4 THE ESSEX NATURALIST.
north of the Thames. Some of this southern material from the
high-level gravels might well find its way into the later Palaeo-
lithic gravels of the coastal plain that lie near or below the
contour of 100 feet O.D.
In this event the southern material would inevitably be
greatly diluted in proportion.
Along the Essex coast the Kentish material is, on the con-
trary, more abundant in the Palaeolithic gravels than it is in the
high-level gravels, and therefore it must represent an independent
and fresh source of supply. This is notably the case with the
lowest deposits of all, such as those of the Clacton and Lion
Point channels, which descend below the level of the lowest
spring tides. In these channel deposits we find not only Green-
sand chert, but also Bullhead flints from the basement bed of
the Thanet Sand, and the peculiar marbled or mahogany flint
characteristic of Northern Kent. These materials are found
in great quantity, and are remarkably fresh and unweathered.
The white cortex of the flints is occasionally nearly as fresh as
that of the flint nodules in a chalk pit. The friable shelly
limestone that forms the basement bed of the London Clay also
occurs in the Clacton channel, and I found one block measuring
9 x 7 x 5 inches.
The underground surface of the Chalk lies at about 200 feet
below O.D. at Clacton, and although this underground Chalk
surface rises fairly rapidly towards Harwich, there is no source
from which this associated group of materials could be obtained
except by rapid river transport from the Lower Thames. It is
quite clear that at the time of which we are dealing a large part
of the North Sea was dry land, and the Thames turned northward
and flowed along what is now the Essex coast.
The Thames Terraces.
As the older river deposits of the Essex Coast are thus a direct
continuation of those of the River Thames, the Thames deposits
must be our basis of correlation.
The researches of Mr. Llewellyn Treacher in the Maidenhead
district have shown that the highest terrace of the Thames, the
terrace of Boyn Hill, yields only the primitive group of palaeo-
lithic implements, comparable with the "Oldest Class" of
Worthington Smith.