Caderno Lab. Xeolóxico de Laxe Coruña. 1995. Vol. 20, pp. 347-380

Bornhardts, Boulders and Inselbergs

Bornhardts, Bloques e Inselbergs

TWIDALE, c. R.

Bornhardts are steep-sided domical hills. The profiles are associated with sheet fractures, and their plan form with steeply dipping fractures oforthogonal or rhomboidal systems. They are well developed in massive rocks, especially, but not only, in granite, and they occur in various climatic settings. They are found in multicyclic landscapes. They occur in massifs as well as in isolation, as inselbergs; in either setting they meet the adjacent plain or valley floor in a piedmont angle or nick. They may be the basic form from which are derived nubbins and castle koppies. Bornhardts are upstanding for various reasons. For example, sorne are tectonic forms, while others are exposed stocks. Such causations have only local validity. Two competing hypótheses find favour as general explanations. Sorne workers conclude that bornhardts are remnants of circumdenudation, monadnocks de position or Fernlinge, residuals shaped by and remaining afterscarp retreat. Others consider that bornhardts are structural forms, Hartlinge, or monadnocks de dureté, which have evolved in two stages and are etch forms. The rock compartments on which they are based resisted subsurface weathering either because of their composition or because of their low fracture density. They are comparable to corestone boulders, with the important difference that whereas corestones are detached, bornhardts remain in physical continuity with the main mass ofcountry rock. TWIDALE, C. R. (Department of Geology and Geophysics, University of Adelaide, South Australia 5005, Australia). 348 Twidale, e, R, CAD. LAB. XEOL. LAXE 20 (1995)

INTRODUCTION PREVIOUS EXPLANATIONS

Bornhardtsare bald, domical hills (PL 1), Sorne early workers saw bornhardt Sorne occur in isolation, as inselbergs, or inselbergs are literal as wellas littoralfeatures island mountains, but many are found in (e.g. albeit briefIy, BORNHARDT, 1900), and many bornhardts are found in coastal ordered groups within massifs (PL2). They settings as for example in the Rio de]aneiro were first recorded inthescientific literature area, and at several sites along the south by Darwin, from the Rio de] aneiro area, in coast of Australia. But it was soon realised 1846, but are named after Wilhelm thatsuchcoastal connectionsare coincidental Bornhardt, who bequeathed to us splendid rather than essential, for not only do many descriptions and perceptive comments on bornhardts stand aboye and beyond the the forms in an inselberg setting from what reach of any feasible sealevel, but there is is now Tanzania (BORNHARDT, 1900; also no evidence ofoceanic contacts during WILLIS, 1934). Why, when and how have the likely period offormation, either in the theyformed, whyare they domical, andwhy recent or distant past, or indeed, in sorne do sorne stand in isolation? instances, at any time. Thus, the domes of the high Sierra Nevada in California, are BORNHARDT CHARACTERISTICS developed ingranitic rocks emplacedduring the Cretaceous(e.g. BATEMAN and Bornhardts are found in a variety of WAHRHAFTIG, 1966; HUBER, 1987) topographic contexts, in aH major climatic and standing as they do thousands ofmetres settings, and in a variety of lithologies, high in the relief, surely cannot have been though they are best developed, preserved touched by the sea after the exposure ofthe and displayed in granitic rocks, in shield host masses. Similarly Ayers Rock formed lands, and in arid or savanna climates. They in the later Cretaceous and Cainozoic times (TWIDALE, 1978a; HARRIS and meet the adjacent plains or vaHey floors in TWIDALE, 1991), but there is noevidence an abrupt breakofslope called thepiedmont of marine incursions into the region since angle or nick, and occur in multicyclic the Early Cretaceous, and even then not into landscapes. In plan, they are defined by the immediate vicinity of the residual steeply dipping fracturesand their convex­ (FRAKES et aL, 1988). upward profiles are associated with sheet Bornhardts are well developed and fractures (KING, 1949a; BIROT, 1950; preserved in arid and semiarid lands, and TWIDALE, 1982 a and b). this convinced many workers that there is a Bornhardts are spectacular, even genetic connection between landform and dramatic, landscape features (see e. g. VAN climatically induced processes. Inselbergs DER POST, 1962, pp.181-182) and they and inselberg landscapes were at one time have long engaged the interest ofgeologists attributed to differential aeolian planation and geomorphologists. They have been at the regional scale (PASSARGE, 1904a, explained in various ways. AII of the 1904b, 1924; see also KEYES, 1912; interpretations advanced in explanation of ]UTSON, 1914), but the concept never bornhardts, however, are based either in achieved widespread acceptance and soon environment or in structure. faded as evidence accumulated pointing to CAD. LAB. XEOL. LAXE 20 (1995) Bornhardts, boulders 349

Pi. 1. Bald, domical residuals or bornhardts, central N amibia.

