure of the weathering front as a result of erosion under the influence of gravity, rivers, wind, waves, or frost action. All stages in the development of corestone boulders from kernels set in a matrix of grus, to partly exposed and free or disengaged boulders, were described by various early workers. It was realised that water penetrated along partings and that the corners and edges of the cuboidal blocks were rotted more rapidly than the plane faces resulting in the conversion of angular blocks of fresh rock to spheroidal masses embedded in a matrix of rotten rock.

30Later it was established that the initial and rapid alteration of mica and feldspar (e.g., Caillère and Henin, 1950; Alexander, 1959) produced hydrophilic clays, which expanded when wet resulting in physical - granular or laminar - disintegration. This caused the hitherto virtually impermeable rock to become more permeable (e.g., Kessler et al., 1940). Such ready access of water induced further alteration (Larsen, 1948; Hutton et al., 1977; see also Ruxton and Berry, 1957), an even greater permeability, and further and intense alteration: a reinforcement or positive feedback effect. The diameter of the spheroidal kernels or corestones diminished as weathering advanced into the blocks of fresh rock until eventually, given uninterrupted time, the fresh rock was consumed, and was replaced by a regolith or mantle of weathered rock, which in granitic rocks is known as grus or fine gravel. Even quartz was dissolved for it disappeared from the upper, longer weathered, zones of grussy profiles.

31The perimeters of corestones, like the surfaces of the still intact bedrock, constitute sectors of the weathering front. Relative rates of weathering and erosion determine the nature of the landform assemblage, whether closely packed, little-rounded blocks, corestones set in a matrix of grus, or a mass of puggy clay (Lewis, 1955). At some sites, where the granite has been reduced entirely to grus, the outlines of former corestones are indicated by concentrations of iron oxide presumably precipitated during pauses in the weathering sequence. Alternatively, such ‘ghosts’ of corestones could be attributed to a brief accession of groundwaters rich in salts of iron.

32Some boulders have been rounded during transport by rivers, and by glaciers. The granitic boulder beaches of the Victorian coast (e.g., Hills, 1970) and eastern Kangaroo Island, the latter locally derived but modified corestone boulders, show that waves have achieved similar results. But assemblages of corestones and boulders with veins intrusive into the granite host intact, demonstrated that the corestones and boulders are in situ and are due to weathering (Kingsmill, 1862). There, many boulders are two-stage forms, for evidence pointing to their subsurface initiation and shaping has been recorded from a range of climatic and topographic settings, and from various bedrocks including basalt, dacite, norite, rhyolite, sandstone, and (crystalline) limestone, as well as granitic.

33One of the early contributors to this store of innovative ideas concerning granitic corestone boulders (Scrivenor, 1913) was J.R. Logan, a naturalist who resided in Penang, in what is now peninsular Malaysia, and who brought together and confirmed earlier articulated explanations concerning the formation of corestone boulders but, and significantly, drew inferences pointing to future developments (Logan, 1851). Considering the granite outcrops of Singapore and adjacent islands (particularly Pulau Ubin, in the Strait of Johor) Logan noted the occurrence of ‘planes of division’ that occur in orthogonal patterns, thus subdividing the rock into ‘cuboidal’ blocks. Like previous workers, Logan realised that these partings were planes of weakness along which water could penetrate and rot the rock with which it came into contact. Despite the obvious but then unquantified relatively minute amounts - now estimated as of the order of 0.6% of the total present on Earth (e.g., Nace, 1960) - of water occurring in rivers and as shallow groundwaters, he nevertheless appreciated the crucial importance of water in shaping the landscape.

34J.R. Logan also and astutely realised that whereas the fresh rock masses still covered by rotted rock or soil remained in contact with retained moisture and continued to be weathered. Even in the humid tropics the exposed corestones dried in the sun, leaving them “above the influence of decomposition” (Logan, 1851, p. 326). He appreciated that ‘dry’ implies stability, but ‘wet’ ensures rotting - what J. MacCulloch (1814, p. 72) evocatively called “this gangrenous process”. Logan (1851, p. 326) expressed this as follows: “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.” Thus, J.R. Logan related causes and consequences, and moved on from the analysis of past events to likely future trends.

35Most hypotheses are suggested by analogy and Logan’s observations and his ideas embrace the essence, first, of the concepts of etching and two-stage development, and second, of the inevitable sequence of structural differentiation, unequal activity, and positive feedback. As will be demonstrated, they are critical also to the development and maintenance of ancient landforms and landscapes.

Extension of the two-stage concept

36In their descriptions and discussions of corestone boulders the early investigators cited did not use the terms etch and two-stage, but they recognised the process and mechanism. A century later, J.D. Falconer (1911) realised that some bornhardts may be of subsurface derivation, and J.T. Jutson (1914, 1934) deduced that the inselberg landscapes of Western Australia had evolved in two stages (his Old and New plateaux) involving subsurface weathering and subsequent erosion and exposure. Later, J. Büdel (1957) described the etching of landscapes at the regional scale as “double planation” for it resulted in surfaces of low relief both at the surface, and at the weathering front.

37Etching can account for plains of remarkable regularity such as the karstic Nullarbor Plain, the Meekatharra Plain of Western Australia, cut in granitic rocks, and in southern Africa, the Bushmanland and Drakensberg surfaces cut across various rock types (e.g., King, 1950). The process is also germane to the understanding of the glaciated high plains that occupy much of the Labrador Peninsula and also the Baltic or Fennoscandian Shield (e.g., Fogelberg, 1985).

