Geoscience Reference
In-Depth Information
Recent detailed interpretation of the Wolstonian surface-blanket succession recognises
an early phase of near-ice-sheet river sediments, followed by ice-laid deposits and then
the deposits of a lake that was probably ice-dammed and extensive enough to be given
a local name: Lake Harrison. These deposits are followed by further ice-laid sediments,
and finally by the river-laid deposits referred to above as the Dunsmore Gravels. Al-
though a vivid story of environmental change has emerged from this locality, it is still
not clear which of the Ice Age cold episodes is represented.
FIG
177.
Former rivers of Area 9.
Some of the early surface-blanket, water-laid deposits of the Avon valley have
been interpreted as the deposits of a now extinct river that has been called the Bytham
(Castle Bytham is 10 km north of Stamford in Area 15). In Area 9, this river has been
reconstructed as flowing northeastwards, roughly parallel to the present Avon, before
turning eastwards near Leicester (Fig. 177). It is then supposed to have continued to
flow eastwards across the present Fens before turning southwards. Along with the an-
cestral Thames, the River Bytham is thought to have been drastically re-routed by the
arrival of the major ice sheet of the Anglian cold episode, about 450,000 years ago.
Other recent work on the landscape history of this catchment is based partly on the
recognition that pebbles in old surface-blanket gravels near Oxford have been derived
from New Red Sandstone outcrops to the northwest of the River Severn, and could not
have been carried by rivers over the present Cotswold Hills drainage divide. Indeed,
the gravels lack pebbles of material that would prove that the Cotswolds were available
for erosion at that time. The suggestion is that very active erosion of the weak mud-
stones of the younger New Red Sandstone and Early Jurassic caused the lower River
Severn to cut backwards and northwards up the Bristol Channel. This erosion was fo-
cused along the valley of the Bristol Channel, decreasing the load on the local crust and