Pi. 2. Bornhardts arranged on adjacent fracture defined blocks, in the Kamiesberge ofNamaqualand, South Africa. 350 Twidale, C. R. CAD. LAB. XEOL. LAXE 20 (1995) the erosional dominance of rivers, even in In arid or semiarid conditions, and in a aridandsemiaridlands (e.g.]UTSON, 1934; dissected landscape, water gravitates to the PEEL, 1941). Many interpret bornhardt water table, leaving the higher zones high, inselbergs as remnants ofcircumdenudation, dry and stable (see e.g. BARTON, 1916); as the final residuals surviving after long whereas at and below the water table, the distance scarp retreat, Fernlinge or bedrock is in contact with water and is monadnocks de position (KING, 1942, altered, rendering it susceptible to erosiono 1949a). King (1957) suggested that scarp Surface wash is also concentrated in the recession is dominant wherever the land piedmont zone. Thus, lower slopes are surface is shaped byrunning water, butmost steepened and the upper undermined, workers consider that the process operates leading to scarp retreat (TWIDALE and optimally in semiarid and savanna MILNES, 1983; TWIDALE, 1983). Thus conditions. The mechanism is certainly the scarp retreat hypothesis is theoretically operative in appropriate structural feasible, and is now, as in the past, the most conditions, namely, where there is a resistant favoured explanation of inselberg and caprock; and it is significant that this situa­ bornhardt formation (e.g. KING, tion is commonplace in southern Africa, 1949a,1953; PUGH, 1956; OLLIER and where King developed the concepto TUDDENHAM, 1962; SELBY 1970). Recession ofscarps is also assisted by aridity For other workers, however, bornhardts or by semiarid conditions, where the are structuralforms (sensu lato): Hartlingeor importance of what moisture there is, is monadnocks de dureté .A few isolated enhanced, and where scarp foot weathering granite residuals are demonstrably upthrust becomes a dominant factor in shaping slope along faults and are tectonic forms. The Pic morphology and in determining slope Parana, in southeastern Brazil, is a well behaviour (TWIDALE, 1967; TWIDALE documentedexample(BARBIER, 1957)and and MILNES 1983). So important is this sorne of the pitonsof French Guyana are factor that scarp retreat can be envisaged evidently ofthe same genre (CHOUBERT, eveninnon-caprocksituations,as for instance 1949). But, far more commonly the fractu­ in granitic terrains, for it can be argued that res that define bornhardts are joints, with no

PI. 3. The Pinnacles, a small group ofinselbergs near Broken Hill, western New South Wales. CAD. LAB. XEüL. LAXE 20 (995) Bornhardts} boulders 351 demonstrateddifferential movement, so that adjacent plains. For these, a maJorlty of though locally valid, this tectonic bornhardts, appeal has been made to explanation has no general application. variations in fracture density (e.g. LE Evidently sorne bornhardts are upstanding CONTE, 1873), with the suggestion that because they are developed in a different the upstanding hills are developed on type of rock which is, implicitly, more massive, compact compartments, and the resistant to weathering and erosion than plains eroded in rock characterised by that which underlies the adjacent plains or numerous open fractures. Sorne early valleys. Thus, sorne of the bornhardts of workers, and notably MENNELL (1904), French Guyana are formed in a leucogranite argued that water could more readily which is more resistant than the biotite-rich penetrate into compartments of high frac­ rocks in which the plains are eroded ture density, with the result that these were (HURAULT, 1963). The Pinnacles (P1.3), eroded to leave the compact masses in relief. near Broken Hill, in western New South They considered that the differential Wales, are buttressed by discrete blocks of weathering took place at the surface, after vein quartz which render the masses more exposure. The exploitation of prominent resistant than the biotite-rich schists that vertical fractures to give acicular forms and have been worn down to form turrets, as in the eastern Sierra Nevada of thesurroundingplains. Sorne domicalforms California (for instance in Unicorn peakand are merely exposed stocks, or batholithic around MtWhitney), the OrganMountains protuberances, whichare more resistant than of New Mexico (see SEAGER, 1981), in the host rock. Several examples have been southern Greenland (SOEN, 1965) and, in cited from Mozambique and Zimbabwe syenite, in the Agulhas Negras of (HOLMES and WRAY,1912; DU TOIT, southeastern Brazil, sustain this 1939, p.61) and in central Namibia a interpretation, for frost shattering, initially rounded boss ofgranite (P1.4a) stands aboye perhaps below the surface but the weaker schist into which it is intrusive overwhelmingly under epigene conditions, (see TWIDALE, 1982a 1982b). In the is responsible (P1.5). Musgrave Ranges of central Australia, Otherinvestigators, however, envisaged Mitchell Knob and associated bald domical that the fracture controlled differential masses (P1.4b) may be intrusive into a weathering took place beneath the surface, metamorphic complex, though the smooth at the base of the regolithic veneer (e.g. continuation of the upper convexities into LINTON, 1955; WILHELMY, 1958; basal overhangs gives the appearance of GODARD, 1977). The concept involving large discrete globules or bubbles ofgranite the exploitation of fracture density, but (cf BRAJNIKOV, 1953; BARBEAU and allowing also for the differential weathering GEZE, 1957). Whatever the method of ofdifferent rock types, was earliest and most emplacement of these crystalline masses, succinctly expressed by Falconer: however, the bornhardts based on them "A plane surface of granite and gneiss merelyconstituteaspecialcase oflithological subjected to long-continued weathering at contrasto base level would be decomposed to unequal Many bornhardts, however, appear to be depths, mainlyaccording tothecomposition of the same rock type that underlies the and texture of the various rocks. When 352 Twidale, C. R. CAD. LAB. XEOL. LAXE 20 (1995)

