THE SUBSIDENCE AT LEXDEN.
5
145 feet to 150 feet. For, as Mr. Fisher remarks, the chalk is lying
nearly flat in the neighbourhood of Colchester. But the depth at
which the chalk lies offers by no means the most conclusive presump-
tion against the existence of a denehole there. This is rather to be
found in the fact that nowhere does the chalk crop out at the surface
nearer than Sudbury and Hadleigh, both on the north; in other
quarters it does not appear at all, at similar distances from Lexden.
Besides, the beds overlying the chalk in this district are both numerous
and variable, and those belonging to the Glacial Drift lie unconform-
ably on the Eocenes and Pliocenes below. Thus, the depth of the
chalk beneath the surface, the fact that its nearest outcrop is about
twelve miles away, and in one quarter only, and the variability and
Fig 4.—From 6-in. Ordnance Map. v Alluvium. g Gravel.
unconformities among the overlying beds, all combine to show the
extreme improbability that any deneholes exist in the neighbourhood
of Lexden.
A glance at the six-inch map (Fig. 4) seems to me to suggest a
much more probable explanation of this Lexden cavity than any of
the three already mentioned. My own opinion is that it was the
result, not of a vertical subsidence, but of a simple landslip. The
river, where it most nearly approaches the site of the cavity, makes a
sharp bend, the current tending to eat into the southern bank, near
which the sinking occurred. Landslips must often have occurred at
this point; but so long as they were small, and only a few feet of