GeoL. Soc. MaLaY.Jia, BuLLetin 42, December 1998; pp. 237-255
Granitic bornhardts: their morphology, characteristics and origins
C.R. TwIDALE
Department of Geology and Geophysics University of Adelaide Adelaide South Australia 5005
Abstract: Bornhardts are bald domical hills either standing in isolation as inselbergs ("island mountains"), or forming components of massifs. Though especially well represented in granitic terrains, they are also developed in other lithologies. Their plan form is determined by systems of steeply inclined fractures, while their profiles are associated with convex upward fractures. They meet the adjacent plains or valleys in a piedmont angle or nick. They are climatically azonal. They occur in various topographic settings and consistently in multi cyclic landscapes. Epigene forms as old as Cretaceous are known, and exhumed forms ranging from Late Pleistocene to late Archaean in age have been recognised. Bornhardts have been explained in various ways: as literal and littoral inselbergs, as climatic (savannah) forms, as minor horsts, and so on. Some are upstanding because they are shaped in rock that is compositionally different from, and implicitly more resistant to, weathering and erosion than that which underlies the adjacent plains and valleys. Some are exposed stocks. Such explanations have local validity, but two hypotheses have been suggested as general explanations. First, bornhardt inselbergs can be construed as remnants of circumdenudation following scarp retreat but much field evidence is not accounted for and some is incompatible with several of the deducible consequences ofthe hypothesis. Second, bornhardts have been interpreted as two-stage forms, which originated at the weathering front as resistant masses due either to low fracture density or to rock type, and which became landforms as a result of the stripping of the regolith and exposure of weathering front topography. This two-stage model satisfies much of the field evidence and the isolated residuals of shield inselberg landscapes are the last remnants of resistant compartments which have survived long-continued subsurface weathering. According to this hypothesis, topography is consequent upon, and subsequent to, the formation of sheet fractures which are associated with compressional stress. Nubbins (block- and boulder-strewn domes) and castle koppies (small blocky hills) are derived from the weathering of bornhardts.
DEFINITION AND HISTORICAL REVIEW international technical literature to denote an isolated, steep-sided hill. Similarly, and again Of the many new landscapes and landforms logically, the basic inselberg form, the domical hill, discovered by Nineteenth Century European or bornhardt, was named after Wilhelm Bornhardt explorers, few created such astonishment as the who bequeathed to us splendid descriptions and isolated hills which, seemingly appearing out of penetrating analyses of these forms as they occur nowhere, interrupted the otherwise featureless in what is now Tanzania (Bornhardt, 1900; see plains that occupy so much of Africa, Peninsular Willis, 1934). India, Australia and South America. Even to as erudite and modern a man as Laurens VanDer CHARACTERISTICS Post (1958, pp. 181-182) they communicated a sense of mystery. Bornhardts, and especially bornhardt inselbergs These isolated residuals were referred to in (Fig. 1a), are spectacular forms, and inselberg similar terms by both British and German explorers, landscapes (Fig. 1b) are at once intriguing and but the English "island mount" (Eyre, 1845, I, p. enigmatic: why are these hills there, surrounded 203 ; Giles, 1889, I, p. 158) never took hold, and, by seemingly limitless featureless plains? Not doubtless because of the major contributions due to surprisingly, they have stimulated considerable the galaxy of German explorer-scientists who were interest over the years. The most dramatic, the so active, mainly in Africa, late last century and early called classical, inselberg landscapes are developed, this (e.g. Passarge, 1895, 1904a and b; Bornhardt, preserved and displayed in arid and semiarid 1900; Kaiser, 1926; Jessen, 1936), its German regions of shield lands, in the ancient granitic cores equivalent, inselberg, became established in the of continents like Africa and Australia, and the
Paper from a GSM Technical Talk preJeIlteo on 25 November 1998 238 C.R. TWIDALE interior of Brazil (e.g. Bornhardt, 1900; Passarge, in the cool uplands ofthe Californian Sierra Nevada, 1904a and b, 1928/29; Jessen, 1936; Journaux, as in the Yosemite Valley and Domeland (Matthes, 1978). But they are also represented in cold shield 1930; Schrepfer, 1933; Huber, 1987), in areas such as Greenland and Finland (Oen, 1965; Mediterranean areas such as the Cape Fold Belt Fogelberg, 1985). Inselberg landscapes are well (Fig. 2a) and in the humid tropics, for instance developed in cold climates in orogenic regions, for southeastern Brazil (Fig. 2b) and southern Nigeria instance in western Iberia and western Scandinavia, (see e.g. Darwin, 1846; Falconer, 1911; Birot, 1958;
Figure la. Domical hills or bornhardts, eroded in granite gneiss, central Namibia.
