PLEISTOCENE GEOLOGY OF THE THAMES VALLEY. 361
in length. Some are smaller than this, but none are larger. One
may be seen between Palmer's Avenue and Sockett's Heath
trending in a general east and west direction. Another has its
head situate about one furlong to the west of the Small-pox
Hospital and trends, roughly speaking, north and south. The
other valleys which occur in this district all follow either the one
or the other of the directions indicated above. They are wholly
confined to that portion of the district in which the High Terrace
Gravel rests directly upon the Chalk or in which only the Thanet
Sand intervenes between the former and the latter. In the
northern portion of the eastern tract, where the more argillaceous
Woolwich Beds and the London Clay come on under the gravel,
the valleys are absent.
That they do not owe their origin to fluviatile erosion
operating since the deposition of the High Terrace Drift is
evident from the fact that they do not cut through the gravel,
which on the contrary invests the valley bottoms as completely
as it does the high ground round about. This objection also
operates against any agent of subaerial denudation which
involves the mechanical removal of detritus from off the surface.
It seems inconceivable that the valleys could have been fashioned
in the Thanet Sands before the deposition of the drift, for in that
case they would either have been filled up and so would have
given but slight evidence of their existence at the surface, or else
they would have been effaced by the ordinary erosive action of
the river.
The first section that threw any light upon the development
of these valleys was one exposed in a gravel pit at Sockett's
Heath. Here the regularly stratified gravels described in
Section III. of this paper were seen, on the western face of the
pit, to have been bent down inwards from the north and from the
south so as to form a syncline of some fourteen or sixteen feet in
depth and about fifty feet in width. This trough was filled up
with confused and unstratified gravel resting upon equally
confused and irregular sand and loam, the whole of this infilling
material possessing the appearance of having been tumbled in
from above (Fig. 2). All the stones as they approached the
centre or base of the syncline were seen to become perfectly
vertical, which plainly showed that it was a downward move-
ment that had placed them in their present position. The axis