38Several other common landforms and landform assemblages such as rock basins, pitted surfaces, clefts, gutters, and flared slopes appear to be of similar origin for examples have been observed already shaped at the weathering front beneath a regolithic cover in situ (Twidale, 2002). Questions remain concerning the processes responsible for the weathering of the initial saucer-shaped depressions (along fractures and at fracture intersections, on concentrations of susceptible minerals?) and for their contrasted development into pits, pans, armchair-shaped hollows and cylindrical hollows following exposure (Twidale and Corbin, 1963; Hedges, 1969); but that many are initiated in the subsurface is beyond doubt.

39Similarly, the bedrock concavities known as flared slopes (fig. 2) clearly are formed at the weathering front for they have been exposed, already shaped, in artificial excavations (Twidale, 1962; see also Clayton, 1956). Some extend laterally into cylindrical foot caves, which morphologically can be regarded as especially deep concavities (Twidale, 1978). Whether this development takes place in the subsurface or after exposure is debatable. They could be formed by sapping, or by localised haloclasty within the flared concavity. Alternatively, they could be an expression of especially intense weathering either in the chemically aggressive near-surface zone, or most likely, at the weathering front where water accumulates above the impermeable fresh country rock and where the initial breakdown of the rock most readily occurs (e.g., Twidale, 1986). But no incipient granitic footcave has so far been located in the subsurface.

Fig. 2. – Wave Rock, a flared slope some 14-15 m high in granite, northern slope of Hyden Rock, Yilgarn Craton, southwestern Western Australia
Fig. 2 – La Wave Rock (“Roche en Vague”), une pente évasée de 14-15 m de haut dans du granite, versant nord du Hyden Rock, Craton du Yilgarn, sud-ouest de l’Australie occidentale

Fig. 2. – Wave Rock, a flared slope some 14-15 m high in granite, northern slope of Hyden Rock, Yilgarn Craton, southwestern Western AustraliaFig. 2 – La Wave Rock (“Roche en Vague”), une pente évasée de 14-15 m de haut dans du granite, versant nord du Hyden Rock, Craton du Yilgarn, sud-ouest de l’Australie occidentale

Black stripes due to blue-green algae colonisation of water seepages.
Les bandes noires sont dues à des algues bleu-gris colonisant des suintements d’eau.

Photo: C.R. Twidale.
Photo : C.R. Twidale.

Implications of etch origin

40Regolithic water is the essential factor in two-stage development and as shallow groundwaters are virtually ubiquitous, so, therefore, are etch forms. Although inheritance has to be borne in mind, availability of water can explain otherwise anomalous occurrences. For instance, granitic nubbins due to the blocky disintegration of the outer shells of bornhardts, are typical of, and are best developed in, the humid tropics. Black Mountain, near Cooktown, and the Metal Hills near Chillagoe, both in monsoonal north Queensland, appear to consist wholly of boulders. But nubbins occur also at favoured sites, as for instance in valleys in the vicinity of Alice Springs, in the MacDonnell Ranges, in the arid interior of Australia, where water drains from the adjacent quartzite ridges. The Devils Marbles, located some 200 km to the north, comprise large residual boulders and nubbins located in an anticlinal valley defined by quartzite ridges. The latter are bevelled and are remnants of the planation surface beneath which the granite was differentially weathered. A mesa capped by ferricrete indicates the location and nature of the weathered former floor of the anticlinal valley.

41Etch forms evolve in two stages and have two ages, one indicating the subsurface phase of development, the second, the period of exposure as a landform. Though the processes at work vary in rate and in detail according to bedrock and regional and local climate, the results are similar regardless of whether developed in a tropical, temperate, or cool environment. Thus rock basins or gnammas and bornhardts are found (albeit under various names) in humid tropical, cool temperate and arid lands. Etching also has produced similar forms also in rocks of different origins but common physical characteristics, namely, rocks that are well-jointed but of low permeability. Thus, inselbergs are well developed in granite, sandstone, conglomerate and crystalline limestone, as are corestone boulders, flared slopes, rock basins and pitted surfaces.

42Investigations of the work of wind, rivers and glaciers by such distinguished investigators as R.A. Bagnold (1941), J.F. Nye (1952), and L.B. Leopold (e.g., Leopold et al., 1964) have proved invaluable. Not only have they allowed an improved understanding of several commonplace landform assemblages but just as the effects of extreme tectonic and extraterrestrial impacts remain imprinted in the landscape, so the significance of extreme process events are better appreciated. The changes wrought by coastal storms and river floods have long been documented (e.g., Johnson, 1919; Tricart, 1960) but the basic fluvial studies of L.B. Leopold, M.G. Wolman, and their colleagues allowed the resolution of long standing and at times acrimonious debates concerning, for example, the Channeled Scablands of the northwestern USA (e.g., Pardee, 1910; Bretz, 1923, 1969; Baker, 1973).

43But such studies have not greatly influenced problems posed by those many landforms shaped at the weathering front, save insofar as they clarify how the stripping phase of the two-stage development is achieved. For instance, how surface agencies such as glaciers erode is germane not only to the understanding of glaciated surfaces (e.g., Boyé, 1950; Bird, 1967; Lidmar-Bergström, 1997), but also explains how pockets of pre-glacial regolith have survived passage of ice sheets (e.g., Bouchard, 1985). Fundamentally, however, it is the chemical reactions of bedrock and groundwaters that provide the key to understanding etch forms (e.g., Loughnan, 1969; Trudinger and Swaine, 1979; Yatsu, 1988; Viles, 1988).

