THE PLIOCENE PERIOD IN WESTERN ESSEX. 265
Brook, an affluent of the Stort, appears to represent the head-
waters of the old Lea tributary, which must have followed a
course analogous to that of the present Stort, flowing first
southwards and then westwards.
The Roding valley exhibits the normal features. The bulk
of the glacial drift is lodged on a broad 200-ft. shoulder which
occurs on both sides of the valley, but there are indications
of further pre-glacial downcutting in that the Boulder Clay
warps over the shoulder and descends to lower levels (150 ft.)
near the river. The phenomenon is not due to slipping, for it
occurs uniformly and on both sides of the valley. The gravels
of Buckhurst Hill and Woodford, which represent the outwash
of the Roding ice-lobe, rest upon outlying portions of the 200-ft.
platform.
The Upminster lobe presents us with a new feature. In the
col between Havering and Brentwood the drift is resting as usual
on the 200-ft. level and this holds as far south as the Chelmsford
Road. Here there is a sudden drop to lower levels, and on either
side of the Ingrebourne valley the Boulder Clay occurs just
above the 100-ft. contour. The sudden drop in the Boulder
Clay level is certainly not attributable to slipping and tempts
speculation as to the existence of veritable. "seracs" at this
spot in Pleistocene times. However this may be, the evidence
afforded of a large pre-glacial depression on or near the site
of the present Thames valley14 is of great importance. That
the drift formerly extended widely in this depression is suggested
by an isolated remnant of Boulder Clay near Childerditch
Street, and by various small outliers of gravel. These facts
lead incidentally to the conclusion that the Boyn Hill terrace
of the Thames is in a literal and direct sense redistributed glacial
outwash, a supposition which might be strongly urged on quite
other grounds.
Passing on to the Herongate lobe we find the normal regime
in evidence. All the drift between Brentwood and Billericay
lies at or just above the 200-ft. level, and the same is true of
the greater part of the drift in the Wid Valley to the north.
It descends to a slightly lower level (160-170 ft.) as traced towards
the river, which, like the Roding, had evidently sunk its course
slightly below the platform prior to the advance of the ice.
14 Cf. J. W. Gregory Geol. Mag., vol. lxiii. (1926), p. 273.