Figure lb. Inselberg landscape in granitic rocks, Namaqualand, northwestern Cape Province, South Africa.
Ceo!. Soc. MaLaYdia, BuLLetin 42 GRANITIC BORNHARDTS: THEIR MORPHOLOGY, CHARACTERISTICS AND ORIGINS 239
Figure 2a. Paarlberg (Pearl Mountain) a complex granite dome, grey in colour and looking not unlike gigantic pearls set in the landscape, northeast of Cape Town, South Africa.
Figure 2b. The Pao deA~ucar (Sugarloaf), a tall, steep-sided bornhardt Decemher 1998 near Rio de Janeiro, southeastern Brazil. The precipitouis flanks are due partly to the exploitation offaults (Brazilian Tourist Bureau). 240 C.R. TWIDALE Kieslinger, 1960). In the latter areas, the form of have been closely investigated. But domical hills the land is less readily discernible than in desert or are by no means restricted to such lithological savanna regions, but bornhardts are present; for settings, for they are well developed also in instance in the rain forests of southern Nigeria, arenaceous, rudaceous and carbonate sediments. Allison (in Allen, 1980, p. 97) noted " ... rocky hills as well as ancient volcanic rocks (Mainguet, 1972; sticking up out of the forests, inselbergs, with Sweeting, 1973; Twidale and Bourne, 1978; rounded bare rocks sticking above the two hundred Jennings, 1985; Campbell and Twidale, 1991). foot tops of the trees". Epigene forms at least as old as Cretaceous have Bornhardts are domical hills in which bare rock been identified (e.g. Twidale and Bourne, 1975a, is exposed over most of the surface (Figs. 1 and 2). 1998) and exhumed bornhardts range in age from The slopes of gneissic forms are ribbed (Fig. 2c). latest Archaean to late Pleistocene (Twidale, 1982a, They are developed in massive rocks and though 1986; Twidale and Campbell, 1985), showing that orthogonal and sheet fractures are well developed, the forms have developed throughout geological open partings are few. In plan their shapes are time. determined by systems of steeply inclined orthogonal or rhomboidal joints and their convex POSSIBLE ORIGINS upward profiles are associated with sets of convex upward sheet fractures. The hills are steep-sided and meet the adjacent plains in an abrupt break of (a) General comments slope or piedmont angle which is partly due to Any general theory needs to account for the steeply dipping fractures but is enhanced by scarp characteristics of granitic bornhardts outline foot weathering and erosion (e.g. Peel, 1941; above, and to apply, in some measure at least to Twidale, 1967, 1978a, 1987a). forms developed in other rock types. These domical hills are found in massifs (Fig. Granitic bornhardts have been interpreted i 3), or as components of ranges, as well as in isolation. several ways (for reviews see Twidale, 1982a; Vidal They occur in multicyclic landscapes (Obst, 1923; Romani and Twidale, 1998) Explanations can Jessen, 1936; King, 1949) and many inselbergs and broadly be categorised as structural (sensu lata ) massifs are bevelled (Fig. 4). Numerous bornhardts including, upfaulting, compositional variation. and inselbergs have been shaped in granitic (and magmatic differentiation, and exploitation 0 other plutonic, crystalline) rocks, and such residuals variations in fracture density (either surficial or
Figure 2c. Bornhardts developed in granite gneiss, northern N amaqualand. The foliation is expressed in faint ridges or ribs, here trending top left to bottom right. Ceo!. Soc. MalaYJia, BuLLetin 4] GRANITIC BORNHARDTS: THEIR MORPHOLOGY, CHARACTERISTICS AND ORIGINS 241
Figure 3a. Part ofthe same area showing adjoining granitic domes, each developed on a fracture-defined block and with sheet structure well developed.
