erosion surfaces

erosion surfaces Erosion (planation) surfaces are cut by erosion across varied geological structures and rock types, and have the general form of plains. W. M. Davis proposed that areas erode to an erosion surface, or peneplain, formed by slope decline. Penck and King proposed that slopes evolve by parallel retreat to form pediments that coalesce to make pediplains. These workers thought that landscapes consist of a series of peneplains or pediplains. Indeed, until the 1950s geomorphology was largely concerned with erosion surfaces. Geomorphologists explained landscapes in terms of cycles of erosion and successions of erosion surfaces or partial erosion surfaces. Other terms are etchplains and panplains. A non-genetic word is palaeoplain.

In the 1960s, work on erosion surfaces was strongly attacked in favour of process studies and systems theory. Some workers even doubted that erosion surfaces existed. Nevertheless, planation surfaces are real. Their existence is evident in many geological sections where the amplitude of topography is trivial compared with the size of folds, faults, and intrusions in the bedrock. They are also clear on the ground, and on continents like Australia and Africa they dominate the landscape.

It is difficult to explain how plains really formed. Moreover, there are many parts of the Earth's surface where only one palaeoplain can definitely be recognized. As C. D. Ollier has pointed out, palaeoplain development takes place on the same timescales as plate tectonics and biological evolution; so in most parts of the Earth there has probably not been enough time for more than one or two erosion surfaces to develop. In south-eastern Australia, for example, the palaeoplain first described by E. S. Hills is preserved along much of the Great Divide, and is probably Mesozoic in age. In other places, for example western South America, there are a number of erosion surfaces, probably because of the greater rates of uplift there.

Erosion surfaces are sometimes dissected into remnants. Observation of these remnants, and early enthusiasm for erosion surfaces, probably caused many phantom surfaces to be reported, although bevelled cuestas are real. At its extreme, accordant summits may be taken as evidence of a former erosion surface (Gipfelflur), but how far such interpretations can be carried is problematic.

Many unconformities in the geological record separate older, folded rocks from younger, nearly horizontal rocks. These unconformities are buried surfaces, some formed by terrestrial, others by marine, planation. Where the younger rocks have been eroded away, the exposed unconformity becomes an exhumed erosion surface. In the Kimberley region of Western Australia the removal of a thin cover sequence resulted in the modern land surface corresponding very closely to the Precambrian erosion surface.

Major rivers developed on erosion surfaces show remarkable persistence in the landscape. In Uganda and eastern Australia some rivers on the palaeoplain pre-date continental rifting. In Australia this is true even of rivers east of the Great Divide that were reversed and now flow into the Tasman Sea instead of to the north-west.

Colin F. Pain

Bibliography

Ollier, C. D. (1991) Ancient landforms. Belhaven Press, Cambridge, Mass.