312 THE ESSEX NATURALIST
Sandy Gravel—
(a) light brown sand with very little
gravel ............................................. 8" 3
(b) Gravel with sand, pebbly but pebble
size smaller than section ref. 2 ............ 10" 4
Sand, hard bedded sand, deep brown with a
reddish tinge .................................... 19" 5
78
No evidence of chalk was to be seen and borings near this
point have indicated that chalk will be contacted at a point
nearly five foot lower than the base, or apparent floor, of the
subsidence. The floor of the subsidence appeared undisturbed,
and the sand spread was possibly quite thick and the texture
was the sarae even on excavating a further foot. The particle
size grading of this sand was interesting: 95 % of the sand was
finer than 25 B.S. mesh sieve, 35 % finer than 52 B.S. mesh sieve,
and 5% finer than 100 B.S. mesh sieve. It can be seen that this
is quite a fine sand, but coarser than the Thanet Sand, which
has a much greater percentage finer than the 100 B.S. mesh sieve
and this is obviously the sandy facies of the Taplow Terrace
deposits.
The thickness of the first gravel bed was a little irregular and
throughout the section below the soil profile, a noticeable
efflorescence was to be seen around the rounded pebbles.
It was probable that the base of the section was only a plug
blocking the shaft, and this plug may in time subside into a
profile similar to that described by Holmes. The original hole
was probably a denehole which was plugged up at some unknown
time—maybe centuries ago, and although the supports had rotted
long ago the accumulative effect of constant traffic with attendant
earth vibrations on the A. 13 had loosened the plug and the force
applied suddenly at this point in a very heavy storm would
start the subsidence. It will be noted that all these subsidences
are located at points where the field is adjoining a road and it
appears likely that earth vibrations caused by vehicular traffic
are one of the major causes in loosening these plugs.
Mr. Meeson reported to the Royal Archaeological Institute in
1869 (in Holmes, 1887) 'that deep cavities known as "daneholes"
existed in every field in the neighbourhood of Grays Thurrock,
below which there was a substratum of Chalk'. The evidence
does indicate a relationship between these subsidences and dene-
holes, but these holes do not appear to be of the same complexity
of those at Hangman's Wood, Grays.
The geological sequence at the recent subsidences is that
capping the Upper Chalk (zone of Micraster coranguinum) uncon-
formably. At this point are the Taploy Terrace Gravels and the
Thanet Sand is not encountered above the chalk for at least a
mile eastwards.