Unequal activity, reinforcement, concatenation

44J.R. Logan (1851) realised that the corestones of fresh rock exposed at the surface and dried by the sun were relatively stable, whereas the corestones still covered are still under attack by moisture retained in the regolith continue to evolve. Exploitation of structural factors induced unequal activity, though that term was not applied to the contrast for another century. In a paper on drainage systems, E. Bliss Knopf (1924, p. 637) referred to headwater regions that were “out of reach of erosion”. This conclusion, based in field observations ran contrary to W.M. Davis’ deductive scheme: “Although the river and the hillside waste sheet do not resemble each other at first sight, they are only extreme members of a continuous series, and when this generalization is appreciated, one may fairly extend the ‘river’ all over its basin and up to its very divides.” (Davis, 1909, pp. 266-267). Bliss Knopf’s contrary inference later found support in field measurements (Dunne and Aubry, 1986), while C.H. Crickmay (1932, 1976) extended its implications in his hypothesis of unequal activity.

45Several well-known landforms, and some less familiar, are expressions of unequal activity and associated reinforcement or positive feedback mechanisms (Behrmann, 1919; King, 1970; Twidale et al., 1974). Reference has been made to the enhancement of weathering when water has initiated disintegration. Once a bornhardt that originated as a massive rock compartment stands in positive relief, it sheds water. It is weathered and eroded only slowly, whereas the rocks beneath the surrounding plains are intensely altered for they receive runoff and seepage from the hill as well as direct rainfall. If baselevel permits, the altered rock is eroded, and the surface lowered. In addition, and alternatively, however, flushing during heavy rains and runoff may cause volume decrease and subsidence in the piedmont (Ruxton, 1958). Chemical weathering may have similar results (e.g., Trendall, 1962).

46Directly or indirectly, in arid lands as well as humid, in glaciated terrains as well as tropical, rivers are responsible for shaping most of the Earth’s surface and river systems are examples of reinforcement at work. Natural selection applies. Some channels, and commonly those that have exploited some structural weakness, are more effectively reinforced than others and become master streams (Twidale, 2004). Once a stream is preferentially incised and positive feedback is induced, it receives more and more seepage and runoff. Thus its existence and dominance are enhanced, perpetuated, and reinforced.

47Some deeply incised streams have been defeated and dismembered by resistant formations and are now represented by opposed but aligned stream and valley remnants, possibly linked by a wind gap. But many rivers have had the power to maintain their courses even when the structural advantage to which they owe their origin and perpetuation were lost though deep erosion into a structural field the nature and geometry of which differ from those in which the system originally developed. Streams tend to persist, which is why some extant rivers and streams are demonstrably of great antiquity.

48At a different scale in a different context, tension scars and gaps are developed in association with several types of mass movement, such as landslips, landslides, and earth flows. Not only are the hill slopes above the scars unbuttressed but seepage at the base of the backwall of the scarp induces its undermining and collapse. This is particularly pronounced in landslips like that on the Hummocks Range near Lochiel, South Australia, where the strata dip downslope (Twidale and Bourne, 2011).

49Thus, some landforms become self-perpetuating and self-enhancing. The inevitable sequence of structural contrast and advantage, unequal activity, and reinforcement responsible for this has been termed concatenation.

Significance of unequal activity

50At a regional scale, and germane to the question of mode of landscape evolution, slope behaviour is crucially affected by unequal activity induced by gravity. L.C. King’s scarp retreat model is a case in point. Runoff on any slope is concentrated on the lower sectors and in the scarp foot or piedmont. The resultant weathering induces basal erosion and the regrading of the slope from the base upwards. Faceted slopes are developed particularly where a caprock is present. Also, once steepened to the maximum inclination commensurate with stability, slopes are worn back and the landscape evolves by stream incision and the recession of valley side slopes (Fisher, 1866; Holmes, 1918; Lehmann, 1933; King, 1953).

51The essence of King’s model involving scarp retreat and pedimentation has proved invaluable in the analysis of landscape. Admittedly the ‘pediment’ aspect is flawed, and is best omitted from consideration in this context. Pediments are not limited to piedmont zones and moreover vary in structure and origin. Some are enigmatic. For instance, the forms known as rock pediments may be of etch type (Mabbutt, 1966), particularly as the evidence of sheet floods cited by W.J. McGee (1897) evidently is suspect (Kirk Bryan, in letter dated March 4, 1924; see Brock, 1977, II, p. 11). Neither are pediments necessarily associated with receding scarps.

52Scarp recession, however, is commonplace. The evolution of plateau landscapes clearly demonstrates scarp retreat, for given consistent structure, but regardless of stage of dissection, the slopes bounding the residuals are of similar inclination and morphology, though the penultimate stages involve the lowering of divides.

53In the Tent Hills region, west and northwest of Port Augusta, for example, widely scattered plateaus and mesas capped by thin quartzites and remnants of an (?) Early Cretaceous/Jurassic surface are prominent. The faceted slopes display closely similar inclinations. Scarp-foot depressions with silcreted false cuestas are common. Corraberra Hill, however, is a butte with a faceted eastern slope preserved on part of the perimeter below a caprock, but the western slope, located below a disintegrated, fragmented, and discontinuous quartzite capping, is graded. Sugarloaf Hill is a smooth domical hill with no coherent capping and with sigmoidal slopes on all sides. It is the penultimate stage of development, prior to the erosion of the shale slope hitherto protected by the quartzite cap, and the lowering of the hill. Similarly, the conical remnants resulting from scarp recession in the Cape Fold Belt of South Africa also represent late stages in the sequence.

