342 PLEISTOCENE GEOLOGY OF THE THAMES VALLEY.
False bedded material here and there occurs and traces of
slight contemporaneous erosion in the form of little filled up
channels are sometimes exhibited by the sections. We have
obtained many boulders of Wealden or Purbeck ironstone with
Cyprides from the gravels in this pit, also a boulder of basaltic
rock, and some pebbles of "schorl-rock." Large sarsens,
which may be seen in situ in the Thanet Sand of the district,
likewise occur, some of them weighing many hundredweights.
In this pit many of the seams of gravel and sand are stained
with a hydrous dioxide of manganese, the staining following the
dip of the false bedding, and hence proving that the manganese
owes its present position to contemporaneous fluviatile deposi-
tion (70).
(b.) A little to the east of Chalk Pit Farm, at the fork of
the two roads, an old gravel pit shows a few feet of gravel
similar to that of Sockett's Heath.
(c.) Hangman's Wood. Gravel is exposed here in an old
pit at the entrance to the wood. The junction with the Thanet
Sand is seen in the shafts of the "Deneholes."
(d.) About a third of a mile from the north-eastern
extremity of Hangman's Wood by the side of the road leading
to Orsett, an old gravel pit shows about seven feet of gravel and
sand from which we have obtained implements,
IV. MIDDLE TERRACE DRIFT.
The Middle Terrace Drift of the Grays Thurrock area is
found extending in a long narrow strip of brickearth, sand and
gravel, commencing near High House on the west, passing
through West Thurrock and Grays Thurrock, and ending off to
the east of Little Thurrock. The continuity of this band of
Drift is broken for a short distance to the west of Grays, by a
deflection in the northern boundary line of the Holocene alluvium
which here cuts inland as far as the Chalk outcrop. Thus the
Drift is divided into two portions—a western and an eastern.
The western tract, with which we shall first deal, has a
total length of about three miles. Numerous sections are afforded
by the cuttings for the tramways of the various chalk quarries
between Grays and Purfleet,