-----
f~t~}}f}f granite Okm 1 ---:::::=:. gneiss I
Figure 3b. Geological sketch of part of the Kamiesberge, Namaqualand, northwest Cape Province, showing fault pattern in relation to bornhardt massifs (drawn from air photographs). December 1998 242 C.R. TWIDALE subsurface); or environmental, including marine of French Guyana (Choubert, 1949), but most setting, arid or semiarid environment; or process bornhardts are evidently not defined by recently dominated, in particular, circumdenudation active faults. Some few bornhardt are exposed following long-distance scarp retreat, which for some stocks (e.g. Holmes and Wray, 1912; Twidale, 1982a, workers at least is associated with savanna or arid pp. 128-131; Fig. 5) and have been left upstanding climatic regions. Many of these suggestions are by reason of lithology, but most bornhardts are locally valid. For instance, Passarge (1895, p. 377) apparently of the same composition as the rock suggested that some inselbergs are horsts, and the that underlies the adjacent valleys and plains. An Pic Parana, in southeastern Brazil is demonstrably so on: some explanations have local or partial ofthis character (Barbier, 1957) as are some pitons application. But only two, those involving scarp
Figure 4. Part ofthe Everard Ranges, in the northern arid interior of South Australia, showing bevelled granite domes.
Figure 5. This rounded stock of Proterozoic Donkahoek Granite, exposed by erosion of schist into which it was injected, forms a small bornhardt in central Namibia.
Ceo!. S oc. MaLaYJia, BuLLetin 4:! GRANITIC BORNHARDTS: THEIR MORPHOLOGY, CHARACTERISTICS AND ORIGINS 243 recession (Holmes, 1918; King, 1949) and subsurface Vidal Romani and Twidale, 1998, p. 198; Fig. exploitation of fracture density (Falconer, 1911), 7a). have been considered as general theories. 2. If bornhardts are the last remnants surviving after long-distance scarp retreat they ought to (b) Scarp retreat be no older than the duration of a cycle. In The suggestion that bornhardts are the last areas of continental extent which include residuals surviving after long-distance scarp classical shield terrains where bornhardts are recession (King, 1949; Fig. 6a) is widely accepted well represented, the duration of a cycle has (e.g. Ollier and Tuddenham, 1962; Selby, 1970). It been estimated as about 33 Ma (Schumm, 1963) is plausible, for scarp retreat has undoubtedly so that no bornhardt ought to be older than occurred in caprock situations and such conditions Oligocene. But some (epigene) residuals are arguably obtain in granitic terrains, and especially demonstrably of Cretaceous age, and are 3-i in aridity and semiaridity: granite is not inherently times older than they ought to be in terms of capped but the concentration of groundwaters at theory. shallow depth beneath the surface implies greater 3. Several field characteristics are not accounted weathering and hence susceptibility to erosion there for by the hypothesis. For example incipient than at the surface (Twidale, 1982a, p. 138). Yet bornhardts occur at shallow depth in the natural all that is plausible is not necessarily true. Several subsurface but have been exposed in artificial lines of evidence argue against scarp retreat as a excavations (Figs. 7b and c). The relationship significant mechanism in the development of between bornhardts and multicyclic landscapes bornhardts. and their occurrence in a variety of topographic 1. Ifbornhardts are associated with scarp retreat settings is not explained. The occurrence of they ought to be found mainly on major divides stepped inselbergs, especially those with flared in maturely dissected landscapes. Though some sidewalls (Jessen, 1936; Twidale, 1982b) residuals are so situated, others occur in the suggest, first, that the wearing back of scarps flanks of narrow valleys and others protrude is limited to a few tens of metres and not the from valley floors (see Thomas, 1966; Jeje, 1973; hundreds of kilometres demanded by inselberg
Figure 6a. Bornhardt inselbergs as remnants of denudation following long-distance scarp retreat.
I I I I \ I I
(ii)
Figure 6b. Bornhardts as two-stage or etch forms. Decemher 1998 Figure 7a. Small granite dome exposed in the valley floor of a river near Malmesbury, north of Cape Town, South Mrica.
