76 CONTRIBUTIONS TO PLEISTOCENE GEOLOGY
contents were being worked for brickmaking just north.'' With regard to "the
one noticed by Prof. Hughes," Mr. Whitaker says (op. cit., p. 411;) :—"I have a
drawing of part of the section in the great pit east of Belmont Castle, made by
Prof. T. McK. Hughes, presumably many years ago, when he was on the
Geological Survey, in Kent, which shows a hollow of Drift cutting deeply into
the Thanet Sand, and consisting of the following beds, to a total depth of nearly
20 feet :—
Soil, with patches of gravel towards the base.
Irregular, wavy beds of brown earth and sand, passing down into grey clayey
sand, with, at one part, a layer of brown clay at the base.
False bedded, coarse yellow sand, with patches of gravel (chiefly pebbles)
at the base."
As noted above, the direction of the channel is north and
south, or approximately at right angles to that of the course of
the Thames. The deposits which fill it are totally unlike those
laid down by the Thames in this district. The gravel at the
base gives us the best evidence; it is composed almost entirely
of black flint pebbles derived from the Eocene Strata, with a few
subangular flints, obtained probably from the destruction of
some of the Thames gravels in the neighbourhood. Hardly a
quartz pebble occurs, and practically none of the other accessory
constituents commonly found in Thames gravels. This gravel
looks as though it had been derived from the Lower Bagshot
beds, which occur in the Brentwood district to-day not far from
the present source of the Mardyke and its little tributaries, and
is mixed with just that sort of material which that stream would
have gathered on its way hither. The clays, too, bear a very
striking resemblance to the Woolwich and Reading clays
which occur to the north. From these facts we feel justified in
saying that the beds filling the channel are markedly distinct
from those forming the High Terrace of the Thames, and that
they in fact represent the ancient course and its contained
deposits of the Mardyke.
The first fact that strikes one on a visit to the district
between Stifford and Brentwood is the enormous amount of
denudation which it has suffered since High Terrace times.
Between the gravel-capped hills to the south, which rise to just
over 100 feet, and the London Clay Hills with their crests of
Bagshot and Glacial Beds at Warley and Brentwood, which are
well over 300 feet in height, is a broad valley whose floor in some
places is less than 20 feet above Ordnance Datum. It is clear
that in High Terrace times this valley did not exist, for it is the