PI. 4. (a) This stock of granite in central Namibia is upstanding because of the preferential weathering of biotite schists into which it is intruded. (b) These ovoid masses ofgranite in the Musgrave Ranges ofcentral Australia are intruded or injected into a gneissic host rock: the most prominent is called Mitchell Knob. CAD. LAB. XEüL. LAXE 20 (1995) Bornhardts} boulders 353

PI. 5. (a) Acicular granitic peaks in the Organ Mountains, New Mexico. (b) Vertical fractures have been exploited to give numerous low turrets on the crest ofpart of the syenitic Agulhas Negras, southeastern Brazil. 354 Twidale, C. R. CAD. LAB. XEOL. LAXE 20 (995) elevation and erosion ensued, the weathered protruding into the base of the regolith. crust would be removed, and an irregular Second, various agents, including wind, surface would be produced from which the glaciers and waves, but most commonly more resistant rocks would project." rivers, effect the erosional stripping of the (FALCONER, 1911, p. 246). regolithic veneer to expose the projecting In sorne instances isolated residuals are compartments, rounded as a result of the left in relief, but where large masses of preferentialweathering ofcorners andedges, resistant rock are affected, weathering upstanding as bornhardts. In reality, the exploits steeply inclined fractures, 'first' stage involves interactions of subdividing the massifinto rows or ordered groundwaters with bedrock at the domes, as in the granitic Everard Ranges, weathering front, and the exploitation of the dacitic Gawler Ranges and the structuresand textures inheritedfrom earlier conglomeratic OIgas complex, all in the magmatic, thermal, tectonicanddiagenetic arid interiorofAustralia, and in the granitic phases, so that etch features can be regarded andgneissic Kamiesberge ofNamaqualand, as multiphase rather than two stage (TWIDALE and VIDAL ROMANI, 1994); inCape Province, SouthAfrica. Theessential but Falconer's point is well taken for it is feature of Falconer's concept is that subsurface weathering and later exposure of bornhardts evolve in two major stages the weathereing front that in an immediate (Fig.1). First, shallow groundwaters cause sense produce etch forms. structurally controlled differential subsurface weathering ofthe rock mass at a COMPARISON WITH BOULDERS regional scale, and so create an irregular weathering front (MABBUTT, 1961), The two stage concept used to explain leaving the more resistant compartments many bornhardts hadearlier beenintroduced

a

b

Fig. 1. The two stage evolution ofbornhardts and inselbergs.In (a) a regolith ofuneven thickness has developed on structurally differentiated country rock, and in (b) removal of he regolith has exposedpreviouslydevelopedprojectionsofbedrockintothe baseoftheregolith as bornhardts. CAD. LAB. XEüL. LAXE 20 (1995) Bornhardts, boulders 355