Figure 7b. Granite dome exposed by quarrying, Ebaka, Cameroun (M. Boye).
Figure 7c. Crest of granite dome (X) exposed near the base ofthe bevelled Leeukop, near Potchefstroom, South Africa. GRANITIC BORNHARDTS: THEIR MORPHOLOGY, CHARACTERISTICS AND ORIGINS 245 landscapes and by King's hypothesis, and, been eroded, the evidence is no longer available. second, that the recession is due to subsurface Bles (1986), however, has shown that surface weathering, not fluvial erosion. Also, they fracture patterns and densities provide a close and strongly suggest that inselbergs persist through reliable indication of fracture patterns and density several cycles of weathering and erosion: they at depth, and, reversing the argument, surface persist because even long-continued erosion does conditions can surely be taken as indicative of not totally reduce land surfaces to baselevel; higher, though now eroded, patterns (Twidale, the slate is not necessarily wiped clean. 1987b)? Field evidence demonstrates abrupt spatial (c) Two-stage development: bornhardts as variations in fracture density, and they are crucial etch forms to the two-stage concept. But why are there Mennell (1904) attributed the blocks and variations in fracture density? Three mechanisms boulders of the Matopos, in what is now Zimbabwe, have been suggested. First, Lamego (1938) to fracture-controlled weathering, implicity at the suggested that the gneissic terranes of the Rio de surface. This undoubtedly occurs but is less common Janeiro area had been folded, and that the crests of and less effective than the exploitation of fractures antiforms, being in tension, had been preferentially by subsurface moisture. Such fracture-controlled weathered and eroded. The deep compressional differential subsurface weathering and subsequent cores of the same structures, however, were differential erosion of the weathered granite or grus resistant and formed the bases of the well known has long and widely been recognised as responsible morros (Fig. 8a). for the boulders and core-boulders that are so Second, the effects of horizontal shearing on characteristic of granitic terrains the world over large orthogonal compartments or blocks has been (e.g. Hassenfratz 1791; Scrivenor, 1931; also cited (Twidale, 1980). Shearing along steeply Twidale, 1978b, 1982a). inclined fractures would induce compression within Most bornhardts are arguably also two-stage the compartments so defined along one horizontal forms (Fig. 6b). They were initiated by differential axis and tension on that disposed at right angles to subsurface weathering, at which stage they were it. Continued or recurrent dislocation would projections of bedrock at the weathering front, or perpetuate the resultant strain patterns and also lower limit of effective weathering (Mabbutt, 1961), cause fracture propagation from the margins of the into the base of the regolith. They were later blocks inwards (Fig. 8b). In thisway compressed exposed as bornhardts through the stripping of the cores would be developed in aligned patterns within regolith. That the bornhardt masses were fracture defined blocks such as are found in several manifestly less susceptible to subsurface moisture areas (see e.g. Twidale, 1982a, p. 140). Such a related weathering can be attributed to two possible mechanism also seems appropriate to parts of the factors. First, they may be developed in a rock Kamiesberge ofNamaqualand, in the northwestern which is compositionally or texturally different from Cape Province, South Africa, where, near Witwater that in which the surrounding plains and valleys for example, the bornhardt massifs are separated are eroded. This is true of some residuals but most by fault-line valleys in which steeply inclined fault bornhardts appear to be of the same rock type as planes are evident (Fig. 3). that of adjacent compartments. Second, they may (d) Tectonic: thrusting and compression be massive in contrast with the high fracture density of surrounding compartments. Such situations can A third possible reason for fracture variation is be demonstrated at several sites (e.g. Jones, 1859; suggested by the thrusting or low angle reverse Twidale, 1964; 1982a; Budel, 1977, pp. 108-109) faulting evidenced in some Iberian granitic masses, and is, increasingly, accepted as the most common has been noted (Vidal Romani and Twidale, 1998, reason for bornhardt development in granitic p. 66; Twidale and Vidal Romani, 1999), for example terrains. in the Mariz Quarry near Guitiriz, and near Lugo, But the question arises whether the present both in Galicia. Such dislocation causes rupture or distribution of fracture density constitutes a valid imbrication and the formation of sheared margins basis for comparison. Fracture density in residuals around large, rounded, commonly ovoid, masses, can be shown to be less than in adjacent some of boulder size but others large enough to be compartments, but it can be argued that what is counted as bornhardts when exposed (Fig. 8c). They significant is not the density beneath the present may be detached from the underlying host mass as plains but the density in the previously existing a result of weathering of sheared zones and may compartments above the present plains and adjacent thus be regarded as gigantic floaters (cf. Brajnikof, to the residuals; for it was these which were 1953). This mechanism, involving dislocation along differentially weathered and eroded. But, having flat-lying, rather than steeply dipping, partings, December 1998 246 C.R. TWIDALE
Figure 8a. Possible reasons for variations in fracture density according to Lamego.
