He suggests that rivers have a strong local influence on erosion and that paleoforms which happen to be between them are not eroded away as fast as the river beds themselves. This results in a tendency to increase relief with time. This idea is contrary to the generally accepted view that landscapes tend to become flatter with time. Other factors which the author feels contribute to the survival of paleoforms are the greater hardness of the rocks, protection from weathering effects in elevated areas compared to low-lying ones and several complex phenomena associated with erosional processes. In the reviewer's opinion, Twidale's ideas make the generally accepted model for the formation of peneplains all the more difficult to accept.
Peneplains are extensive plains of the past.
They are quite common in some sedimentary deposits, but their mode of origin has been much debated. If, as Twidale suggests, time increases relief patterns, the presence of extensive flat-like peneplains in ancient sediments may well represent features where little or no time for erosion has occurred, having been buried rapidly after deposition. This is a concept which Twidale and most other geomorphologists might vigorously reject. Twidale's paper is particularly gratifying because he shows an unusual degree of independent thought, being quite free to challenge well-established concepts. The author feels that it is dangerous to extrapolate present-day rates to the past.