PI. 6. (a) Granite corestones and boulders near Chazeirollettes, near Aumont, southern Massif Central, France. (b) Granite corestones, Selangor, West Malaysia. 356 Twidale, C. R. CAD. LAB. XEOL. LAXE 20 (995) to account for boulders. More than two aboye the surroundinghillcrest, anddespite centuries ago, Hassenfratz gave an the weathering of vertical fracture zones explanatoryaccount ofa granite exposure he remain in substantial contiguity with the had noted near Aumont, in the southern underlying granite (PI.8). Bornhardts are Massif Central (PI.6a). He noted boulders very large pillars rather than large boulders. strewn over the surface, and corestones still embedded in the weathered granite or grus. He interpreted the assemblage as an EVIDENCE AND ARGUMENT evolutionary sequence, and wrote: Several of the hypotheses outlined have "... on aper\oit tous les intermédiaires local or specific application, but only two entre un bloc de granit dur contenu et have found favour as general explanations, enchassé dans la masse totale du granit that is, as hypotheses that have the capacity friable et un bloc entierement dégagé." toaccount not necessarilyfor aH, butcertainly (HASSENFRATZ, 1791, p. 101). for most, bornhardts; these are the scarp The number ofexposures, from all parts retreat andthe two stage hypotheses. Various of the world, in which corestones are lines of evidence and argument can be contained in a matrix ofgrus (PI.6b), leaves adduced wherewith to test the field data little doubt that most granite boulders have against the deducible consequences of the formed in this manner. Most are derived two major hypotheses (TWIDALE, 1982a from joint blocks defined by fractures of and b). orthogonal systems. Sorne are due to the If bornhardts are two stage forms, and disintegrationofsheetfractures, eitherbelow assuming that the same processes and thesurface, or, andperhapsmore commonly, mechanisms that operated in the past are after exposure, and to the subsequent still active, then besides those that have subaerial rounding of the resultant blocks, been subjected to both stages and are but mostoriginated as corestones, and are exposed, there ought to be sorne that have two stage forms. Weathering also exploits passed through only the first stage, that foliation, producing domical hills with have beenformed by differential weathering ribbed slopes, each "rib" consisting of beneath the surface but are not yet exposed. numerous penitent rocks, monkstones, There ought to be nascent bornhardt masses Büssersteine or tombstones (PI.7). projecting into the regolith but not yet Corestones, however, are isolated and no naturally exposed by stripping.of the longerinphysical continuitywiththeparent regolith. There are. Of sorne, like that mass, and though Brajnikov (1953) has exposed in the Vredefort Brick Pit, Orange suggested that sorne ofthe morrosofBrazil Free State, South Africa (PI.9a), there is no have become detached, bornhardts are more indication at the natural surface: the properlycompared to therockpiHars exposed incipientbornhardtcarne tolightonlyduring at Murphy Haystacks, on the northwest excavation. Others were minimally exposed coast of Eyre Peninsula (TWIDALE and as a small platform or low large radius dome CAMPBELL, 1984). These pillars, most of which on excavation proved to be the crest them with flared sidewalls, stand4-5 m of a bornhardt, as at Ebaka, Cameroun CAD. LAB. XEüL. LAXE 20 (995) Bornhardts, boulders 357

Pi. 7 (a) Bornhardt in foliated granite, with ribbed slopes, Kamiesberge, Namaqualand, South Africa. (b) Penitent rocks in eastern Mt Lofty Ranges, South Australia. 358 TwidaleJ C. R. CAD. LAB. XEOL. LAXE 20 (995)

PI. 8. Granite pillars, Murphy Haystacks, west coast ofEyre Peninsula, South Australia. CAD. LAB. XEOL. LAXE 20 (1995) Bornhardts, boulders 359

PI. 9. (a) Partofnascent dome (X) exposed inVredefort BrickPit, Orange Free State, South Africa. (b) Incipient bornhardt exposed by excavation at Elkington Rock, north of Minnipa, Eyre Peninsula, South Australia. 360 Twidale, C. R. CAD. LAB. XEOL. LAXE 20 (1995)

(BOYÉ and FRITSCH, 1973); adjacent to the weathering front (TWIDALE 1962; the Leeukop, near Vredefort; at Midrand BOYÉ and FRITSCH, 1973; TWIDALE (the former Halfway), in the central andBOURNE, 1975a,1976). Notallminor Traansvaal, South Africa; at Calca Quarry, forms originate at the weathering northwest coast of Eyre Peninsula, and at front.Some, including sorne basins and Elkington Rock, near Minnipa, flutings, indubitably develop wholly under northwesternEyrePeninsula(Pl.9b). Inthese the influence of epigene processes (e.g. examples the thickness ofregolith increases LAGEAT et al., 1994). Others, though in all directions away from the platform; initiated at the weathering front, largely and similar situations have been develop, orthey diversify, after exposure, as demonstratedbyaugeringatotherplatforms exemplified by tafoni and rock basins on Eyre Peninsula. respectively. Sorne, like rock doughnuts A subsurface initiation of bornhardts, and rock levees, are due to the unequal the host mass, is consistent with the irre­ weathering ofexposed and covered surfaces futable evidence that several minor forms (e.g. Twidale, 1988) and others, namely commo~lyassociated with bornhardts, such plinths, owe theirexistence to protection by forms as pitting, basins, Rillen, and flared overlying blocks. Yet others, like displaced slopes, are also initiated in the subsurface, at blocks, owe their origin to gravity

PI. 10. Flared slope, and incipient flared slope exposed in reservoir, at Yarwondutta Rock, near Minnipa, Eyre Peninsula, South Australia. CAD. LAB. XEüL. LAXE 20 (995) Bornhardts, boulders 361