(j) T~__ /c 4
. ' "-1,\. 0 ~ t:;XI t:n ./ --~ C T
(ij)
Figure 8b. Possible reasons for variations in fracture density according to Twidale.
surface
Figure 8e. Large boulders and bornhardts formed by shearing along low angle plane of dislocation (after Twidale and Vidal Romani, 1999).
GeoL. Soc. MaLaYJia, BuLLetin 42 GRANITIC BORNHARDTS: THEIR MORPHOLOGY, CHARACTERISTICS AND ORIGINS 247 provides another explanation for variation in bornhardt terrains, and including West fracture density. Malaysia (Tjia, 1978) are in substantial In his magisterial monograph on the granites compression (e.g. Denham et al., 1979). of the northeastern United States, Dale (1923) noted 3. Association of sheeting with minor forms the possibility that granitic domes could be associated with compression e.g. A-tents or pop developed as a result of a granitic mass being ups (e.g. Twidale and Sved, 1978; Wallach et laterally compressed from two contrasted directions. al., 1993), reverse faults, wedges, etc. (Fig. 9c). This is analogous to the dome and basin (or "egg 4. Indications that sheet fractures predate the box") structures envisaged as evolving in land surface with which they are related sedimentary terrains as a result offold interference (Twidale, 1971; Gerrard, 1974). (Ramsay and Huber, 1987, pp. 494-495). This Also, it is pertinent to ask why expansion due suggestion is of particular interest in areas like to oftloading has not been accommodated along West Malaysia which is in substantial compression pre-existing fractures, and how oftloading fractures from two main directions, namely east-west and can develop in rock sequences that have never been NNW-SSE (Gobbett and 'fjia, 1973; 'fjia, 1978). deeply buried? The resultant landscape would be tectonic. If In the laboratory, compression of partly subjected to subsurface weathering and subsequent constrained masses produced upwardly convex erosion the distribution of strain implies that fractures (Holzhausen, 1989) and this experimental domical forms could develop only as a result of situation may be compared to bedrock protrusions relief inversion. projecting into the base of the regolith being subjected to compressive stress: the crucial evidence (e) Rounding of masses and association of the subsurface origin of many bornhardts is with sheet fractures exposed in excavations where domical rock masses, The convex-upward profiles of bornhardts are nascent or incipient bornhardts, are seen to be associated with sheet fractures and structures (Fig. already formed beneath the surface, at the base of, 9a), but the nature of that association is debatable. and projecting into, the regolith (e.g. Falconer, 1911; In terms of scarp retreat, bornhardts are rounded Boye and Fritsch, 1973; Twidale, 1982a; Vidal because of the preferential weathering of corners Romani and Twidale, 1998). Several minor and edges and sheet fractures developed as a landforms characteristic of granitic terrains also consequence of the rounded shape: they are demonstrably are initiated below the land surface, oftloading or pressure release joints in the sense of at the base of the regolith (e.g. Logan, 1849, 1851; Gilbert (1904). So widely is this interpretation Twidale, 1962; Twidale and Bourne, 1975b, 1976). accepted that the fractures are commonly referred (f) Discussion to as oftloading or pressure release joints; and this despite the mechanism having been rejected by The two-stage hypothesis is consistent with the engineers (Wolters, 1969; Brunner and Scheidegger, azonal distribution ofbornhardts, for groundwaters 1973). are ubiquitous and only the rate of development In terms of the two-stage hypothesis and may vary with climate; with the occurrence of variations in fracture density related to crustal bornhardts in various topographic settings, for stress, however, sheet fractures are due to resistant masses can emerge at the surface in compression and the shape of the residuals is uplands as well as plains. It