THE PLIOCENE PERIOD IN WESTERN ESSEX. 267
elongated sub-glacial depression along the northern side of the
Chelmsford Road, between that place and Colchester, immediately
behind the ridge of Tiptree Heath. There can be no doubt that
this depression was in part of pre-glacial fashioning, for the
upper as well as the lower surface of the drift-sheet descends
into it. It represents in fact a strike-valley comparable with
the Upminster depression along the south of the ice front. At
the same time it is probable that it was the locus of excep-
tionally severe glacial erosion due to the piling of ice behind
the Tiptree fender. Thus at Kelvedon a well-record gives
the sub-drift surface at —100 ft. O.D. This seemed an improbable
figure to Whitaker,16 who suggests that the thickness of the
drift was over-estimated. However, when we consider it in
the light of the figures for neighbouring localities, it seems
likely that the record is valid.
The 200-ft. platform can be traced north-eastwards beneath
the Boulder Clay as far as Great Saling, west of Braintree.
Beyond this the general level drops slowly eastwards. Thus at
Braintree and Sampford it is 180 ft., while on the eastern side
of the Colne valley at Halstead it is 170 ft. In the case of
isolated figures like these it is impossible to be certain that the
true plateau surface has been encountered, but it is certain
that it descends eastwards towards the East Anglian sea-board,
as is apparent from the record of the Suffolk wells. The figures
available indicate a gentle and regular eastward warping, and
the first signs of this warping appear along a N.W.—S.E. line
running through the Braintree district.
A full discussion of the age and origin of the 200-ft. platform
would involve a wider area than that embraced by the present
paper, but certain of the more important conclusions may here
be noted. A marine origin for the platform is quite inad-
missible for the region under notice, and since the feature extends
beyond the glaciated area it cannot be attributed to the work
of ice (see also p. 262). In regard to age, the Essex evidence is
particularly important in that it establishes the platform as
definitely pre-glacial, in the local sense at least. The Roding,
the Wid, etc., had entrenched themselves to some extent in the
platform before the advance of the ice (p. 265), so that some con-
siderable interval elapsed between the completion of the plain
16 Water Supply of Essex (Mem. Geol. Surv., 1916), p. 202.