Geoscience Reference
In-Depth Information
ka
-1
. Global solid (suspended) sediment transfers are
shown in
Figure 13.2
.
Denudation cycles and chronology
Episodic uplift and the presence of large continental areas
of low relative relief, even at moderate altitudes (1-2 km),
prompted the formulation of model
denudation cycles
and
denudation chronologies
to chart land surface
development. The most enduring models 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 13.3
shows the '3,000 ft' erosion
Plate 13.3
The '3,000 ft' remnant summit plateau of Y
plain. Ridge-push forces, as the Atlantic continued to widen,
elevated it tectonically by 1 km less than 25 Myr ago, in the
mid-Cenozoic era.
Photo: Ken Addison
LONGITUDE
180°
120°
60°
0°
60°
120°
80°
80°
60°
60°
40°
40°
20°
20°
0°
0°
20°
$QQXDO<LHOG t km
-2
750-1,000
250-750
50-250
0-50
Hot and cold deserts
20°
40°
40°
60°
60°
60°
120°
180°
120°
60°
0°
Source: After Walling and Webb; in Gregory (1983)