is compatible with consequent on structure. This point of view found their presence in multicyclic landscapes, for the expression about a hundred years ago (e.g. Niles, latter imply deep erosion and the exposure of deep 1872; Merrill, 1897; Hankar-Urban, 1906), and compressional zones of antiforms, and also time for Gilbert also acknowledged that compression could deep (differential) erosion). Their development in account for sheeting in and around Stone Mountain, sedimentary terrains is accommodated in some Georgia, USA (in Dale, 1923, p. 29). Evidence degree (e.g. Twidale and Bourne, 1978), though not pointing to compression being responsible for at entirely, for other mechanisms may apply in some least some sheet fractures includes (Twidale, 1964; instances. For example, the Sugarloaf, northwest Vidal Romani et al., 1995; Twidale et al., 1996): of Port Augusta, South Australia, is a remnant of 1. Inverse relationship between topography and circumdenudation following scarp recession (see structure, with rounded hills underlain by Twidale et al., 1970, Twidale, 1978a). The classical synformal structures, implying deep erosion and inselberg landscapes of shield lands become inversion of relief (Fig. 9b). comprehensible in terms of prolonged subsurface 2. Measurements of stress which show that many weathering to which such regions have been parts of the continental crust, including subjected, the great age of some bornhardts is
December 1998 Figure 9a. Sheet fractures and structures exposed in the littoral of one of the Pearson Islands, eastern Great Australian Bight.
Figure 9b. Granite dome underlain by synform along which sill has been intruded and later been faulted, Joshua Tree National Monument, southern California, USA.
Figure 9c. A-tent, or pop up, on lower slopes of Kokerbin Hill, a granite in bornhardt in the southwest of GRANITIC BORNHARDTS: THEIR MORPHOLOGY, CHARACTERISTICS AND ORIGINS 249 consistent with the suggested mode of development, boulder-strewn nubbin (or knoll) and the castellated and the development of bornhardts throughout koppie (Twidale, 1981). Nubbins are due to the geological time also becomes understandable for subsurface disintegration of the outer shell or shells water, and groundwaters, have long been present of rock defined by sheet fractures so that the core on Earth. dome becomes covered by a mass of blocks and Many bornhardts (and boulders, and other boulders (Fig. lOb and c). Such forms are typical of minor forms) apparently evolve as a result of the humid tropics, especially monsoon lands such subsurface weathering, followed by erosion of the as northern Australia and Hong Kong, where the regolith and exposure of the topographically regolith has been largely stripped to expose the differentiated weathering front. Thus they can be core boulders set in a matrix of weathered granite regarded as two-stage forms. But in many instances or grus. Massifs and ranges consisting entirely of subsurface weathering is guided by fracture block- and boulder-strewn hills occur in arid patterns or by other structural features which southern California (for instance, in the Mojave originated long before there was significant contact Desert and in the ranges inland from San Diego) with groundwaters. Thus the Gawler Ranges, where they are thought to be inherited from periods located in the arid interior of South Australia, is a oflater Tertiary humid climate (Oberlander, 1972). massif of bornhardts exposed during the Early Nubbins are also found in moist niches such as Cretaceous but developed on a mass of intermontane basins and valleys in such arid regions MesoProterozoic (ca. 1.582 Ga) ignimbrites. The as the MacDonnell Ranges of central Australia and subsurface weathering to which the bornhardt in parts of Namaqualand, southwestern Africa. complex is due was determined by a system of The transformation of bornhardts to nubbins orthogonal and rhomboidal fractures which by the disintegration