70 CONTRIBUTIONS TO PLEISTOCENE GEOLOGY
have a quieter period indicated, during which the river occupied
this part of its valley, which probably formed some backwater
in which the Anodon peacefully flourished, and, lastly, the water
of the river again retired, only visiting this spot when in flood,,
and depositing the loams forming the upper part of the section.
The stage in the history of the Thames Valley marked by the
Middle Terrace was brought to a close, and a further epoch of
active erosion was opened by the further lowering of the
effective base-level of erosion. This movement does not seem
to have gone very far at first, for just below the level of the base
of the Middle Terrace occurs evidence of a pause in what is
known as the Third Terrace". In entering into this part of our
subject we are handicapped to some extent by the fact that the
deposits of this stage are comparatively but little known, and
where they have been studied they have usually been classed
with the Middle Terrace.
Between Purfleet and Gray's Thurrock, to the south of the
high-road generally, and between it and the Holocene Alluvium,
we meet with a thick deposit of coarse gravel, sections in which,
were described in Part I. at p. 344. In that part we classed this
gravel with the Middle Terrace, but the further study we have
made of it shows in our opinion that this view was wrong, and
that it really forms a Third Terrace. In the Geology of London,.
vol. i., p. 419, Mr. Whitaker, speaking of the tract between
Purfleet and Grays, says :—"When the Drift here was mapped
no one would have thought of drawing its boundary so high up-
the slope, in the absence of sections, and that line was then
drawn, by feature, at a lower level, where one expected the
Drift to end off."
When we were working at our field map of the district we
endeavoured to trace the boundary between the brickearth series
and the gravel series, and this we did easily by feature and the
aid of the sections, the line which we drew agreeing very closely
with the line on the Survey Map. On examining the tramway
sections wherever they were clear, at their southern ends we
invariably found that the gravel abutted against the brickearth
series, and that in no case did the latter rest upon the former.
Furthermore, it was evident that the base of the gravel was much
below the level of the base of the brickearth series, i.e., there is
9 Figure 2, "."