(TWIDALE et al., 1991) and a suite of they are of a rock type different and more forms, in which A-tents are prominent, are resistant thanthatworndown toform plains, associated with the release of compressive and this has been demonstrated ina number stress (TWIDALE and SVED, 1978). But of cases. It has been suggested that others many minor granite forms are initiated at are upstanding by virtue not oflithological the base of the regolith, at the weathering contrasts but of their being massive, i.e. front. because ofvariations in fracture density. If Though they are perhaps a special case, this were so, there ought to be evidence of flared slopes unquestionably form at the such variations. There is. The evidence is weathering front, in the scarp foot zone not plentiful, for suitably positioned (PI. 1O), for they have been uncovered from excavations are required, but thepresence of beneath a regolithic veneer in situin rocks subdivided by numerous fractures excavations at several sites (TWIDALE, located immediately adjacent to compactor 1962, 1982a). These concavities represent massive compartments (Pl.11) has been an undermining ofthe bases ofrocky slopes described from Ucontitchie Hill, on and they well illustrate, albeit in extreme northwestern Eyre Peninsula, near Garies in form, the mechanism responsible for the N amaqualand, around Blackingstone Rock, piedmont angle or nick, the abrupt break of on eastern Dartmoor, and at Kolar, near slope between hill and plain which is Bangalore, inpeninsularIndia(e.g.] üNES, particularlypronounced inaridandsemiarid 1859; TWIDALE,1964, 1982a; BÜDEL, lands (e.g. HILLS, 1955). Scarp foot 1977, pp.1ü8-1ü9). weathering induces erosion of the It might be argued that such evidence is basalsectors ofhillslopes, the steepening of irrelevant because the relevant contrast in scarps and hence a sharp transition from hill fracture density is not that between the to plain (PEEL, 1941; TWIDALE, 1967). present bornhardt and the compartment In sorne terrains, and especially those beneaththepresentplain, butratherbetween developed in granitic or other well jointed the bornhardt and the adjacent rocks, structural factors play an important compartments, located at the same level as role, and induce trulyangular nicks, but the the bornhardts, that have been eroded. The abrupt change from hill to plain is also well point is well taken, but BLES (1986) has represented in plateau areas, for example shown that near surface fracture densities where the hill-plain junction is developed provide a reliable guide to fracture density in one rock type. The basal sapping and pattern at depth, and extrapolating responsible for· the piedmont angle vertically upwards rather than in depth, it undoubtedly takes placein the shallow can be suggested that they also provide a subsurface, but may also evolve at depth, as guide to conditions in the eroded water gravitates along slopes on the compartments, so that the evidence cited is weathering front to the bases ofprojections, germane to the discussion (TWIDALE, where it may well sitand circulate aboye the 1987). impermeable mass offresh rock. Variations in fracture density have been Sorne bornhardtsare upstanding because explained in terms ofstrain distribution in 362 Twidale, C. R. CAD. LAB. XEOL. LAXE 20 (1995)

a b = =

b

Fig. 2. Developmentofstressedcoreorincipientresidualwithina fracture- defined blockbyshearing and fracture propagation. In (a) a single orthogonal block is repeatedly sheared, and in (b) several such blocks are depicted. ,

