THE PLIOCENE PERIOD IN WESTERN ESSEX. 257
Blackmore. Records of their occurrence in still other localities
would be a most valuable contribution to the vexed question of
the older Pliocene Period in Essex. It is possible, though as
yet not proved, that these red sands are a remnant of a wide-
spread formation referable either to the Lenham Beds proper—
or to the Pebble Gravel group of slightly younger age.
3. Late Pliocene Times. The 200-FT. Platform.
If we are correct in attributing the Pebble Gravel to the
work of an immediately post-Lenhamian river-system, it is clear
that we must recognize at least two stages of post-Lenhamian
uplift. The first was relatively of slight amount, merely elevat-
ing the Lenham Channel into land, and thus originating the
regime of the Pebble Gravel, but the second was more marked,,
causing a phase of downcutting in the river-system, during
which the landscape was moulded into a form essentially identical
with that which it now possesses. This was the second cycle
of denudation in S.E. England, the first having culminated in
a peneplain at the end of Miocene times. The Lenhamian
transgression intervened between the two cycles. The second-
cycle has continued through Pleistocene and later times, during
which other uplifts have occurred. The record is therefore
complex, and it is possible to recognize several pauses in the
downcutting process. Throughout Essex, and, indeed, over a
much wider area, there is a marked platform at 200-250 ft.
which seems to mark such a pause, representing the general-
base-level reached by the rivers of the district at or near the
end of Pliocene times.
In the south of the county, beyond the limit of the Glacial
deposits, the land is low and it is only in the plateau of the
Rayleigh hills that any remnant of the 200-ft. base-level is
preserved. Here there are extensive spreads of chert-bearing
gravel resting on the platform, and of these the writer has
already given some account.8 It must suffice here to point
out that they represent material carried northward by the
Medway at a date prior to the excavation of the Thames valley
as it now is, but later than the stage represented by the higher
and older Laindon gravels.
8. Proc. Geol. Assoc, vol. xxxiv. (1923), p. 320.