INSELBERG

(from German insel, island, and berg, mountain), isolated hill that stands above well-developed plains and appears not unlike an island rising from the sea. Although the term is usually applied generally, it is sometimes restricted to hills that are formed under tropical and subtropical conditions, composed of plutonic rocks such as granite and gneiss and characterized by steep, curved sides of smooth, bare rock. Inselbergs occur as ridges, ranges, or isolated individuals. Their formation is a result of the resistance to erosion by structural irregularities. Bare rock domes, which may stand 450 m (1,500 feet) above the plains, are more specifically called bornhardts and display an onionlike peeling (exfoliation) of concentric rock shells (in thickness from a few centimetres to 6 metres [from about an inch to about 19 feet]) from the surface. A pediment, or low-angle slope slightly concave upward, is usually present at the base of the inselberg; the hill-plain junction is usually quite sharp. In its later stages an inselberg may develop into a kopje (koppie), i.e., a residual hill composed of a pile of large boulders, in form similar to a tor. Isolated hills similar to inselbergs but formed under nontropical conditions are known as monadnocks. (from German Insel, island, and Berg, mountain), isolated hill that stands above well-developed plains and appears not unlike an island rising from the sea. The early German explorers of southern Africa were impressed by such features, and they dubbed the domed or castlelike highlands inselbergs. Spectacular examples include Ayers Rock and the Olga Rocks in central Australia. Inselbergs are relict features. They have maintained their relief as the adjacent surrounding landscape was lowered. C.R. Twidale of Australia demonstrated the role of subsurface weathering in shaping the flanking hillslopes and pediments of granitic inselbergs. The occurrence of inselbergs implies immense variations in the rates of degradational activity on the land surface. These structures are one of several varieties of landform called paleoforms that can survive with little modification for tens of millions of years. In inselberg landscapes, the active erosional processes are confined to valley sides and valley floors. Victor R. Baker