54The scarp retreat concept also finds support not only in observations indicating that slopes are developed from below and are worn back, maintaining the steepest inclination commensurate with stability (e.g., Penck, 1923; Tricart, 1957: Twidale and Milnes, 1983), but also in the statistical work of T.J.D. Fair (1947, 1948). And though in his enthusiasm, its author has at times applied the model inappropriately (e.g., King, 1949, 1966, 1975) it provides a plausible explanation of, and indeed was suggested by, particular suites of landforms. Unequal activity resulting from the downwards percolation and flow of water is a critical factor in slope development.

55The contrast between moist basal and dry upper slopes is most pronounced in arid and semiarid lands. But it develops everywhere the landscape is shaped by running water. A soil and vegetation cover reduces the contrast between upper and lower slope sectors in humid lands but the tendency to scarp recession remains. Only in very weak rocks such as argillite or a friable regolith, and particularly in humid lands where these geological conditions obtain, where baselevel is essentially stable and where the land surface carries a cover of soil and vegetation, may upper slope lowering outrun recession.

56Unequal activity finds expression also in several landforms and assemblages that can be attributed to the contrasted effectiveness of the etching process. Subsurface weathering may affect only part of an outcrop, or affect it unequally. For instance, the conversion of domical bornhardts to castellated koppies can be explained as an expression of unequal activity. Many bornhardts originated as two-stage forms that were initiated at the weathering front as a result of differential weathering of the country rock and later exposed. If during exposure, and as with J.R. Logan’s corestones, the crest of a domical rock mass - an incipient bornhardt - was exposed and a period of standstill then ensued, the crest would remain stable and largely unchanged while subsurface weathering continued all around. Particularly if major steeply-dipping joints were present to be exploited, the covered flanks would be weathered and steepened, later to be revealed delimiting a steep-sided but laterally reduced castellated form, castle koppie, or in cold lands, tor (Twidale, 1981).

57At a smaller scale, rock doughnuts and rock levees can be explained in terms of the contrasted weathering of covered and exposed surfaces. Rock levees (Scott, 1967; Lister, 1973) are susceptible of similar explanation with the river channel forming a drain that leaves immediately adjacent sectors of the regolith less moist and less weathered, causing the development of bedrock rims on each side of the linear depression (Twidale, 1993). On the coast, the water held in beach sand, particularly shell grit, around rock basins drains into the depressions. The beach in the immediate vicinity of a basin is relatively dry so that the rate of weathering of the bedrock with which it is in contact is low compared to the interface between rock and beach sand located some little distance from the basin. Thus an annular bedrock rim or doughnut (Blank, 1951) develops around the basin. It stands higher than the beach sand and is dry at times whereas the covered rock rock surface remains in contact with moist sand, and to be weathered and thus susceptible to lowering. The doughnut persists and the lowering of the adjacent platform results in an increased relief amplitude and eventually in the development of a font or “bénitier (Coudé-Gaussen, 1981), which arbitrarily may be defined as a doughnut the height of which exceeds its diameter (Twidale and Campbell, 1998).

Episodic exposure and increased relief amplitude

58The occurrence at various levels on the flanks of many of the residuals of zones of flared slopes and the alcoves that are lateral extensions of such concavities, as well as breaks of slope, or indentations caused by intense weathering, is suggestive. Flared slopes demonstrably are piedmont forms shaped in the shallow subsurface. The upper shoulder of the concavity marks the former hill-plain junction. The presence of flared slopes on the flanks of inselbergs shows that the residuals have been exposed in stages. There being no evidence of uplift, the adjacent plains must have been recurrently lowered. Such episodic exposure (King, 1966; Twidale and Bourne, 1975) has produced stepped forms and landscapes (Jutson, 1914, 1934; Jessen, 1936; King, 1949; Twidale, 1982; Bourne and Twidale, 2000). They imply, first, that local relief amplitude has increased through time and second, that the bornhardts have persisted through several phases of landscape revival. This sequence has developed because of unequal erosion initiated and dictated by the structural contrast that caused the bornhardts to develop in the first place. Once upstanding, reinforcement mechanisms intervened and the height of the hill relative to the plain increased.

59Thus, concatenation is responsible for several aspects of landscape development, but unequal activity and its consequences are particularly germane to the question of very ancient landscapes and their persistence.

Time - survival and antiquity

Convention and heresy

60J. Hutton (1788) postulated constant landscape change, and W.M. Davis (1899) argued that erosion extended over all the landscape, even to the divides between stream catchments. Such were their reputations and influence that with few exceptions models of landscape evolution implied youthfulness (King, 1942; Hack, 1960; Hack and Goodlett, 1960). W.Q. Kennedy (1962)’s model involving the interplay of what he termed uplift, (stream) erosion and denudation (or slope wasting), and which led to the conservation of perched landscape elements in some instances, was an exception.

61Overwhelmingly, however, the conventional wisdom was and remains that with the exception of those features that have been recently exhumed, most landforms date from the later Cenozoic (Ashley, 1931; Wooldridge, 1951; Thornbury, 1954; Schumm, 1963). Landscapes were regarded as of Quaternary or latest Tertiary age. Certainly none predated the Eocene (Brown, 1980). Such conclusions would appear to be well founded, for how can any land surface survive exposure to the elements for more than a few millions of years at most? Yet for almost a century there had been those whose observations and deductions indicated otherwise (for review, Twidale, 2007).

62Direct dating of erosional land surfaces of great antiquity is not possible at present, but ages - really age-ranges - can be closely ascertained using stratigraphic and topographic data. The topographic relationship of surfaces to each other is informative for unless disturbed higher surfaces are older than the lower (Rütimeyer, 1769; Baulig, 1928), a law of topographic sequence which is the converse of the Law of Superposition. The relationship of surfaces to genetically related and dated deposits, or to dated volcanic lavas, or by correlation with stratigraphically dated duricrusts, also can be revealing.

