160 THE ESSEX NATURALIST.
It was at one time believed that all these pebble gravels were
remnants of a sheet of marine shingle. But their local variation
within short distances is out of harmony with marine distribution.
Moreover, the various local types form bands, running in a
roughly S. and N. direction across the country. As many of the
stones they contain have, without question, been derived from
the Weald, there seems no reasonable doubt that they represent
a series of north-flowing streams from Kent and Surrey, before
the lower Thames took its present course.
Flint Pebbles and Flint Nodules.—The earlier gravels
of the Thames Basin are mainly composed of ready-made
Tertiary flint pebbles. As denudation reduced the available
supply, and exposed larger areas of chalk, so fresher flint nodules,
together with angular and sub-angular fragments, increased in
relative proportion. It would appear that the pebbly type of
gravel lasted to a later date in S. Essex, and in the lower Lea
Valley, than further W. This is in accordance with the theory
of the former course of the Thames, because all the chalk flint
from the Chilterns and the Upper Thames basin would be
swept along the Hertford-Ware route. The supply of fresher
chalk flint from the Kentish tributaries was relatively small in
bulk, and it was the Great Eastern Glaciation that transported
large quantities into Essex. It follows that any gravel in our
region which contains a large proportion of fresher flint is not
earlier than the Great Eastern Glaciation.
Bunter Drift [Sherlock, 1912, p. 204, etc.].—Certain
gravels, which may have been an outwash of the "Western
Drift," swept through the Goring Gap at about 400 to 450 O.D.,
and these contain more Bunter pebbles than flint. Enormous
quantities of these pebbles must thus have been introduced
into the London Basin as so many are present in later gravels,
and in the glacial deposits.
The Upper and the Lower Gravel Trains.—These are
the names given by Wooldridge [1938] to two stages that
come between the Bunter Drift and the 200-feet Platform to the
W. of London.
In our region there are two well-defined datum lines, namely
the high-level Pebble Gravel stage, and the 200-feet Platform
(q.v.). Between these clear stages there was a period of down-
cutting by the rivers to a vertical depth of about 200 feet. There
are several gravels that come within this period, and as examples
I will take those at Cheshunt, Buckhurst Hill and Rayleigh.
The Cheshunt Pebble Gravels.—There are rather large
patches of pebble gravel in the Cheshunt and Hoddesdon