ON A RECENT SUBSIDENCE AT MUCKING, ESSEX. 245
about 100ft., though the depth to the Chalk varies from 8ft. to
150ft.
At the Mucking Hole we have not merely the full thickness
of the Thanet Sand, but also that of the Woolwich and the
Blackheath beds between the surface and the top of the Chalk.
For the strata seen in the sides of the pit evidently belong to
the basement bed of the London Clay. And the Geological
Survey map shows London clay around the spot where the pit
exists, while it evidently would also be found where the older
depressions appear in the field westward, beneath the gravel
forming the surface.
As regards the "Basement-bed" of the London Clay, the
following details may be useful. Mr. W. Whitaker remarks in
his Memoir, The Geology of London and of Part of the Thames
Valley, Vol. I., p. 238 :—"Immediately at its base the London
Clay commonly contains a greater or lesser admixture of green
and yellow sands, generally mixed with rounded flint pebbles,
and not unfrequently cemented by carbonate of lime into
semi-concretionary tabular masses. These mixed beds, how-
ever, never exceed a few feet in thickness, and pass upwards
rapidly into the great mass of the London Clay. To this part of
the formation Mr. Prestwich has given the name of the 'base-
ment bed.'"
The nearest section that I can find which will illustrate the
nature and thickness of the strata between this basement bed of
the London Clay and the top of the Chalk in this district is that
of the well at Broad Hope Farm, about a mile eastward of
Stanford-le-Hope. It is given in Mr. Whitaker's collection of
Essex Well Sections (Part IV.) Essex Naturalist, Vol. IX,,
p 178. For our present purpose a somewhat condensed account
of it will suffice:—