Evidence of landscape antiquity

63In his seminal analyses and syntheses of Western Australian landscapes, J.T. Jutson (1914, 1934) recognised that most of the high plains (his New Plateau) of that State were of etch type. They were derived from the stripping of the lateritic regolith that was Jutson’s Old Plateau and remnants of which are preserved throughout the southwest, and especially in sector preserved in the Darling Range or Plateau. This feature was stated to be of great age, but the author was no more specific in print (e.g., Jutson, 1934, p. 201). In his correspondence with Professor E. de C. Clarke in 1930 and 1931, however, Jutson was made aware of, and showed great interest in, Early Cretaceous/Jurassic strata preserved in valleys scored in the Darling Scarp, and their implications for the age of the laterite preserved on the higher Darling Plateau. This interest he shared with D.W. Johnson, who visited Western Australia in the early 1930s (see Brock, 1977, II, pp. 28, 34, 117, and 120; also Playford et al., 1976). The later discovery of occurrences of Eocene marine strata in palaeochannels incised in the New Plateau also pointed to a Mesozoic age for the lateritised surface of the Old (Van de Graaff et al., 1977; Clarke, 1994; Twidale and Bourne, 1998).

64In South Australia, P.S. Hossfeld (1926, p. 13) intuitively suggested that as the horst block of the Mt Lofty Ranges was flanked on both east and west by Tertiary strata, and as to the west they include marine beds of Eocene age, both the drainage and the lateritic summit surface of the horst must predate the Tertiary. This found later support in stratigraphic studies (e.g., Miles, 1952; Campana, 1958). Lateritic debris occurs in the basal Middle Eocene marine sequence (Glaessner and Wade 1958). Further, the laterite appears to predate the Wisanger Basalt, which is of Middle Jurassic age (Daily et al., 1974; McDougall and Wellman, 1976). Basalt of the same suite was extruded lower in the local topography, and is therefore younger than, the adjacent lateritic plateau near the north coast of the Island east of Penneshaw (Tilley, 1921).

65O.T. Jones (1931) speculated that the summit surface of central Wales may be of Triassic age, but the first claim for landscape antiquity based in hard stratigraphic evidence is due to E.S. Hills (1934, 1938) who pointed out that the prominent summit bevels widely preserved in the uplands of eastern Victoria must be older than the volcanic rocks, some of Eocene age, that occupy the valleys incised below the high plain remnants. Hills’ deduction concerning the age of the high plains of eastern Victoria, caused the similarly situated palaeosurfaces previously identified in southeastern New South Wales and putatively dated as Pliocene (Craft, 1932), to be re-considered and correlated with their Victorian counterparts.

66E.S. Hills was quite clear as to what was meant by an ancient land surface. It was not one untouched by the elements but one that had nevertheless retained its essential morphology, both in itself and with respect to the local terrain. Regarding the high plains of eastern Victoria, he wrote: “While these surfaces of low relief clearly represent preserved relics of old surfaces which have escaped deep dissection, they have naturally suffered some reduction and modification in detail during the long periods of time to which they have been exposed to weathering and erosion, but this is relatively minor…” (Hills, 1975, p. 300).

67This is fair comment, but “minor” may be misleading in some instances. It is also necessary to recall that etch forms have two ages, one referring to the time of initiation, the other to the phase of exposure.

68With regard to the degree of modification suffered by specific forms, the bevelled bornhardt landscape of the Gawler Ranges is an etch complex that was developed by differential subsurface weathering during the earlier Mesozoic (Triassic-Jurassic) and exposed in the Early Cretaceous. There has been some localised and shallow dissection of silicified marginal piedmonts and some anthropogenically induced gullying within the Ranges, but the topographic framework remains as it has been since the Early Cretaceous (Campbell and Twidale, 1991).

69Similarly the ridge-and-valley topography of the Flinders Ranges, is a fold mountain belt developed in mainly Proterozoic and Cambrian strata. It was shaped by differential weathering and erosion of strata beneath an Early Cretaceous surface which is preserved partly on the crestal bevels of quartzite ridges but also in argillite exposed in the deeply eroded but buttressed core of the regional anticline that forms the central part of the upland (see also Twidale and Bourne 1996). That the ridge-and-valley topography of the southern part of the upland was in existence by the Middle Eocene is demonstrated by lacustrine sediments of that age that were deposited in the lake impounded in what is now the northern Willochra Plain, and remnants of which tongue up pre-existing valleys adjacent to the Plain. The pre-Eocene topography is preserved, but valleys floors have been lowered by some 5–10 m so that in detail relief amplitude has been increased through Cenozoic time.

70By contrast, the arkosic residual that is Uluru (Ayers Rock) stood as a low hill above the plains of central Australia in the Late Cretaceous. As a topographic high it shed rainwater and thus became a comparatively dry and stable site, though a regolith was stripped to expose a dimpled and grooved erstwhile weathering front. The strata beneath the adjacent plains however remained in contact with a moist regolith and the inclined bedrock surface at the base of the front was sculpted by hydro-weathering processes and worn down both vertically and back or laterally.