of the outer shells of rock, and originated at least 1 Ga and probably 1.4 Ga, yet their conversion to masses of core-boulders and which eventually effectively determined the size grus, is crucial to an understanding of the granitic and plan form of the bornhardts (Campbell and terrains of West Malaysia. There, granitic terrains Twidale, 1991). The volcanic massif was at least carry a regolithic cover so thick that only on the 1.5 km thick (Blissett, 1975). The stresses steepest hillslopes are faintly convex, bald, smooth responsible for the orthogonal fracture system rock slopes exposed. This is a function of the affected the entire mass, but only the uppermost prevalent hot humid conditions which are conducive 800 m or so would have been in substantial contact to rapid alteration of rocks in general and granites with groundwaters until the Mesozoic (Campbell in particular (see e.g. Caillere and Henin, 1950; and Twidale, 1991). For such reasons it has been Alexander, 1959; also Colman and Dethier, 1986). suggested that many two-stage forms are in reality But sheet structures are developed, as is multistage features (Twidale and Vidal Romani, demonstrated in road cuttings, for example, on 1994). Such an approach serves to direct attention Langkawi Island, and Logan (1848, p. 102), Wallace to the longer term links between geology and (1869, p. 24), and Scrivenor (1931) among early landform evolution. workers, reported steep rounded bare rock slopes. Logan for example noted that «Amidst the luxuriant RELATED FORMS forest that always covers granitic hills and mountains, the explorer suddenly finds himself Pillars can be construed as miniature facing a high perpendicular wall of rock, ... " (Logan, bornhardts for, unlike corestone-boulders they 1848, p. 102). These rock faces, commonly slightly remain in physical continuity with the bedrock (Fig. convex outward, are undoubtedly exposed sheeting lOa). Fracture control of overall size and shape is planes. Such exposures suggest the presence clearly demonstrated in minor towers or pillars beneath the regolithic veneer of incipient such as Murphy Haystacks (Twidale and Campbell, bornhardts. 1984). Major steeply inclined fractures have in The presence within the regolith of core places been exploited by frost or nival action, boulders, the abundance ofwhich is best appreciated resulting in steep sided jagged towers of acicular where there have been landslides or where the land form, in such areas as southern Greenland (Oen, has been cleared (Fig. 11), suggests, however, that 1965), the Organ Mountains of southern New Mexico the hills are incipient nubbins, with the regolith (e.g. Seager, 1981), around Mt. Whitney in the still largely in situ. The skyline "tors", blocks, eastern Sierra Nevada, California, at Cathedral boulders, pinnacles and miniature towers (such as Rocks in the Yosemite Valley of the same upland, the Batu Gumbar Orang; Ahmad, 1980) that occur and in The Needles, South Dakota. on crests such as those of Buki Chemragong and Bornhardts can be regarded as a basic form Bukit Panbunga, in Pahang (Ahmad, 1976) and from which are derived two others, the block- or Gunong Gelumut, in Johor (Rajah, 1986) represent December 1998 I\) 01 o
------
~ .:' /~~. " '0' -:regolith ,-, /P-- ::. -:- , //-t /-">-- J _~'- ~o. !...-Lj J~ t -l~ t--- '\~ '\
~ o O_O~~- -~LP-j~-~-=---"::",:,,, 0 p I #' :0 Figure lOa. Granite pillars, Murphy's Haystacks, near Streaky Bay, west coast of Eyre Peninsula, South Australia. -, ~; ~7"~ -----, -- ~ Ci :/,C L-/--. ~ -- ~\. :to - /' -\-+- r m
~ r-'" VJ "'" ~ ~ ""<.. Figure lOb. Suggested development of nubbin ~. from bornhardt by intense subsurface weathering.
"-i? ..,.~ s· ~
Figure 10c. Nubbin in northwest Queensland. Figure 11. B 0 U 1 d e r s exposed by soil erosion following clearance of vegetation in preparation for plantation arboriculture, near Tampin, West Malaysia.
Figure 12a. Castle koppies Mrewa district of central Zimbabwe.