Wudinna Hill \ I ~l Little Wudinna I • HiII, •\

N

2 - OI I km

Fig.3. Plan pattern ofgranite outcrops north ofWudinna, Eyre Peninsula, South Australia. CAD. LAB. XEüL. LAXE 20 (1995) Bornhardts, boulders 363 antiforms and synforms (LAMEGO, 1938), eroded below the early Cainozoic African and as a consequence of shearing planation surface (WHlTLOW, 1978-9; (TWlDALE, 1980a). The second LISTER 1987),andthatthebornhardtmassif mechanism in particular (Fig.2), implies that is the Gawler Ranges occurs below the that bornhardts ought to be in compression level of an. etch surface of Early Cretaceous along one horizonal axis and in tension age (CAMPBELL and TWlDALE, 1991). along the other horizontal axis normal to Theassociation betweenmultipleplanations the first; there is sorne evidence, at present and bornhardts can be linked to crustal sparse, that this is so. In these terms also stress, for a multicyclic landscape implies (Fig.2), bornhardts ought to be disposed in not only timefor the differential weathering plan patterns, in alignments, rather than essential to the two stage concept, but also scattered randomly in the landscape. They profound erosion and the possible exposure are, for not only in massifs, but also in of the deeper zones of compression found inselberg landscapes, the residuals stand in below the neutral plane in antiformal ordered rows (e.g. Fig.3).In sorne areas, such structures. as the Kamiesberge of Namaqualand, that Turning to sorne of the implications of the fractures defining major compartments the scarp retreat hypothesis, if bornhardt are faults is demonstrated bydisplacements, inselbergs are the last remnants of slickensides, recrystallisation, and so on. circumdenudation, they ought to be AIso, a genetic relationship between preserved on major divides. They are bornhardts and implied compressive stress not.Evidence from several parts ofthe world is consistent with the characteristic shows that bornhardts are found adjacent to occurrence on and within bornhardts of rivers and valleys , as in parts of Nigeria sheet fractures and structures, and the (THOMAS, 1966), and in the Yosemite of common development on the residuals ofa the Sierra Nevada in California (HUBER, suite of forms associated with stress and 1987);partlyexposed inthesides ofrelatively strain(MERRlLL, 1897;TWIDALE, 1964, narrow valleys as in the Thompson Valley 1973, 1982a; TWIDALE et al., 1995). near Estes, Colorado, and oftheWitrivierin The occurrence of bornhardts in the eastern Transvaal (Pl.12); and even in multicyclic landscapes, is implicit in the floors of valleys, as near Malmesbury, FALCONER'S (1911) two stage concept, north ofCape Town (Pl.13). and has been noted by BORNHARDT lf bornhardts are the last remnants of (1900), WILLIS(1936),]ESSEN(1936)and circumdenudation, none ought to be older KING (1949a). Planation surfaces can be than the duration of a geomorphic cycle interpreted as implying stability and time operating in a region of continental or for deep (differential)subsurface weathering, subcontinental extent, i.e. about 33 my and multicyclicity implies deep erosion and (SCHUMM, 1963). No inselberg that has exposure of the differentiated weathering not been exhumed ought to be older than front ,including residual hills. Thus it is no Oligocene. Sorne are. The Humps, in the coincidence thatinZimbabwe, for example, southern Yilgarn ofWestern Australia, are about 80% of the bornhardts and other at least ofEocene age (FAIRBRlDGE and inselbergs ofthat country occur on surfaces FINKL, 1978; TWIDALE, 1986). The 364 TwidaleJ e. R. CAD. LAB. XEOL. LAXE 20 (995)

PI.. 11. Reservoir at U ncontitchie HilI, Eyre PeninsuIa, South Australia, showing contrast between fracture density beneath the plains, as indicated by the diameters ofthe bouIders exposed in excavation, and that ofthe adjacent residual. CAD. LAB. XEüL. LAXE 20 (1995) Bornhardts, boulders 365

PI. 12. Granite domes exposed in valley side slopes (a) Near Estes, Colorado, USA, and (b) near Witrivier, easternTransvaal, South Africa. In (b) X indicates King's Africanplanationsurface, ofessentially Paleogene age. 366 Twidale, C. R. CAD. LAB. XEOL. LAXE 20 (1995)

PI. 13. Granitic dome exposed in valley floor near Malmesbury, north ofCape Town, Cape Province, South Africa.

