PLEISTOCENE DEPOSITS IN BILLERICAY, ESSEX 165
This has led to some modification of previously mapped boun-
daries and the recognition of special basal structures in the Pebble
Gravel. The exposures are now concealed or obliterated and it
seems desirable to place descriptions of them on record. They
show that the Pebble Gravel fills channels and circular depres-
sions incised into Bagshot Sand, and that younger glacial deposits
occur topographically only a few feet below these gravels. The
two Pleistocene deposits have not been found in contact. The
outcrops of these various beds are depicted in Fig. 1. Here only
the junction between the London Clay and a 'Bagshot sand facies'
is shown, since the local Claygates contain many sand layers of
Bagshot-type and are not readily separated from the Bagshots
proper. Pebble Gravel was seen 'in situ' at between 305 and 320
feet (91 and 97 m) O.D. This altitude compares closely with those
of almost identical deposits at Stock, Galleywood, N.E. of Ingate-
stone, Warley and Langtons, while at Brentwood (Middlemiss,
1955) and Laindon (Woolridge, 1926) similar gravels occur at a
little over 350 feet (106 m) and 365 feet (111 m) respectively.
Woolridge (1923) has also described Pebble Gravel at Rayleigh at
about 225 feet (68 m).
2. The Pebble Gravel
The junction between the Pebble Gravel and the Bagshot
sands is an unconformity, with the pebbly deposit cutting sharply
across the nearly horizontally bedded sands and containing in its
lowermost foot or so a great deal of reworked Bagshot debris. The
maximum thickness of Pebble Gravel observed was about 10 feet
(3 m) and this occurred in deep channels cut into the Eocene
Sands. Normally, however, the Gravel is about 3 or 4 feet (.9 or
1.2 m) thick. Frequently Bagshots crops out at the present
relatively flat land surface, surrounded by gravel. Locally the
gravel consists of well rounded ovoid flints of a variety of colours
including many of Blackheath Bed type, all measuring up to about
15 by 8 by 6 cm and comprising about 50 per cent of the deposit.
These are set in medium-grained highly ferruginous quartz sand.
The pebbles show no preferred orientation and there is little in-
dication of bedding—though, occasionally, discontinous near-
horizontal sandy lenses or clay seams occur.
The size distribution of the peebles and their matrix is strongly
bimodal and is illustrated in Fig. 2. Bucket samples from several
exposures were analysed and the size distribution found to be
markedly consistent. Pebbles above 0.5g were weighed individu-
ally, and smaller sizes were divided into sieve fractions. Along
with the ovoid flints there were about 2 per cent of rounded but
highly irregular and embayed flints, including some rotted and
white a few nearly spherical pebbles of spherulitic chert reminis-
cent of the Portland Chert Series, and vein quartz up to 4 cm in
diameter. A few ironstone fragments and nodules appear to have
been derived directly from the local Bagshots but there were also
a few large subrounded tabular blocks of well cemented coarse