Figure 12b. Castlekoppies Hay tor, on Dartmoor southwestern England. ' 252
weathered sector fj. C.R. TWIDALE W h d s rom which fl as e downslope Tb b mes have been field evidence for th glimpses of the dO~ical :or::ebrock slopes are merely and for thei: dra e grt' eat age-range of the forms rna IC occu '. ' outer shells of rock and b Ideneath the weathered 1andscapes. Bornhardts rrence m m selberg h ,au ers are exposed onl n ~n t be coast and in river channels, and where ther~ basic form from wh' h tc.: be re~arded as the as een accelerated soil erosion. residuals namely n~~b ' 0 erd promment granitic evolved '. . ms an castle kopies have . Falcon~r (1?11, p. 247) presaged such an . III speCial (subsurface) weathering enVIronments. mte.rpretatlOn with his account of what he termed k~PJ~s, but what are here referred to as nubbins, w~thi:r: the walls of Kana, in Hausaland, northern ACKNOWLEDGEMENTS Nlgena: . "Kogan Dutsi, the larger of the two flat-topped The gist of this paper was presented at a meeting hllls of diorite, although deeply decomposed still of the G~ological Society of Malaysia, in Kuala preserves in its lower part detached bould:rs or Lumpu~, m November, 1998, and thanks are due to cores of u,:weathered rock. If the subsequent erosion t~~ SOCiety for providing that opportunity. Dr. H.J. had contmued until the weathered material had TJla offered discerning comments after the lecture been entirely removed, the flattened hill would have and Dr. Jennie Bourne made valuable criticisms of been. replaced by a typical kopje of loose boulders the paper in draft form, and the author is grateful restmg upon a smooth and rounded surface of rock to both. He also expresses his thanks to officers of below." the Geolo~cal ~urvey of Malaysia, and particularly Castle koppies are apparently derived from the Mr. Ibrahim Bm Auman, for facilitating some of the field studies reported. marginal subsurface weathering of platforms, which can be regarded as the exposed crests of otherwise covered domes. This results in the rotting of the REFERENCES shallow subterranean bedrock mass, the reduction in the lateral extent of the solid mass and the AHMAD, JAFAR BIN, 1976. The geology and mineral resources development of steep bounding slopes. Such of the Karak and Temerloh area, Pahang. Geological Survey of Malaysia District Memoir 15, 138p. castellated forms typically consist of an angular AHMAD, JAFAR BrN, 1980. The geology and mineral resources mass constructed of a few large orthogonal blocks of the Sungai Terris area, Pahang. Geological Survey of in situ, and in toto about as big as a house. They Malaysia District Memoir 18, 148p are typically small and are for good reason known ALEXANDER,F.E.S., 1959. Observations on tropical weathering: also as inselbergs de poche (pocket inselbergs). They a study of the movement of iron, aluminium, and silicon are found in two topographic settings. They are in wea thering rocks a t Singapore. Quarterly Journal of the well developed in relation to very old palaeosurfaces Geological Socieh} of London, 115,123-142. ALLEN, c., 1980. Tales from the Dark Continent. Macdonald beneath which there has been prolonged and intense Futura, London, 200p. weathering, resulting in the reduction of resistant BARBIER, R, 1957. Amenagements hydroelectriques dans Ie compartments to very small dimensions (Fig. 12). sud du Bresil. Comptes Rendus Somma ire et Bulletin Societe This is why castellated residuals are so small, for Geologique de France 6, 877-892. they represent the final resistant cores of resistant BrROT, P., 1958. Les domes crystallins. Centre National de la compartments. Koppies are also found in areas of Recherche Scientifique Memoires et Documents 6, 7-34. present or recent intense frost action where epigene BLEs, J.L., 1986. Fracturation profonde des massifs rocheux and shallow subsurface action causes reduction and granitiques. Documents du Bureau de Recherches Geologiques et Minieres 102, 316p. steepening ofthe solid rock mass. The castle koppies BUSSElT,A.H., 1975. Rock units in the Gawler Range Volcanics, (tors) of Dartmoor (e.g. Linton, 1955), the Bohemian South Australia. Geological Survel} of South Australia. Massif (e.g. Demek, 1964), central and western Quarterly Geological Notes 55, 2-14. iberia (Vidal Romani, 1989) and other Hercynian BOYE, M. AND FRITSCH, P., 1973. Degagement artificiel d'un massifs, and of the high Pyrenees, are typical. dome crystallin au Sud-Cameroun. Travauxet Documents de Geographie Tropimle 8,69-94. BORNHARDT, W., 1900. Zur Oberfltichenges taltung und Geologie CONCLUSION Deutsch Ostafrikas. Reimer, Berlin, 595p. Bornhardts are well represented in granitic BRAJNI1
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------.~.~.-~-..-.------Manuscript received 26 January, 1999
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