PI. 14. Stepped northwestern slope of Yarwondutta Rock, near Minnipa, Eyre Peninsula, South Australia. CAD. LAB. XEüL. LAXE 20 (1995) Bornhardts, boulders 367 summit bevels ofAyers Rock and the OIgas the bounding slopes of which display complex are probably of latest Cretaceous alternations ofscarp and platform, step an~ (Maastrichtian) age, though the steep tread. Suchforms, developedatvarious scales bounding slopes are Cainozoic features (Fig.4) are quite widely distributed (see e.g. (TWIDALE and HARRIS, 1977; JESSEN, 1936;TWIDALEandBOURNE, TWIDALE, 1978a; HARRIS and 1975b; TWIDALE, 1882a) The TWIDALE, 1991). The Gawler Ranges northwestern slope of Yarwondutta Rock, massif was an upstanding topographic nearMinnipa, northwesternEyre Peninsula, feature during the Eocene, and probably provides a clear, compact example (PI. 14). dates back to the Early Cretaceous The steps are, in fact, flared slopes (see (CAMPBELL and TWIDALE, 1991). aboye, andTWIDALE, 1962). Exposures of Ifbornhardts are due to scarp recession, contemporary flared weathering fronts there ought to be sorne bounding slopes confirm thattheshouldermarkingtheupper that are distant from majorpartingswhereas limit ofthe concavity represents the former in reality most are located close to fracture hill-plain junction. Extrapolating this zones: there has been a certain amount of conclusion to the stepped slope, the exposed wearing back of slopes, for example in the flared slopes denotealternationsofsubsurface Gawler and Everard ranges, butnot mucho scarp foot weathering and erosion, though It is asking much, and perhaps too much, of whether this took the form of lowering of coincidence thatslopes shouldhave stabilised the entire plains or simply a phased erosion onornear fractures, and moreover stabilised of the plains surrounding the residuals is long enough for flared slopes and scarp foot notknown.Thesteppedmorphologyimplies depressions to have developed (see e.g sorne recession ofthe steep slopes, but this PUGH, 1956; CLAYTON,1956; is due not to gullying, as supposed by KING(1949b), buttosubsurface (moisture) TWIDALE, 1962). weathering; and the recession amounts to at Stepped inselbergs provide crucial most a few tens ofmetres and not the scores evidence concerning the reality, as opposed or even hundreds of kilometres demanded to the theory, ofscarp recession in granitic by King's theory, andmore importantly, by terrains. Stepped inselbergs are residuals inselberg landscapes. Evidently the landscape slate is not wiped clean during each phase or cycle of erosion, for YarwonduttaRock has survived at least two such alternations ofweathering and erosion (TWIDALE, 1982c). Retardation of recession due to diminution of the contributingcatchment(TWIDALE 1978b) may account for this in sorne measure. On the other hand, reinforcement or positive feedback effects (TWIDALE et al., 1974) Fig. 4. Inselbergwithdistinctbevelorshoulder, tend to perpetuate and enhance relief. the Bongoberg, Angola. (After Jessen, 1936). KING (1966) pointed out that in many 368 Twidale, C. R. CAD. LAB. XEüL. LAXE 20 (1995) areas the height ofthe bornhardts aboye the forms, is all but covered by blocks and plains far exceeds the depth of weathering boulders (Fig. 5a; Pl.15b). Such intense below them, and suggested that this was weathering can be explained in terms ofthe inconsistent with the requirements of the abundant rainfall and high ambient two stage theory. But stepped forms imply temperaturesofthehumid tropics.Nubbins that sorne bornhardts, at any rate, have thatoccuroutside these regions reflect either evolved not in a single alternation of climatic change, as in the American subsurface weathering and erosion, but in Southwest (OBERLANDER, 1972) or several (TWIDALE and BOURNE, 1975b; locally abundant subsurface moisture, as see also TWIDALE, 1978a), so that the may be the case in valleys and basins discrepancy between height ofresidual and underlain bywell fractured granites inparts thickness of regolith in any one area is not ofNamaqualand(Pl.15c), and northofAlice critical. Springs, in the Macdonnell Ranges ofcen- tral Australia. Castellated forms or castle koppies RELATED FORMS (Pl.17a), most commonlyoccur inisolation, The value of a principIe or hypothesis as in Zimbabwe, and on many of the lies in the number of features it explains. Hercynian massifs ofwestern Europe. In the The scarp retreat explanation ofbornhardts Traba Massif of Galicia, and in the,Sierra is refuted by much field evidence and leaves Geres of northern Portugal (e.g. the Corga das Negras), however, theyare closely spaced other aspects unexplained. The two stage and stand in ordered, fracture-controlled hypothesis on the other hand accounts for groups(Pl. 17b) They are best explained as the field evidence, local, regional and glo­ the innermost cores ofbornhardts, the crests bal. It is also capable of accommodating of which were exposed and thus relatively related forms and features. dry and stable, but the buried flanks of Thus, inthese terms, block- andboulder­ which were weathet;ed in the subsurface, strewn nubbins(orknolls)andthecastellated causing a reduction in the mass by lateral forms known as castle koppies are simply centripetalmigrationoftheweatheringfront variants ofbornhardts TWIDALE, 1981a). (Fig.5b). That such marginal subsurface Nubbins (Pl.15a) are prominent in the weathering has taken place is indicated by humid and seasonally humid (monsoonal) the presence offlared segments on sorne of tropics. They commonly occur in ordered the bounding slopes (Pl.17c)This marginal groups. Though sorne blocks andboulders weathering may have taken place either reflect epigene weathering ofexposed sheet over a long period, as in Zimbabwe, or structures, sorne break down undoubtedly intensely and rapidly, as for example in takes place beneath the surface (Pl.16) and cold, or formerly cold, areas such as the most nubbins are best interpreted as due to moors of southwestern England, the rapid disintegration in the upperpart ofthe Pyrenees, and the uplands ofnorthern Por­ regolith of the outer one or two shells or tugal and southern Galicia (Pl.17d), where sheet structures, so that the inner domical frost action and ground ice were and are core, which can be seen in many of the active inthe upperor near surface zone ofthe CAD. LAB. XEOL. LAXE 20 (1995) Bornhardts, boulders 369

a

iii ----...-----

iv----

¡¡

b i~

ii~ ~

iii

Fig.5. Sequential development of(a) nubbin (b) castle koppie. 370 Twidale, C. R. CAD. LAB. XEOL. LAXE 20 (1995)

PI. 15 (a) Granite nubbin, western Pilbara ofWestern Australia. (b) Degraded nubbin, Devils Marbles, central Australia, showing residual blocks on dome. CAD. LAB. XEOL. LAXE 20 (995) Bornhardts, boulders 371

PI. 15 (e) Granite nubbin, Namaqualand, South Africa.

