244
RIVER-DEVELOPMENT IN ESSEX.
By Miss B. R. SANER, B.A., and S. W. WOOLDRIDGE, D.Sc., F.G.S.
(Read 24th November, 1928.)
THERE is an underlying unity in the geology of Essex which
results from the fact that it lies across the open or sea-
ward end of the London Basin and that, as a consequence, most
of its Tertiary rock materials have been carried into it from the
west along the line of the main syncline. At the beginning of
Tertiary times it lay to seaward of the great Eocene delta and
the rocks of this period represent the finer-grained sweepings
of land to the west, mixed with varying amounts of material
from the Weald and the Midlands. At a later date the
geographical setting, though radically modified in detail, was
the same in its major features. The sea had retired and the
ancestor of the Thames made its way seaward across the county.
It thus becomes necessary to test all ideas of river-development
in the central and western parts of the Basin by their application
to Essex, a region somewhat neglected in this respect.
If we take a general view of the post-Pliocene drift deposits
of the London Basin we see that, with the exception of some
relatively insignificant patches, they fall within two main belts.
Near the western end of the Basin, where the Thames emerges
from Goring Gap, these two drift belts overlap, but they diverge
markedly eastwards. The southern belt, which comprises the
several terrace deposits of the Thames, follows the course of
the river to its mouth, though extending over a much wider
area than the present flood-plain. The northern belt runs
steadily north-eastwards, becoming completely separated from
the belt of terrace-gravels near the line of the lower Colne valley
and passing thence through the Vale of St. Albans to the margin
of the Boulder Clay country on the Essex border. The gravels
of this northern belt, while showing no obvious relation to the
glacial deposits in the west, become closely involved with them
in the east, and, broadly speaking, appear to pass under the
Boulder Clay on the edge of the truly glaciated country.
The record of the Thames during the succession of recognised
terrace-episodes is known in outline. It is known that a large
part of these deposits accumulated during an important inter-
glacial period and they give evidence of a warm climate