254
THE ESSEX NATURALIST.
recently appeared in the Romford Memoir,7 but it will be well to
note its salient features here for a slightly wider area. It occurs
in a series of outliers, compactly grouped, in the region north
and east of Brentwood. For the most part it is found resting
on the Bagshot Sands, though northwards at Ongar Park Wood
the same or a similar gravel rests on the Claygate Beds. It
consists largely of rounded flint-pebbles, showing deep zonal
alteration, but not markedly bleached externally. A high
proportion of these pebbles are essentially cylinders with rounded
terminations, a feature which imparts a characteristic appearance
to the gravel as a whole. In addition, small proportions of
sub-angular flints occur, together with Lower Greensand Chert.
The latter, however, is very rare and sporadic in its occurrence,
and may have been glacially introduced. Small quartz pebbles
of the "Hertfordshire type" are conspicuously absent, though
as in most gravels a few small pieces of rounded vein-quartz,
ranging down almost into coarse sand, may be found. In
regard to structure, the pebbles are generally closely packed in a
mottled clayey matrix, and, as is well known, many of them are
disposed vertically. This is believed to be a "frost and thaw"
phenomenon; it is, in any case, secondary and fails to bear
on the question of origin. Sometimes there is a small amount of
sand, somewhat coarser than the local Bagshot Sands, occurring
interstitially or in irregular masses. Bedding is always of the
most rudimentary (or vestigial) character and often is conspicu-
ously absent.
At Billericay, Stock, Writtle Park, etc., on the eastern
side of the group of outliers, the base of the deposit is but little
above 300 ft O.D.; it rises slightly westwards. Bearing in mind
that the Hertfordshire plateau is inclined eastwards, there
seems to be no escape from the conclusion that the Warley Gravel
rests on essentially the same surface as does the Pebble Gravel
in the west of the county. It must also be recalled that only
13 miles to the south, in Kent, Lenham Beds occur at just above
400 ft. The Lenhamian base must have continued its northward
descent across the Thames into Essex and upon any assumption
it can only just have cleared the summit of the Brentwood hills.
In these circumstances it must be admitted that the Warley
Gravel presents a most perplexing problem and one for which
7 Mem. Geol. Survey (1925), pp. 23-25.