RIVER DEVELOPMENT IN ESSEX. 247
the high-level gravels at Danbury (365 ft. O.D.), it refers to a
date earlier than that dealt with by Sherlock and Hawkins,
but Gregory implies that a similar plan persisted until the later
stage, for he takes the Ouse-Lea-Chelmer through the broad
depression which leads eastwards from the valley of the lower
Stort and so into the Chelmer lowlands. The general floor-
level of this depression (beneath the glacial deposits) is 200 ft.
O.D. and thus clearly it is a later surface than that preserved
on the hill-top at Danbury.
It is not our present intention to discuss these several views
exhaustively. In fact, as noted above, the different recon-
structions refer to different dates and they are not necessarily
mutually exclusive. There is other evidence that in Pliocene
times—the epoch of the Pebble Gravel—the Thames system
was of the general form which Gregory pictures for the Danbury
stage, though the detailed reconciliation of his views and those
published elsewhere on the Pebble Gravel presents some diffi-
culties. Leaving aside these older conditions, our concern is
with the course followed by the Thames immediately prior to
the main glaciation or during its earlier stages.
In this connection the crucial area is undoubtedly the Henley
loop, where the two drift-belts mentioned above overlap and
supplement each other's record. Starting here on the Flood
Plain of the present Thames, we may pass up in regular succession
to the Taplow terrace, the Furze Platt terrace of Mr. Le Treacher
and the Boyn Hill terrace. All the gravels above the latter
terrace (and indeed some which occur at that height and which
should be included with the Boyn Hill terrace) are coloured
pink on the maps of the Geological Survey and are very generally
regarded as "glacial gravels." But, as noted many years ago
by H. J. O. White,2 they resolve themselves on close examination
into a series of well-individualised terrace-spreads which evidently
continue the record of the Thames backward in time and upward
in elevation until a height of about 400 ft. is reached. Now, as
we have seen, the two drift-belts diverge abruptly at the line
of the lower Colne valley, and herein must lie the major evidence
in favour of Sherlock's views. If we confine our attention to
gravels at 300 ft. and higher, we can trace a fairly continuous
2 The Geology of the country around Hen]ey-on-Thames (Mem. Geol. Surv., 1908).