Geoscience Reference
In-Depth Information
Another branch of the northern glacier, keeping to the west of the Lincoln ridge, and
reinforced by the North Sea ice, moved towards Doncaster and up the Trent basin to the
vicinity of Derby, where it met the Derwent glacier, and thence crept southward along the
valley of the Soar into Warwickshire.
In the west, the Cheshire plain was invaded by ice from the Irish Sea which, diverting
the glaciers descending from the mountains of North Wales towards the south, carried a
large number of Scottish and Lake District erratics into the northern part of the basin of the
Lower Severn, heaping them also upon Cannock Chase, and upon the high land near
Wolverhampton.
In South Wales, Dr. Strahan and his colleagues have shown that the ice descended in
great thickness from the Brecknock Beacons towards the Bristol Channel, reaching the
shores of the latter near Swansea, filling the Neath and Taff valleys to overflowing and
rising to a great height on the intervening hills.
Evidence has also been found of the invasion of the southern part of this district by ice
from the Irish Sea, which is supposed to have travelled up the Bristol Channel from west
to east, and to have crossed the Pembrokeshire peninsula from St. David's Head towards
Gower and to the neighbourhood of Cardiff: erratics believed to have been derived from
the first-named locality having been found nearly 100 miles to the eastward of their
probable source.
The depth of St. George's Channel between St. David's Head and Ireland, however,
exceeds 50 fathoms, and the natural course of the Irish-Sea glacier, joined by those
descending the western slopes of the Welsh mountains, should have been southward along
the deep submarine valley opening out to the Atlantic. The distribution of the erratics just
mentioned seems therefore to indicate that the volume of ice, approaching the narrowest
part of St. George's Channel was too great to enable it wholly to escape in that direction,
some of it being forced by lateral pressure to travel eastward up the Bristol Channel.
It seems worth considering whether so important an ice-stream would not have blocked
the entrance to the estuary of the Severn, the result being an accumulation of sedentary ice
in the valley of that river, derived partly from the glaciers of Central Wales and partly
from Atlantic blizzards, which I think, for meteorological reasons, may have prevailed at
that epoch. Such blizzards might have filled portions of the Severn basin, then unoccupied
by moving ice, with masses of snow that would have eventually consolidated.
This view may possibly throw light on the origin of the great alluvial and lake-like plain
of Glastonbury, and of the gorge at Clifton. It may explain also why Arenig boulders
[Ordovician age] have been piled up on the Clent Hills, southwest of Birmingham, to a
height of nearly 900 feet. It is difficult to understand that this could have occurred, if at
that time the Welsh ice could have followed an unobstructed course along low ground
towards the Bristol Channel.
The conditions here sketched out, namely, of ice moving upon Central England from the
sea in a direction opposed to that of the natural drainage, are precisely those under which
Glacial
lakes
with
their
accompanying
over-flow
channels
would
have
naturally
originated.
3.5 Pro-glacial Lakes, Cañon-like Valleys and Gorges
Towards the close of the Pleistocene epoch ice sheets over much of Britain had
blocked the northward flow of rivers in northern and central England, causing their
waters to be ponded back to form extensive pro-glacial lakes such as Lake Lap-
worth and Lake Humber. At about the same time, rivers normally flowing to the
