GLACIAL STRATIGRAPHY OF WEST ESSEX 327
recognised; S. V. Wood (1877) first classed them as "Mid-Glacial"
(the Chalky Boulder clay constituting the "Upper Glacial") and
this broad sub-division was followed by Whitaker (1889) and
White (1932). Clayton designated the same deposit "Chelmsford
Gravel" and regarded it, following the earlier views of Boswell
and Harmer, as the outwash product of meltwater streams eman-
ating from the advancing Chalky Boulder clay ice sheet.
The present borehole data reveal that the distribution of
these sub-boulder clay gravels is less widespread and more dis-
continuous in west Essex than previously envisaged. In particular,
the Old Series mapping (Fig. 1) greatly over-simplified the problem
by including many gravels, sands and sandy boulder clays of non-
equivalent age under the blanket term "Glacial Gravel". Clayton's
mapping (1964) is more discriminating, and only one minor error
was noted (at 113-114).
Attenuated patches of chalk and flint gravel associated with
variable sands and sandy clays occur on the buried interfluves, but
the main mass of the deposit (10-40 feet thick) is confined to a
broad depression from east of Harlow (95) to Great Hallingbury
(130). The relations here conform to a similar gravel distribution
in the Hertford area (Sherlock and Pocock, 1924 p. 32). This
"mid-Essex depression" is believed to have been occupied by a
former course of the Thames (Wooldridge and Henderson, 1955),
so that the concentration of coarse sediment may be due to fluvio-
glacial reworking of extensive, pre-existing fluviatile gravels.
Intra-glacial sediments
In the Sheering, Hallingbury and Stanstead areas sands,
gravels and laminated silts are found intercalated with boulder
clay in impersistent, lenticular seams. Sometimes these localised
variations are exposed at the till surface but are not considered
to be the result of in situ weathering or solifluctional disturbance
of the boulder clay itself. Three interpretations are possible: —
(a) That they are the result of multiple glaciation. Since
the sediments in question are wholly or partially
enclosed within lithologically-uniform boulder clay
and since there is no suggestion of an inter-glacial
horizon, this explanation cannot be readily admitted.
(b) That they indicate multiple ice advance and retreat
within the Chalky Boulder clay glaciation and are to
be attributed to the normal ephemeral proglacial
environment giving rise to laterally-variable fluvio-
glacial, lacustrine and ice stagnation deposits.
Several phases of advance and retreat must have
occurred to account for their development at
different levels within the one till sheet.