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Bornhardts, Bouldes and Inselbergs

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Bornhardts, Bouldes and Inselbergs 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
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