PI. 16. Section at Paarl Quarry, near Cape Town, South Africa, showing sheet structure already disintegrated beneath the naturalland surface. 372 TwidaleJ C. R. CAD. LAB. XEOL. LAXE 20 (995)

Pi. 17. (a) Castle koppie, central Zimbabwe. (b) Castle koppies in the Traba Massif of Galicia. CAD. LAB. XEOL. LAXE 20 (995) Bornhardts, boulders 373

PI. 17. (c) Castle koppie in central Pyrenees.

regolith. In any event the rock mass was The most characteristic inselberg reduced to a small size to give what French landscapes are found in the tropical shield workers appropriately call inselbergs de lands. The reason is that the shield lands are poches. As with nubbins, local conditons stable, so that whatever the climatic may produce koppies in anomalous conditions, there has been timefor subsurface weathering to reduce all but the most situations, as with the Devils Marbles, resistant compartments to regolith. Most of located in the core of an anticlinal valley the surviving resistant masses have been inthearid interiorofAustralia(TWIDALE, reduced to small size, and only especially 1980). durable, originally extensive or recently uplifted massifs, have survived as inselbergs INSELBERGS (PI. 19). In weaker lithologies such as limestone, extreme etch planation is more Thoughbornhardts occurwithinmassifs, readily and rapidly achieved (as on the dram~tic they find their most expression in Nullarbor Plain - TWIDALE, 1990) but in isolation, in inselberg landscapes (Pl.18); granitic rocks featureless plains argue and it is ironic that these comparatively duration of weathering, and antiquity of small forms have given their name to entire landform. landscapes. Many, perhaps most, inselbergs are 374 Twidale, C. R. CAD. LAB. XEOL. LAXE 20 (1995)

PI. 18. Inselberg landscape in granite, Namaqualand, South Africa.

PI. 19. The Groot Spitzkoppe, central Namibia. CAD. LAB. XEOL. LAXE 20 (1995) Bornhardts, boulders 375

Pi. 20 (a) Mantled pediment in granite, occurring just below the surface, beneath a thin regolith, as exposed in river bluff, Granite Mountains, southern California, USA. (b) Part of the rock platform or pediment extending from the base of Corrobinnie Hill, northern Eyre Peninsula, South Australia. 376 Twidale, C. R. CAD. LAB. XEOL. LAXE 20 (1995)

PI. 21. Part ofthe Meekatharra Plain, a virtually featureless plain eroded in granitic rocks in central Western Australia.

PI. 22. Rolling planation surface in granite, northern Transvaal, South Africa. CAD. LAB. XEOL. LAXE 20 (1995) Bornhardts, boulders 377 fringed by pediments. Sorne are mantled irregular weathering front of which (TWIDALE, 1981),andcarryathinregolith, bornhardts are a prominent feature. Sorne consisting mainly ofweathered bedrock in inselbergs are tectonic, but most are situ(PI.20a). The regolith has been stripped structural and etch features.The rate of from others to expose a rock pediment or weathering doubtless varies according to platform(PI. 20b). As is well known, KING the physical, chemical and biotic character (1942) believed that pediments have of the shallow groundwaters (and coalesced to form pediplains, but even in his atmospheric climate may well inf1uence typeareas therequiredmulticoncaveupward these), but the end products are everywhere morphology is not evident (TWIDALE, similar. As groundwaters are ubiquitous, 1983). similarforms develop theworld over. Inlike Sorne ofthe extensive, featureless plains manner, though the mineralogy of the ofinselberg landscapes are also etch forms; bedrock inf1uences the rate ofdevelopment, certainly extraordinarily f1atgranite plains similar physical characteristics in the like the Bushmanland Surface (PI.18) and bedrock lead to similar weathering forms the Meekatharra Plain of the interior of regardless of petrological classification. Western Australia (PI. 21) are ofthis type. Hence the lithological azonality of many Where the regolith has not been completely landforms. The nature of the agencies stripped, plains oflow relief, and ofbroadly responsible for stripping ofthe regolith has rolling morphology(PI. 22 - see also BIROT varied both spatially and temporally, but etal., 1974), have been eroded ingrus, as for preeminently it is rivers and running water example in the southernYilgarnofWestern that have exposed etch forms (TWIDALE, Australia, and on Eyre Peninsula, in South 1990). Australia, where, however, the surface is Thetwostageoretchhypothesisaccounts protected by a carapace ofcalcrete. not only for the typical site and regional features of bornhardts, boulders and insel­ CONCLUSION bergs butalso for theirglobal characteristics. It is true that any theory that fits all the facts On balance, the evidence suggests that is bound to be wrong, ifonly because sorne most bornhardts have developed in two ofthe data are f1awed, but the two stage or stages, the first involving structurally etch concept is nevertheless a powerful tool controlledsubsurfaceweathering, thesecond in the interpretation not only of granitic the stripping of the regolith to expose the terrains but of landscapes generally. 378 Twidale, C. R. CAD. LAB. XEOL. LAXE 20 (1995)

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