258
THE ESSEX NATURALIST.
(In the present paper it is the northern or glacial area which
particularly claims attention.
It is apparent from inspection of the geological map that
the greater part of the Boulder Clay plain in Essex is excep-
tionally flat, lying between 200 and 300 ft. O.D. Thus in the
west of the county the 200-300 ft. zone on the Boulder Clay is
over 12 miles wide, and though narrower in the east it is nowhere
less than eight miles wide. Further, it may readily be determined
that the glacial deposits have an average thickness of 40-50 ft.,
though marked local variations occur. From these broad facts
it becomes clear that the sub-glacial surface is virtually a level
plain at about 200 ft. O.D. Nevertheless, the glacial deposits
are seen to descend to lower levels in places, e.g., at Upminster
and Chelmsford, whence it appears that the plain had suffered
either differential warping or erosion prior to the advance of the
ice. So much may be gathered from simple inspection of the
map, but a more detailed examination of the facts serves greatly
to enforce and amplify these general conclusions and also to
throw light upon some purely glacial problems. Accordingly,
the following account is offered as a contribution to the Glaci-
ology of the district, as well as being an essential part of the general
argument of the paper.
The accompanying map (fig. 2) shows the inferred position
of the ice-margin in Hertfordshire and central Essex. It is,
to the best of the writer's knowledge, the first attempt to lay
clown this boundary accurately, though a similar map of a
larger area on a smaller scale has been published by R. M. Deeley.9
The line is not drawn simply to enclose the existing outcrops
of the Boulder Clay. Denudation has removed part of the
edge of the original sheet, but even so, such a line would give a
fair approximation to the actual boundary. Surer evidence
may be obtained, however. The greater part of the line has
been fixed by tracing (or projecting) the Boulder Clay against
sharply rising ground. In addition, the intervening tracts of
higher ground have been carefully surveyed for glacial debris.
At the dissected margins of the sheet, and on the lower slopes
formerly covered by Boulder Clay, the ground is littered with
heavy, far-travelled debris, including the ubiquitous Bunter
quartzites. When an area is shewn on the map rising above the
9 Geol. Mag., Dec. 6, vol. iii. (1916), p. 58.