Geoscience Reference
In-Depth Information
Investigations of Quaternary deposits in the Vale of St Albans (
e9
) have identified
glacial till overlying gravel deposits that appear to have been laid down by an ancestor
of the River Thames. This suggests that, rather than flowing generally eastwards as it
does today, the early River Thames used to flow in a generally northeasterly direction
towards Essex and Suffolk. When the Anglian ice later advanced into the area from the
northeast, the path of the original Thames was deflected to approximately its present-
day position some 30-40 km to the south, creating two glacial lakes at the ice front as
it did so (Fig. 208). The evidence for these lakes is found in a series of finely lamin-
ated silts and clays in the Ware and Watford (
e10
) areas north of London. The lamina-
tions in these sediments are known as varves, and they are typically formed by annual
freeze-thaw cycles experienced by large lakes in cold regions. By counting the number
of cycles, sedimentologists have been able to establish that the lakes must have existed
for several hundred years.
FIG
208.
Slope map showing the limit of the Anglian Ice, routes of the past and present
Thames and the ice front lakes at Ware and Watford.
With the Thames diverted to the south, a number of large outwash streams de-
veloped at the ice front, flowing southwards into the new Thames. As the climate be-
came milder at the start of the next interglacial, these rivers became charged with large
volumes of meltwater, cutting valleys to eventually form the rivers Colne and Lea as
we know them today. Flow rates in the Thames itself also increased and a number of
very coarse gravel deposits date from this time, carried into position by the fast-flow-
ing waters. Examples include the coarse gravel beds of the Black Park terrace (Fig.
204).
