GLACIAL STRATIGRAPHY OF WEST ESSEX 323
its upper weathered zone) and enclosing abundant fragments of
chalk, flints and other erratics in angular to rounded conditions.
The clay matrix is notably sandy in three areas — Little Halling-
bury (111-113), Birchanger (128-132) and Stanstead Park (142-145)
— which probably accounts for erroneous mapping of "Glacial
Gravels" by the Geological Survey in these localities.
A yellow-brown to brown stony loam is found ubiquitously at
the surface signifying the upper zone of decalcification. Apart
from its customary depth of 1 to 2 feet, this leached layer deepens
in places to 12 feet or more, while occasionally it may be com-
pletely absent owing to surface erosion. At greater depth (from 5
to 35 feet) a change in colour and consolidation characteristics
takes place—from firm brown or brown-grey into stiff grey silty
clay. This junction indicated on the cross-section (Fig. 1) marks
the maximum depth of weathering influence and must to some
degree reflect the shallower surface leaching profile. Intense
stream erosion has exposed unweathered boulder clay at the
surface just north of 105.
Despite their interruption by impersistent sand and gravel
seams (discussed below) all borehole records of appreciable depth
demonstrate a consistent and predictable change in colour and
consolidation with depth i.e. no subsurface weathered horizons
were detected. This is taken to imply that the till sheet as a
whole was laid down during a single phase of deposition and has
subsequently undergone a common loading and weathering his-
tory. 'Common' weathering however, does not imply 'uniform'
weathering, for it is certain from the section that neither surface
leaching nor maximum weathering influence operates at a con-
stant and spatially uniform rate, else they would parallel the
topographic profile more exactly. Local variations in surface
run-off and the level of the water-table will necessarily impose
different rates of weathering within the same till sheet. On this
question Thomasson (1961 p. 295) adds that "tills of similar age
may vary considerably in chalk content and texture, which will
obviously affect the ease with which decalcification takes place."
Consequently, the surface condition of boulder clay at any one
point may bear no constant relationship to the age of the till as
a whole; in other words, decalcification is not a direct function
of time elapsed since glacial deposition.
These considerations have direct bearing upon the validity
of the scheme of Chalky Boulder clay subdivision proposed by
Clayton (1957). His succession, based on the type sites of Chelms-
ford and Harlow, is as follows: —
6. Springfield Till
5. Chelmsford Gravels
4. Maldon Till
3. "Long period of erosion"
2. Danbury Gravels
1. Hanningfield Till