Some Residual Hillslopes in the Great Fish River Basin, South Africa

Abstract

Although their gross morphology may be explicable in a gross fashion, land? forms are mad up of a number of smaller, even miniature features which directly reflect the processes forming them. It is to these that inquiry must be directed in attempts to explain the formation and maintenance of slopes. In Southern Africa some distinctive landforms are associated with the Karroo Dolerites. These basic hypabyssal rocks are conspicuous in the landscape over a quarter million square miles (Du Toit, 1920, p. 2). Only slightly transgressive, almost horizontal or gently undulating, dolerite sheets now appear as cappings surmounting steep slopes across predominantly horizontally bedded sandstones and mudstones of the Karroo system (Walker and Poldervaart, 1949). The structural influence of a hard capping on the maintenance of steep slopes has often been stressed (e.g. Dixey, 1955, p. 267, Pallister, 1954, p. 196) and the general manner of retreat of such hill features is by detachment of residual blocks and their final elimination by all-round erosive attack. These characteristics are seen clearly in Plate IV, 1 (pp. 404-5). Residual hills of this nature occur widely enough in Southern Africa to be worthy of study in their own right. As King (1963, p. 59) has pointed out, 'the abundance of mesas and buttes in the Cape Province and Orange Free State, cap? ped with resistant sandstones or dolerites, is shown in the frequency with which the corresponding Afrikaans names, Tafelberg and Spitskop, occur'. Walker and Poldervaart (1942, p. 56) have even designated remnant features undergoing the last stages of removal of their dolerite capping (Plate IV, 1) the 'Theebus and Koffiebus type of mountain'. Because of their distinctiveness, because of their importance in the retreat of major features, and most importantly because so much of their area is made up by steep slopes, several such mesas and buttes, in the Great Fish River Basin of the Eastern Cape Province, (Fig. 1), here designated the Great Brak residuals, were examined to determine if they exhibit still smaller constituent features relevant to the question of hillslope retreat. Annual rainfall for the area is about 13 inches (Republic of South Africa, Weather Bureau) with more than 70 per cent falling during summer, mainly from thunderstorms of a short, violent nature and limited areal extent; the dominant vegetation growth is the low stunted Karroo bush, spreading from the west and replacing the richer grassveld to the east. Major elements of the residuals.?The Great Brak residuals have a general appearance described elsewhere by Bryan (1940, p. 261), with a conical form rising abruptly above the smooth and relatively low gradients of the surrounding water shaped pediment. They rise approximately 1500 feet from a general level of 4000 feet. The dolerite capping has well developed columnar joints and produces not only an untidy bouldery slope (see Fair, 1947, p. 107) but also an imposing buttressed rampart of a freeface over 100 feet high (Plate IV, 2). Above this almost vertical free-face the extent of gently undulating 'table-top' varies with the over-all size of the residual. Immediately below the free-face is a talus slope varying from 300 to 400 with a range of boulder size from 3 feet to over 5 feet, while the slope customarily changes at the pediment in a short, graceful concave-upwards curve, from i1*0 on the pediment to 8xa? on the residual's lower slope. This break of slope, King's 'hydraulic discontinuity' (King, 1963, p. 46), is com? monly assumed to mark an abrupt change of process, the sharper the break the more abrupt the change. It has also been stated that a break of slope occurs at the foot of a