266
THE ESSEX NATURALIST.
The Wid, tributary to the Chelmer-Blackwater system, is
anomalous in that it flows northwards against the regional dip.
There seems no doubt that on the surface of the 200-ft. plain it
flowed southwards through the Herongate gap to the Thames,
its later reversal being due to capture by the subsequent River
Cann, the Mar Dyke, still a considerable stream, representing
its beheaded trunk.
The most easterly of the definite ice-lobes, that of the Han-
ningfields, may be briefly dismissed, for its relations are in all
respects identical with those already noted farther west. All
the drift lies at a level of about 200 ft. on a marked platform
overlooking the present Crouch valley, on the further side of
which, and at the same elevation as the glacial deposits, occur
the fluviatile gravels of the Rayleigh Hills. Of the outwash of
the Hanningfield lobe few certain traces remain. It is
significant that no glacially-transported debris reached the
Rayleigh Hills, less than six miles from the presumed ice-edge,
and this strongly suggests that the depression, of which we have
found evidence at Upminster, passed north-eastwards along the
present Crouch valley, so as, in effect, to trap the outwash and
to prevent its further passage southwards. This conclusion
receives strong support from the fact that round the Tolleshunts,
at Mersea Island and Brightlingsea, the glacial outwash of the
Blackwater and Colne valleys descends below 100 ft.
Having now gained some idea of the pre-glacial topography
in the neighbourhood of the ice-edge, we may consider the
main tract of Boulder Clay country lying to the north and
east. Here we are largely dependent upon well-records for know-
ledge of the sub-glacial surface, but fortunately these are
sufficiently numerous to yield a definite picture of the buried
landscape. The level of the sub-glacial floor at various points
is shewn by the figures15 on the accompanying map (fig. 2). It
is seen that there is a widespread 200-ft. platform beneath the
drift, which, however, gives evidence of appreciable trenching
by the pre-glacial rivers. The existence of a pre-glacial valley
of the Rib has already been noted. A similar drift-filled trough
extends, as shewn by the figures, along the line of the Stort
valley, and deepens northwards into the well-known buried gorge
of the Cam. A further suggestive point is the occurrence of an
15 Some of the figures are generalized, representing a group of adjacent wells.