71The crest of Uluru is about 70 million years old, and is of etch origin. The steep flanks with their varied decorations (fretting, tafoni, breaks of slope, flared slopes), were shaped beneath the then land surface during the latest Cretaceous and earliest Tertiary culminating in the deep weathering and silicification of the plains in Eocene times (Mabbutt, 1965; Wopfner et al., 1974; Wopfner, 1997). The plains have been extensively dissected causing and the 200-250 m high steepened flanks of the inselberg and in particular the row of gaping mouth caves, flares, and breaks of slope shaped by intense weathering at the weathering front, to be exposed on the southern face. By contrast with the other piedmonts, which are shaped in arkose, the southern is underlain by appreciable thicknesses of permeable mixed sediments and alluvial and aeolian detritus. It has also received a disproportionately high runoff from the inselberg. Thus, this piedmont is and has been relatively moist: hence the distinctive gaping-mouth caves and flared zones standing 30-65 m and 4-5 m above the present plain and indicating past water table zones (Twidale, 2010). Another phase of subsurface weathering is in progress on this southern aspect.

72The prominent summit high plain of the Hamersley Ranges is an etch surface of Eocene age, prepared by weathering in the later Cretaceous and thus of an age similar to the summit bevel of Uluru (Twidale et al., 1985; Hocking et al., 1987). Landscapes predating an Early Cretaceous marine transgression are preserved at several sites around the margins of the western shields and of the Great Australian Basin. Many are exhumed but others are of epigene/etch type, as for instance in the Arcoona Plateau and Tent Hills regions, Kakadu, and the MacDonnell, Musgrave and Everard ranges (Twidale, 2007).

73In areas of undisturbed strata high plains conceivably may have formed in relation to structural baselevels provided by resistant formations rather that having been degraded and then dissected. But the lateral extent of such resistant strata is limited, and the upland surfaces are not coincident with bedding. Also, high plains are developed in rocks such as granite, gneiss, and dacite, that lack structural baselevels, or over areas of mixed lithology, as in the Drakensberg Surface of southern Africa and the summit bevel of the Mt Lofty Ranges, and even in areas like the Arcoona Plateau the summit bevels are cut across various very gently dipping quartzites.

74Thus, studies based in stratigraphic and topographic relationships have suggested that substantial elements of the Australian landscape are some 120-130 my old, i.e. of Early Cretaceous age. There is some suggestion and implication - no more - of surfaces dating from the Triassic (e.g., Jenkin, 1988; see also Campbell and Twidale, 1991; Twidale, 2000, 2010).

75Working in southern Africa, F. Dixey (1938) and L.C. King (1950, 1962) concluded that the high plains and plateaus of southern Africa are of considerable antiquity. Later stratigraphic work confirmed the Mesozoic age of the higher and older surfaces, some which are of etch character (Partridge and Maud, 1987; Twidale, 1988; Birkenhauer, 1991; Partridge, 1998). P. Michel (1978) has identified a Jurassic surface in West Africa. The application of stratigraphic and topographic criteria has allowed the identification of very old land surfaces also in the Americas, and in Europe (e.g., Curtis et al., 1958; Poag and Sevon, 1989; Briceno and Schubert, 1990; Battiau-Queney, 1997; Demoulin et al., 2005).

Problems posed by survival of old surfaces

76Given the evidence that some landscape elements have persisted through all Cenozoic time, and in some instances much more, it is reasonable to speculate as to how any land surface can have survived constant exposure to the elements. It is, after all, contrary to common sense that any surface should withstand weathering and erosion for such extended periods as have been suggested. Consideration of the characteristics and distribution of known old landscapes suggests several factors that are conducive to their conservation (Twidale, 1976; Young, 1983). They are preserved on resistant bedrocks that are either compact like the Gawler Range dacite or are permeable and/or porous and do not retain water, such as quartzite or sandstone. Some surfaces are protected by a duricrust such a laterite.

77Many ancient assemblages are preserved in structurally favoured sites such as the deep cores of regional anticlines, or of antiforms in granite. Structural advantage introduces concatenation, i.e. unequal activity and reinforcement. Episodic exposure is clearly conducive to the conservation of uplands, whether local or regional, for they become relatively dry and therefore stable sites. Many old palaeosurfaces are preserved in areas of relative tectonic stability and in particular areas that have not experienced post Palaeozoic orogeny. On the other hand, it can be argued that gentle epeirogenic or isostatic uplift is conducive to survival, for forms and surfaces have thereby been raised beyond the reach of serious water attack. This suggestion is applicable to the Yilgarn Craton of Western Australia.

78Most old remnants stand high in the local relief and are not only relatively dry sites but also sites many of which do not favour luxuriant vegetation growth. It is ironic that when R.J. Russell (1958, p. 5) pointed out that the Earth has never before been so well “armored” or protected by soil and vegetation against processes of weathering and erosion and by implication protected, he was right, as witness the devastating accelerated soil erosion resulting from the destruction of that protective ‘skin of the Earth’. Yet he was referring to the cover that accounts for much of the weathering and eventual erosion of the land.

79This is significant for most rocks are stable when dry but rot in contact with water charged with chemicals and biota. J.R. Logan (1851) realised this as did W.F. Willmott and D.L. Trezise in their brief account of the geology and landscapes of the Chillagoe district of north Queensland. Referring to the Metal Hills nubbins they state: “the hills may be quite old. Once the boulders are isolated from the soil and groundwater, they will be subject to little further erosive attack apart from rainfall” (Willmott and Trezise, 1989, p. 17). Moreover, any landscape features that already stood high in the local relief at the beginning of the Cenozoic were favoured because they rose above the effect of biota that proliferated following the end-Cretaceous extinction (e.g., Twidale, 2011b). On the contrary it was on the high ground that earlier species survived (e.g., Hopper et al., 1996; Twidale and Bourne, 2004).

