320
THE ESSEX NATURALIST
4. Pebble Gravel.
3. Bagshot Beds.
2. Claygate Beds.
1 London Clay.
The presence of high-level boulder clay and Pebble Gravel
at Coopersale Common has been known for a century now (since
Whitaker first mapped the area). More recently, Clayton (1964)
apparently confirmed the old Geological Survey boundaries and
referred the boulder clay to Hanningfield Till type. Yet neither
author identified a local "head" facies as a separate formation,
preferring rather to regard such sandy, stony clays as residual
weathered products of the other two drifts.
The main evidence for Chalky Boulder clay is in the northern
extremity of the section (69) where it comes on in force towards
Cripsey Brook. Within the Coopersale Common area itself though
chalky till is subordinate, being detected in only one borehole
(GB 27) as follows: —
to
topsoil 9"
Surface Drift Soft brown silty clay 1'6"
Stiff mottled brown and grey silty
Boulder Clay clay with stones and chalk frag-
ments. 4'6"
Firm mottled brown and grey silty
"Head" and sandy clay containing scat-
tered stones. 13'0"
Firm brown fissured silty clay 14'9"
London Clay Stiff blue-grey fissured silty clay.
At the north end of the section, stratigraphic relations are
a little complicated, and it may well be that "head" material
consists in part of decalcified chalky till. However, the super-
position in the above description is significant since, in terms of
consolidation and chalky content, it represents the reverse se-
quence of a normally weathered boulder clay. This implies that
the lower deposit is not genetically connected to the Chalky
Boulder clay.
Further south, Pebble Gravel is encountered some 10-15 feet
below the surface, and consists of characteristically coarse sand
and gravel in a matrix of brown fine clayey sand or sandy clay,
resting uncomfortably on the sandy transition beds of the Eocene
clay. Dines and Edmunds (1925 p. 22) state that the Pebble
Gravel here is on average 3-4 feet thick and seldom over 6 feet;
the present section shows it to be rather greater in thickness,
being on average 6 to 8 feet thick, and attaining a maximum fi-
gure of 12 feet.
Also contrary to previous geological mapping, the section
indicates that Pebble Gravel outcrops only briefly at the sur-