Planation Surface

planation surface, any low-relief plain cutting across varied rocks and structures. Among the most common landscapes on Earth, planation surfaces include pediments, pediplains, etchplains, and peneplains. There has been much scientific controversy over the origins of such surfaces. Because genetic implications are so often associated with various names, it seems best to refer to these features as simply planation surfaces.

Figure 1 shows a spectacular planation surface that bevels sandstone cuestas in the James Range in central Australia. Clearly an erosive process cut across rocks of varying resistance. The rock structure would never have developed such a flat surface unless a lateral erosive process had been at work in the past at a particular base level. Where many beveled cuestas of this sort line up at the same approximate elevation, they define the regional planation surface.

Planation surfaces, such as that shown in Figure 1, are especially common in the ancient, tectonically stable land masses of the Southern Hemisphere. The South African geomorphologist Lester C. King identified several phases of cyclic planation, which he correlated on a global basis. The oldest surfaces he recognized, termed Gondwana, were Mesozoic in age and related to the ancient landmass of Pangaea and its subsequent breakup during the Mesozoic. A younger surface, called the African or Moorland, developed during the Late Cretaceous and Early Cenozoic by the stripping of weathered materials from the ancient Gondwana surfaces. Younger surfaces developed during the Neogene and Pleistocene (about 23 million to 11,700 years ago) as incomplete planation at levels below the remnants of the ancient plains.