80Old palaeosurfaces have been reported mainly from low and mid latitude lands not critically affected by nival periods of the Quaternary, though as previously cited, there are exceptions as in the Sierra Nevada of California, the Appalachians, and in the European Alps. The compact shapes and the areal extent of such continents as the Americas, Africa and Australia has limited the impact of stream rejuvenation and erosion consequent on sea level lowering of the Quaternary glacial phases. However, the Permian ice age, in addition to Cretaceous marine transgressions, dated duricrusts, and (hot spot) volcanicity has proved invaluable in the dating of old surfaces in Australia.

81Where scarp recession is active, remnants of the initial surface endure until late in the cycle and further retardation at the local and site scales occur as result of gully gravure (Bryan, 1940), inversion of valley floors such as those previously referenced, and slope protection caused by the diversion of wash and streams (Twidale and Bourne, 2000c). But these conservative factors are of minor importance. The survival of remnants of very old landscapes is a function of concatenation.

Opposed views

82Understandably the ancient landscapes concept has been received with disbelief by many workers. The idea is considered ‘unreasonable’ (e.g., Bourman, 1989, 1995), but no specific objection has been articulated. Evidently some members of the Australian geoscience community look askance at the idea but no informed discussion has been initiated and indeed has been suppressed. The professional reputations of those advocating landscape antiquity have been traduced. This is not now unusual: witness how those sceptical of anthropogenically-driven climatic change are labelled deniers and their arguments prohibited or ignored. On the other hand, it is interesting that evolutionary botanists find the ancient landscapes concept useful. Old landscape elements form refuges for ancient plant species, as for instance Jurassic eucalypts on the higher - and older - reaches of granitic bornhardts in the Yilgarn region of the southwest of Western Australia. The concept is indeed an integral component of the OCBIL (very Old, Climatically Buffered, Infertile Landscapes) theory (e.g., Hopper et al., 1996; Twidale and Bourne, 2004; Hopper, 2009). The situation is reminiscent of the 1930s when arguments of those - mainly geologists - who considered the location of the continents and ocean basins to be permanent and unchanging, confronted those - mainly biologists, though with support of such geologists as L.C. King, A.L. du Toit and S.W. Carey - who supported the idea of continental drift.

83Nevertheless, it may well be that some of the evidence and argument are in error, or are susceptible of alternative explanation, but it takes only one surface of demonstrated great antiquity to upset the conventions concerning the essential youthfulness of the Earth’s land surfaces.

Discussion and conclusions

84It has been said of some investigators that they had seen what others had seen but thought what no others had thought. In the instances discussed here, crucial observations were made, causes deduced, and consequences imagined and tested. New ideas have evolved, partly as consequences of observations in hitherto neglected parts of the world, partly as a result of technological advances, but partly flowing from ideas and conclusions published long ago.

85Assuming that the usefulness of one hypothesis is judged by problems it resolves or clarifies, the early workers who cited evidence concerning refinements of structural impacts, and pointing to two-stage development, unequal activity, and reinforcement, are surely deserving of credit, as are those later investigators who envisaged previously unimagined structural impacts and an outrageous antiquity of landform and landscape.

86Most ideas have been thought of before. The difficulty was and is to think of them again. Several ideas cited here as germane to general theory are of relatively ancient origin. Almost certainly, the workers concerned did not, either at the time or subsequently, realise the significance of what they had concluded, for in many instances their comments took the form of incidental asides or footnotes. But the seeds of ideas were planted to the eventual benefit of later generations.

87It is prudent to recollect that many landforms are convergent. One hypothesis or explanation of a particular form may be of wide application but there are commonly a few examples that, though morphologically similar, are of different origin. Though many bornhardts appear to be of two-stage origin, a few are upfaulted blocks. Likewise, most submarine canyons are best explained in terms of turbidity currents (e.g., Heezen, 1956) and the suggestion that they are drowned terrestrial gorges can be discounted as a general theory. But exceptions have been demonstrated (e.g., Hsü, 1972; Clauzon, 1978; Said, 1993; Clauzon et al., 1996). Few geomorphological hypotheses are absolutely applicable.

88As several workers have commented, in science the credit for an idea frequently goes not to those who either consciously or inadvertently broke new ground, to those to whom the idea first occurred, but to those who convinced the world (Osler, 1904; Darwin, 1914). Others have suggested that it is not important who did the work as long as it is done, and there is merit in this, for no matter how plausible and attractive, most ideas eventually will, in whole or in part, be rendered untenable by new evidence or argument, and will perforce be modified or abandoned. Yet the derivation of concepts that either at the time point directly to a solution or which form the basis of understanding surely ought to be noted. To do otherwise is to accept the constant, unacknowledged and unjust re-invention of many wheels.

The concepts discussed in this paper have arisen from field studies in many parts of the world, but particularly Australia. I have enjoyed the stimulating company of many colleagues, several of whom are cited in my reference list. I thank them all, but I am especially grateful to my two longest-serving and most frequent collaborators, the late Liz Campbell and the ever-present Jennie Bourne. My thanks are offered also to anonymous reviewers for several constructive and readily acceptable suggestions and to Yvonne Battiau-Queney for translation of various sections into French.

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Annexe

Version abrégée en français

La trilogie davisienne « structure, processus et durée », fournit toujours un cadre utile à l’analyse des formes et des paysages. Dans le détail, cependant, le schéma davisien a été radicalement révisé et virtuellement abandonné. A sa place plusieurs concepts largement appliqués ont pris le devant. Certains, signalés il y a plusieurs siècles, furent négligés avant d’être remis au jour quand des faits d’évidence vinrent les conforter. Cependant, des hypothèses sont nées des progrès de la cartographie topographique et géologique, bien que le développement des idées sur l’ancienneté de certains reliefs fut rendu possible ou conforté par des schémas anciens confirmés par les techniques d’acquisition numérique de données. Les changements majeurs comprennent la structure, les processus et le temps-durée.

