Environmental Engineering Reference
In-Depth Information
envisaged elevated land surfaces wearing
down
(W. M. Davis, 1890s) or wearing
back
(W. Penck, 1920s) to an eventual
peneplain
, or through parallel slope
retreat
to a
pediplain
(L. C. King, 1950s). The cycle was rejuvenated through renewed uplift.
Davis's model had mountains like the Appalachians in mind and, in common with those
which followed, employed concepts of
youth
(waxing slopes),
maturity
and
old age
(waning slopes). In this way, for example, much of upland Britain and individual regions
such as Wales, the Weald and the downlands of south-east England were thought to
reflect multiple peneplanation and rejuvenation cycles with accordant remnant summits
and plateau surfaces. Plate 13.1 shows the '3000 ft' erosion surface in North Wales, one
of the highest and on Davis's terms therefore one of the oldest in Britain. Modern
evidence suggests that it is a lowland surface, uplifted in mid to late Tertiary times by
sustained ridgepush forces on the European continental margin as the Atlantic continued
to widen. It was subsequently breached by Quaternary glaciation and the region is still
seismically active.
Denudation cycles and chronologies are now largely discredited but they were
formulated long before sea-floor spreading, volcano-tectonic activity and the dramatic
events of the Quaternary were understood. These clarify the complex nature and rates of
uplift and denudation, the speed and frequency of disturbances to denudational 'rhythms',
and means of dating events. Older models projected whole land system effects without
comprehending the total landsystem and were rather parochial. Davis's 'normal cycle'
assumed fluvial conditions in temperate environments where glaciation was seen as a
climatic 'accident' and King worked largely in semi-arid southern Africa. However,
Davis recognized the progressive transformation of potential to kinetic energy as his
cycle proceeded and his
graded
river profile reflects exponential energy decay and the
progressive reduction of elevation, relief and slope angles.
FORCE AND RESISTANCE
Debate shifted away from denudation cycles to explore the validity of a
morphogenetic
basis for modern geomorphology. Its pervasive presumption of climatic control on
