PALAEOLITHIC INDUSTRIES, CLACTON AND DOVERCOURT. 3
wide sheets of gravel and sand at abnormal levels, scour out
new channels and probably silt them up again and re-arrange
parts of the older deposits, much to the confusion of the simple-
minded geologist who expects the facts to be in accordance
with text-book orderliness.
The cutting and re-silting of tributary channels of later date
across the older terraces is also a fruitful source of difficulty in
interpretation.
As an illustration of the latter complexity one may take
the Wansunt Channel described by Messrs. Chandler and Leach1.
This is a channel cutting into the Dartford Heath gravel, filled
up with a later gravel and yielding a much later flint industry
(namely that of the "Twisted-ovate" group) than that of the
Dartford Heath Gravel itself.
In illustration of the former complexity there are many river
deposits at the same level that are not of the same date. For
example, there are the two noted deposits of the Grays district, or
more properly of the Thurrocks, namely those of Little Thurrock
and West Thurrock respectively. These deposits are described
by Messrs. Hinton and Kennard in our Journal for 1900, and,
although they both belong to the Middle, 50-foot, or Taplow,
Terrace as it is variously called, the fauna shows them to be of
different dates.
The Kentish Chert of the Essex Gravels.
It will be remembered that Prestwich concluded that there
was an early "Southern Drift" prior to the Glacial Period, but
exactly how long before the glaciation still remains uncertain.
During the Southern Drift stage the chert from the Greensand
exposures around the margin of the Weald, together with other
rocks from the South of England, was drifted at a high level
across what is now the Thames Valley into Southern Hertford-
shire and Southern Essex.
Possibly the "Southern Drift" represents a time when the
Thames flowed at a level of some 400 feet, and considerably to
the north of its present course. The southern tributaries would
then bring the Kentish material into the valley, beyond the
present course of the river. However that may be, a certain
amount of southern material occurs in the high-level gravels
1 Proc. Geol. Assoc., vol. xxiii., 1912, p. 102.