THE DRIFTS OF SOUTH-WESTERN ESSEX. 161
districts at from 1 to 3 miles to the W. of the Lea, and at levels
between 400 and 200 O.D. I once lived on one of these spreads
which covered about half a square mile, between the levels of
200 and 250 O.D. I saw many temporary gravel pits about
10 ft. deep, and also sewer trenches along the roads. I was
formerly at a loss to explain the considerable train of pebble
gravel capping the spurs between the minor tributaries of the
Lea, but one can now realise that a precursor of the Lea, then
flowing N., had cut its valley down to, or below, the 200-ft.
contour, and the advancing ice crept over its gravels, leaving
them mantled by Boulder Clay.
It is reasonable to assume that these gravels, representing
200 ft. of vertical down-cutting, cover a substantial period of
time.
Rayleigh Hills [Wooldridge, 1923].—The gravels here
also fall into line with the position of the Cheshunt pebble gravels.
They consist of coarse, current-bedded sand and gravel up to
10 ft. thick. Their constituents include flint pebbles of Black-
heath type, together with an abundance of "Cretaceous stones."
They may safely be identified with the Medway, and suggest a
possible link with the Cromerian channel near Harwich, although
the two may not be of precisely the same date.
Buckhurst Hill (220 ft. to 240 ft. O.D.).—There is glacial
gravel here, but I have also seen exposures of the older pebble
gravel as described by Prestwich [18901. I would class
it broadly with the Cheshunt and Rayleigh gravels, but it repre-
sents an exclusively Tertiary gathering ground ; "Cretaceous
stones" and glacial derivatives are alike absent. Thus one
can conclude that the river that deposited it did not come from
the Weald and may have been the Roding, while its date is pre-
Glacial in the local sense. The close similarity between the
Buckhurst Hill and the Roding pebble gravels will be noted in
spite of the latter having been re-arranged at a later date.
The 200-feet Platform [Wooldridge, 1928].—The next
phase in the history of the London Basin is characterized by a
sufficiently long period of stability to allow the rivers to broaden
their valleys near base-level and thus to cut well-marked platforms
at about 200 feet above the sea in our region with a gradient
rising inland. In central Essex it is largely obscured by Boulder
Clay.
I will only refer to one example, namely the Fairmead-
Chingford plain of Epping Forest. This is slightly blocked by
a miniature moraine of glacial gravel which forms Strawberry