ON THE LEA VALLEY.
227
Thus it appears that the average depth from the surface of
the London-clay, in the puddle-trenches of the new reservoirs, is
about 18 ft., and that its normal variations are from g to 10 ft. on
the one hand to about 25 ft. on the other. And this result coin-
cides with that obtainable either from Dr. Woodward's remarks
on the puddle-trenches of the more southern reservoirs, or from
the sinkings and borings given in Mr. Whitaker's Memoir.
Thus, while in the boundary of the Banbury reservoir the
London-clay cannot be considered to be at an abnormal depth
anywhere, in the "Lockwood Reservoir" it appears to be
abnormally deep at its north-western corner, where it is 57 ft.,
and on its eastern side, close to the northern end of the "Low
Maynard Reservoir," where it is 36 ft. below the surface. And
a line drawn across the reservoir through these two spots would
doubtless mark the position of the narrow subterranean hollow
in the London-clay which may, or may not, attain its greatest
depth at the north-western corner. A continuation of this line
northward would bring it to a point a little westward of the most
westerly point in the northern reservoir where, as we have seen,
the London-clay was deeper than at any other spot in its boun-
dary bank.
I am informed that where the thickness of the beds above
the London-clay was greatest, in this depression, the material
composing them consisted of gravel, sand and silt, much resem-
bling that of the river deposits elsewhere. And the direction of
the narrow line of depression seems to be nearly due north and
south, or very slightly west of south and east of north. But
whether the somewhat unusual depth of 26 feet, at which the
London-clay was found at the western end of the Banbury
reservoir, indicates the nearness of a more noteworthy depression
beneath the Rifle Butts, we have no evidence to determine.
The paper "On a Deep Channel of Drift in the Valley of
the Cam, Essex," by Mr. W. Whitaker,3 appears to me to
describe some drift-filled hollows which are all akin to that
beneath Lockwood reservoir, while one example has a specially
strong resemblance to it. Mr. Whitaker gives details of wells
at Quendon and Rickling, Newport and Wendon, which all
show a very great and wholly unexpected thickness of the
Glacial-gravel and sand, which underlies the Boulder-clay,
3 Q. J. Geol, Soc, vol. lxvi., p. 333 (1890). Essex Naturalist, vol. iii., p. 140 (1889).