Geology Reference
In-Depth Information
Interglacial
Glacial
Para-
glacial
Interglacial
Cold stage processes
Erosion
Warm stage processes
Erosion
Formation of soils
and deep regoliths
Glacial
deposition
Formation of soils
and deep regoliths
Time
Figure 14.2
Suggested changes in geomorphic systems during a glacial-interglacial cycle.
Source:
Adapted from Starkel (1987)
conditions, similar to those of the past several tens of
thousands of years, whereas regional wind deflation and
aeolian deposition prevailed during periods of prolonged
drought.
A study of Early and Middle Pleistocene fluvial and
coastal palaeoenvironments in eastern England showed
that changes in river energetics accorded with the rela-
tive importance of geomorphic processes operating in
river catchments determined by orbital forcing (Rose
et al
. 2001). The size distributions and lithologies of
deposits indicate a shift from low-energy systems com-
prising mainly suspended-load sediments and locally
important bedload sediments to higher-energy bedload
and bedload assemblages containing much far-travelled
material with a glacial input (Figure 14.3). This shift
correlates with a switch from low-amplitude climatic
change dominated by the 21,000-year precession cycle
to moderate-amplitude climatic change dominated by
the 41,000-year tilt cycle. The low-amplitude, high-
frequency climate lasted through the Pliocene to about
2.6 million years ago, and the moderate-amplitude,
moderate-frequency climate from 2.6 million years ago
to about 900,000 years ago. It seems that the shift from
low to moderate climatic variations, and especially the
trend towards a colder climate, would have favoured
the operation of cold climate processes, such as gelifluc-
tion and glaciation. Peak river discharges produced by
seasonal meltwater under this climatic regime were able
to carry coarse-grained sediment along river channels
and through river catchments as bedload. The longer
duration of the climatic variations would have given
enough time for gelifluction and other slope processes to
take material from hillside slopes to valley bottoms, and
for glaciers to develop to a large size and subglacial mate-
rial to reach the glacier margin. Such conditions would
enable material in the upper reaches of river networks
to arrive at the lower reaches. It seems likely that the
nineteen orbitally forced cold episodes in the 800,000-
year-long period dominated by moderate-amplitude,
moderate-frequency climatic variations allowed bedload
to move from the upper Thames catchment in Wales
and an inferred Ancaster river in the Pennines to the
western coast of the North Sea in East Anglia. Similarly,
in cold episodes during the next 1.3 million years, bed-
load moved through the river systems. The arrival of
the Anglian glaciation some 480,000 years ago, with
ice up to 1,000 m thick that reached as far as London
and Bristol, was associated with large-magnitude, long-
duration 100,000-year eccentricity cycle driven climatic
changes. It radically altered the catchments and the
topography.
Alluviation in the Holocene USA
Prior to about 1890, American geomorphologists
ascribed
alluvial river terraces
to movements of the
