PLEISTOCENE DEPOSITS IN BILLERICAY, ESSEX 171
(No. 7) showed that the gravel was cross-bedded on a large scale
and at one point the cross-bedding was strongly contorted and
overturned. The junction with London Clay was also overturned
in conformity with the contortions. A comparison with Clayton's
(1957) analysis of the glacial gravels of further north strongly
suggests that this upper sheet may be part of the Hanningfield
gravels. The lower gravel has been found as low as 150 feet (46
m) O.D. This corresponds closely with the height of an outflow
channel for Hanningfield melt waters recognized by Clayton—it
also corresponds with Clayton's estimate of the base-level of the
pre-till surface. In addition the small streams that dissect the
gravel run almost parallel to the base surface of the spread. It
seems therefore that the present land surface along the northern
edge of the Crouch Valley may be close to the pre-Hanningfield
topography: an inference supported by the occurrence at locality
8 of a lobe of glacial gravel that transgresses the Bagshot Sand-
London Clay junction on a steep slope.
4. Discussion
At Billericay the matrix of the Pebble Gravel was not derived
by comminution of the pebbles. Indeed, the pebbles were almost
certainly little abraded in getting to their present position. They
were, therefore, probably not carried far before deposition, were
rapidly set down and not much reworked after deposition. Thus
they may be in part derived from the slightly higher level pebble
gravels in the Essex area. This hypothesis would be very much
in keeping with the concept of a Pleistocene lake or sea regressing
towards the east and north-east and with the conclusion reached
by Wooldridge (1926, 1960) that the gravel sheets are marine
rather than fluviatile. There are several dissected, horizontal
benches preserved between 200 and 350 feet (61 and 107
m) at both Laindon and Billericay, and these may reflect pulsed
lowering of water level towards the base level of the Hanningfield
Till. However, the presence of large unabraded nodular flints
casts some doubt on this hypothesis of littoral origin for the
Gravel. Wooldridge (1923) found such flints in the Pebble Gravel
at Rayleigh and suggested that they may have been deposited
from floating ice. At Billericay, precisely the same argument
could be used. On the other hand, the Grays-Thurroek Anticline
of mid-Tertiary age is but a few miles to the south and the late-
Tertiary stripping of this structure could have exposed flint-
bearing chalk and overlying pebbly Blackheath Beds on the con-
temporary land surface. Both the nodular flints and the pebbles
of Blackheath type could have been obtained directly from this
source. The frequent occurrence of glauconitic and ferruginous
sands in the lower Tertiaries of Essex may be the source of part
of the ferruginous sandy matrix of the gravel, but the cherts and
vein quartz pebbles appear to have come from much further
afield.
The bottom structures of the gravel may well be interpreted
as resulting from strong current or tidal scour. But it seems likely