322
THE ESSEX NATURALIST
As to its origin, three explanations may be considered. They
are (a) in situ weathering of the underlying formations, (b) soli-
fluctional disturbance, or (c) glacial derivation. As far as the
first is concerned the "head" undoubtedly contains a high pro-
portion of locally-derived Tertiary and Pebble Gravel material;
but it must be distinguished from underlying Pebble Gravel as
a separate and later deposit by virtue of its relatively high clay
content. Similarly, a solifluctional origin must be precluded since
no higher source of material is available. Nevertheless, clear
evidence of slumping to the south does point to postdepositional
surface disturbance of a periglacial nature. In this context, a
consideration of the patch of till (decalcified) south of Theydon
(46) is pertinent. For here boulder clay overlies a pocket of
gravel at about 100 feet O.D. which may be assumed to be equiva-
lent to that 750 yards to the west mapped by the Geological
Survey as Taplow Terrace gravel. Since we cannot admit the
local till to be of post-Taplow age (i.e. Weichselian) this is there-
fore a clear case of a reconstructed drift mass derived from the
higher Theydon till by solifluctional disturbance during the
Weichselian. The "head" material may have been superficially
disrupted during the same phase.
The third alternative — that of glacial origin — cannot be
demonstrated with any certainty from the present data, but the
clay's amorphous structure as noted above, is certainly remini-
scent of glacial till. That the deposit represents the ground
moraine of a western glaciation intermediate in age between the
Pebble Gravel and the Chalky Boulder clay seems to provide the
most satisfactory explanation for the available facts.
Turning now to the glacial deposits sensu stricto, they can
be considered under three headings:—
1. Chalky Boulder clay.
2. Sub-boulder clay sands and gravels.
3. Intra-glacial sediments.
Chalky Boulder Clay.
The Chalky Boulder clay forms a virtually continuous sheet
of till from Coopersale Common as far north as Wendens Ambo,
with outliers to the north (on the summit of the Chalk escarp-
ment) and south (at Theydon Bois). On average it is 30 to 40
feet thick and rests directly on the solid formations, except
between Pincey Brook and Great Hallingbury where it is under-
lain by thick fluvioglacial gravels. The sub-boulder clay surface
has considerable range of altitude; beginning at 330 feet O.D.
near Great Chesterford, it drops to below 140 feet near Sheering
(where boulder clay exceeds 85 feet in thickness), ascending to
340 feet on the Epping Forest ridge and finally falling to 150
feet in the vicinity of Theydon.
Typically, the deposit is described as firm to stiff silty clay,
varying in colour from dark grey (at depth) to yellow-brown (in