ABSTRACT
According to Penck’s arguments, slopes may either recede at the original gradient or else flatten, according to circumstances. Many textbooks claim that Penck advocated ‘parallel retreat of slopes’, but this is a false belief (see Simons 1962). Penck (1953, 135-6) argued that a steep rock face would move up slope, maintaining its original gradient, but would soon be eliminated by a growing basal slope. If the cliff face was the scarp of a tableland, however, it would take a long time to disappear. He reasoned that a lower-angle slope, which starts growing from the bottom of the basal slope, replaces the basal slope. Continued slope replacement then leads to a flattening of slopes, with steeper sections formed during
earlier stages of development sometimes surviving in summit areas (Penck 1953, 136-41). In short, Penck’s complicated analysis predicted both slope recession and slope decline, a result that extends Davis’s simple idea of slope decline (Figure 15.1). Field studies have confirmed that slope retreat is common in a wide range of situations. However, a slope that is actively eroded at its base (by a river or by the sea) may decline if the basal erosion should stop. Moreover, a tableland scarp retains its angle through parallel retreat until the erosion removes the protective cap rock, when slope decline sets in (Ollier and Tuddenham 1962).
