Geoscience Reference
In-Depth Information
The absence, on land, of bedrock of later Tertiary age suggests that the land sur-
face of Southern England was uplifted at this time (beginning about 20 million years
ago), allowing river and stream erosion to begin to carve the scenery. Indeed, some fea-
tures of the present-day scenery are thought to date from mid-Tertiary times, and the
local evidence for this is covered in detail in the Area discussions that follow.
FIG
96.
Dorset cross-section from the northwest to the southeast, located on Figure 92.
Note the Hampshire Basin and the Isle of Wight monocline.
Modification under Ice Age conditions
The whole of the South Coast Region lies south of the areas covered by ice sheets dur-
ing the Ice Age. However, there is no doubt that surface conditions during much of the
Ice Age must have been periglacial (near-glacial), similar to those of Arctic Canada
and Russia at the moment. The subsoil and upper bedrock would have been perman-
ently frozen (forming the
permafrost
layer), while the upper layer of the soil thawed
during spring and summer and froze in the winter. Weathering processes such as the
freeze-thaw shattering of rocks and down-slope mass movement, due to the unfrozen
top layer of soil slowly sliding over the permafrost layer, began to shape the landscape.
These Ice Age conditions also help to explain the dry valleys that are such a fea-
ture of some local scenery, particularly where Chalk forms the bedrock, for example in
the North and South Downs. The dry valleys were initially carved by seasonal water
flow and slope movement when the ground was frozen at depth, preventing the water
from seeping away into the usually permeable Chalk. When the Ice Age ended, the dis-
appearance of the permafrost layer allowed water to percolate down through fractures
in the bedrock, leaving the valley floors dry. Some Chalk streams today are
winter-
borne
, only flowing when the water in the Chalk is at a high level, and then becoming
dry in the summer and autumn.
