4
GEOLOGICAL NOTES.
as shown in a gravel pit, from 8 to10 feet deep and from 30 to
40 yards broad, in the beds lying above the London Clay at the
top of the hill. The gravel is largely subangular, and resembles
in its general character that of the marshes below. And the
loamy material by which it is more or less covered is evidently
Fig. 1. Higham Hill Gravel, &c. Height of Section 6ft; length 20ft.
the equivalent of that which forms the surface bed in the reser-
voir excavations. (Fig. 1). But the Higham Hill deposits are
shown only in an ordinary gravel pit. While the gravel, loam,
&c., in the area occupied by the reservoirs, have been displayed
in sections hundreds of yards in length and ranging in every
direction. Consequently the latter gave not only exceptionally
good opportunities for noting variations in details, but their
length, and the changes in their direction, combined to throw
much light on the local changes of conditions to which the varia-
tions in the nature and thicknesses of the beds were due. An
additional advantage which they possessed, of an exceptional
kind, lay in their cutting across a deserted channel of the Lea.
Then, while the largest gravel pits or brickyards usually show
Fig. 1a. Higham Hill Gravel, &c. Height of Section, 8ft., length, 30ft.
clear sections over but a fraction of their total expanse, the
reservoir excavations were almost everywhere fresh and distinct,
and they were free from the sloping which is so geologically
pernicious in the case of new railway cuttings.
The depth from the surface of the sections in the reservoir
excavations varied from about 9 to 11 feet. They always showed
loam or clayey loam at the surface and gravel at the bottom,
though the thicknesses of the two beds varied considerably. In
one place, for perhaps 150 to 200 yards or more, gravel, capped
by from 1ft. 6in. to 2ft. of loam and soil, constituted the whole