GLACIAL STRATIGRAPHY OF WEST ESSEX 319
M.11 Borehole data
The M.11 Soil Survey data (kindly made available by the
Ministry of Transport) consist of 622 boreholes, drilled during
1966, along a 36 mile route from Woodford to Great Chesterford.
Only the southern part of the complete route is presented here
(Fig. 1). While most bores penetrate to a depth of 20-30 feet,
being spaced on average 500 feet apart, both the density and
depth of borehole coverage greatly increases at certain points
where road intersections are envisaged, thus providing clusters
of more detailed information. The generalised cross-section
(Fig. 1) was constructed by projecting each borehole profile
orthogonally on to the median line of the motorway route. While
certain errors may have been introduced by this method of con-
struction, these are regarded as minimal, and the section as a
whole gives a consistent picture of Stratigraphical relations.
That the original borehole samples were no longer avail-
able for sedimentary and petrographic analysis proved a severe
limitation, so that the present study is based largely on the bore-
hole log descriptions given by the original soil surveyors. In ad-
dition, however, certain standard soil mechanics data were avail-
able, and these have been employed in a discussion of textural
variability in the Chalky Boulder clay.
The succession of deposits
The drift sequence encountered in the bores can be sum-
arized thus:—
(a) Recent alluvium (Roding, Pincey Brook, Stort)
(b) "Brickearth" (Roding).
(c) Terrace gravels (Roding).
(d) Boulder clay and intra-glacial sediments.
(e) Sub-boulder clay sands and gravels.
(f) "Head" (Coopersale Common).
(g) Pebble Gravel (Coopersale Common).
(h) Solid formations.
A comparison of the generalised cross-section with previously
mapped boundaries (plotted on Fig. 1) shows a fair degree of
correspondence, although notable discrepancies are apparent
at Coopersale Common, Cripsey Brook and Birchanger.
Drift deposits capping Coopersale Common (60-70)*
This particular assemblage of deposits, lying towards the
southerly margin of the boulder clay tract, provides the only
evidence for the Eocene-Pleistocene time interval. Two adja-
cent cross-sections are available here and together reveal 6 dis-
tinct formations:
6. Chalky Boulder Clay.
5. "Head"
*figures refer to chainage plotted on the cross-section.