262
THE ESSEX NATURALIST.
responsible for lodging gravels on its summit.10 Harmer,
from inspection of the contoured map, was led to suggest that the
ridge represented the terminal moraine of the "Great Eastern
glacier." This implies that the whole ridge is built of gravel,
which is not the case, the London Clay forming the core of the
hills, though largely obscured by a thick downwash of gravel.
However, interpreted in the spirit rather than the letter, the
suggestion contains a large element of truth.
The glacial gravels of Woodham Walter, Maldon, Tolleshunt
D'Arcy and Mersea Island are remnants of an outwash from the
ice edge, down the Blackwater valley. Similarly, beyond the
north-eastern end of the ridge, the Boulder Clay margin trends
somewhat southward and there are large masses of outwash
gravel in the Colne valley, extending to Brightlingsea and St.
Osyth.
We may turn now to the consideration of the sub-glacial
topography. It may in the first place be remarked that the
erosive powers of the ice in this region must have been strictly
limited. This is proved by the fact that the Boulder Clay is
largely made up of far-travelled material. Autochthonous
debris is of course present to some extent, but its amount is
always small and in many sections it is difficult to recognise it
at all. The feeble erosive powers of the ice in its sluggish ter-
minal portions may be attributed (a) to its small thickness;
(b) to the low gradients down which it moved; and (c) to
heavy loading with transported debris. In the result it appears
that no major modifications of topography were effected by
it in this tract; the sub-glacial features are essentially pre-
glacial.
Dealing first with the area near the ice margin, we may
note that, in the west, the Colne valley lobe passed down a flat-
bottomed trough whose floor over a wide area is at or about
200 ft. O.D. Inspection of a contoured map of the present
surface suffices to establish this fact, but fortunately there
are well-records which enable the level of the true sub-glacial
floor to be determined. This level is shewn at various points
on the map (fig. 2). The sub-glacial surface rises towards
the edges of the Boulder Clay tongue, but the flat floor is nearly
two miles broad. Near the median line of the trough the level
10 See, however, J. W. Gregory, Geol. Mag., Dec. 6, vol. ii (1915), p. 529.