THE PLIOCENE PERIOD IN WESTERN ESSEX. 251
The foregoing review is a necessary precedent to a considera-
tion of the Pliocene history of Essex, for though, as in other
matters, the latter county adds its distinctive quota to the
problem, it cannot on that account be divorced from its natural
geographical environment. The surface of the Hertfordshire
plateau has a very slight eastward gradient and continues
beyond the Lea Valley into Essex. Here the elevations in all
cases fall short of 400 ft., but there can be no doubt that the
summit-plane of the Essex hills is a continuation of the corres-
ponding surface west of the Lea. Since in the latter region
deposits of two distinct ages occur, the question naturally
arises as to the recognition of the equivalent of either or both
of these in Essex.
Immediately east of the Lea Valley, high-level gravels occur
on the crest of the Epping ridge at High Beach and round the
Wake Arms. Though these two masses of gravel are so near
to one another there are significant differences between them.
At High Beach Greensand Chert is rare or absent, while small
quartz-pebbles, though present, are far from numerous. In
the Wake Arms outlier, both quartz and chert are plentiful and
indeed the gravel compares in all respects with the chert-bearing
gravels of Eastern Hertfordshire. There are trustworthy records
of a similar quartzose chert-bearing gravel at Coopersale
Common,5 but the gravels now exposed are of a different and
simpler type, containing little else than flint-pebbles, recalling
that at High Beach. If, then, we are justified in distinguishing
the High Beach type of gravel from that at Jack's Hill, we
may conclude that the line separating the two types runs from
near High Beach to Coopersale Common, where the two types
are in close juxtaposition. It might appear that in thus
separating the gravels, undue emphasis is laid upon mere trivial
varietal differences, but there is abundant warrant for regarding
the difference as real and significant. The accompanying map
(fig. 1.) shows the line (e—f) of separation in question ; it is
carried on northwards of Ongar Park Wood, where the shingle
cap is destitute of chert. Lines also appear upon the map
representing respectively the northern limit of small quartz-
pebbles in the Pebble Gravel (a—b), and the clean-cut north-
western limit of the chert in Hertfordshire (c—d). The chert-
5 Cf. Prestwich, Quart. Journ. Geol. Soc, vol. xlvi. (1890), p. 136.