Structure. En plus des effets depuis longtemps reconnus de la structure active et passive et de la « fabrique » des roches (Gefilgerelief), il y a eu des avancées significatives dans la prise en compte de la complexité de l’état des contraintes et des forces, avec l’identification de phénomènes corrélés avec les contraintes et la mise en perspective des effets de la pression lithostatique. A l’échelle du globe, les effets des linéaments et des structures annulaires sont de plus en plus reconnus (Hills, 1956 ; O’Driscoll, 1980). Aucune partie de la surface de la Terre n’est stable, bien que certaine régions soient plus instables que d’autres : les phénomènes néotectoniques sont largement répartis et existent même dans les régions cratoniques. De plus, des phénomènes auparavant interprétés différemment sont maintenant considérés comme étant d’origine tectonique (Dale, 1923 ; Twidale et al., 1996) et les implications de l’érosion profonde et du sous-jacent préétabli sont désormais reconnues (Hills, 1961 ; Saul, 1978). De plus en plus de formes de relief ressemblant à des phénomènes tectoniques sont signalées.

Processus. Quelque soit le climat, l’eau est le facteur le plus décisif pour l’évolution du relief. Les évolutions en deux étapes avec relief d’altération différentielle (etchforms) se trouvent partout (Hassenfratz, 1791 ; Falconer, 1911 ; Jutson, 1914). Beaucoup de formes de relief familières sont azonales au plan climatique et lithologique. En particulier, des plaines d’une étonnante régularité ont été façonnées par l’altération différentielle (etching), soit dans un substrat peu résistant ou à la suite d’une longue érosion sous-jacente à la surface. L’inégale activité (de l’érosion) et l’accroissement des irrégularités (s’ajoutant tous deux à l’influence structurale initiale, connue sous le nom de contraposition ou « concatenation » en anglais) sont des facteurs significatifs de l’évolution du relief (Logan, 1851 ; Bliss Knopf, 1924 ; Crickmay, 1976), de même que l’altération chimique partielle ou inégale et que l’affleurement épisodique, ce dernier engendrant un relief en marches d’escalier. Dans certaines régions des aires cratoniques, l’amplification de l’énergie de relief a prévalu sur le long terme, plutôt qu’un aplanissement généralisé, à la fois à l’échelle locale et régionale (Twidale et Bourne, 1975).

Temps-durée. Beaucoup de formes de relief non-exhumées sont très vieilles, initiées par exemple au Mésozoïque (Hills, 1934; King, 1950). Ce résultat va à l’encontre de l’enseignement de sommités telles qu’Hutton, Lyell ou Davis et aussi de l’opinion commune. Si bien que beaucoup de géomorphologues désapprouvent ces idées, mais les preuves topographiques et stratigraphiques sont convaincantes et difficiles à contredire. Les botanistes travaillant sur l’évolution trouvent le concept utile car ces anciens éléments de relief auraient constitué des refuges pour des espèces de plantes antiques. Les facteurs de survie comprennent la stabilité tectonique, des avantages de localisation et de structure, comme un substrat résistant, une localisation perchée dans le relief local et la concaténation.

La prise de conscience de certaines de ces conclusions est relativement nouvelle, mais il faut reconnaître que dans certains cas, les preuves et arguments critiques s’articulaient depuis longtemps dans un schéma cohérent.

“There are no new truths,
but only truths that have not been recognised
by those who have perceived them
without noticing.” (McCarthy, 1962, p. 155).

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Table des illustrations

Titre Fig. 1 – A-tent in granite developed on lower slope of Kokerbin Hill, Yilgarn Craton, southwestern Western Australia Fig. 1 – Une forme de tente triangulaire développée en bas de pente de la colline de kokerbin, Craton du Yilgarn, sud-ouest de l’Australie occidentale
Crédits Photo: C.R. Twidale. Photo : C.R. Twidale.
URL http://geomorphologie.revues.org/docannexe/image/9900/img-1.png
Fichier image/png, 1,0M
Titre Fig. 2. – Wave Rock, a flared slope some 14-15 m high in granite, northern slope of Hyden Rock, Yilgarn Craton, southwestern Western AustraliaFig. 2 – La Wave Rock (“Roche en Vague”), une pente évasée de 14-15 m de haut dans du granite, versant nord du Hyden Rock, Craton du Yilgarn, sud-ouest de l’Australie occidentale
Légende Black stripes due to blue-green algae colonisation of water seepages.Les bandes noires sont dues à des algues bleu-gris colonisant des suintements d’eau.
Crédits Photo: C.R. Twidale.Photo : C.R. Twidale.
URL http://geomorphologie.revues.org/docannexe/image/9900/img-2.png
Fichier image/png, 573k
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Pour citer cet article

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Charles Rowland Twidale, « Landscape analysis: derivation and rediscovery of ideas », Géomorphologie : relief, processus, environnement, vol. 18 - n° 3 | 2012, 259-277.

Référence électronique

Charles Rowland Twidale, « Landscape analysis: derivation and rediscovery of ideas », Géomorphologie : relief, processus, environnement [En ligne], vol. 18 - n° 3 | 2012, mis en ligne le 04 novembre 2014, consulté le 24 novembre 2017. URL : http://geomorphologie.revues.org/9900 ; DOI : 10.4000/geomorphologie.9900

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Auteur

Charles Rowland Twidale

School of Earth and Environmental Sciences, Geology and Geophysics - University of Adelaide - G.P.O. Box 498, Adelaide South Australia 5005 - Australia (rowl.twidale@adelaide.edu.au)

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