EROSION IN ESSEX AND SUFFOLK.
225
given very little information as to the varying depths at which
the London-clay was reached beneath the river deposits.1 It is
true that here and there at the bottom of the excavations the top
of the London-clay was either visible, or the presence of frag-
ments of Septaria nodules in the gravel suggested the nearness of
the clay. But, as we shall see, the general impression derived from
visits to the reservoir excavations, as to the depth of the London-
clay, needs correction from the additional evidence obtained
during the construction of the puddle-trenches in the centre of
the boundary banks.
I am greatly indebted to Messrs. Sharrock and Spencer, the
engineers representing Messrs. Pearson, the contractors, for
information as to the strata met with in the puddle-trenches, of
which they have preserved a most careful account. But I do
not propose here to touch upon the details of the river-deposits,
or even to note minutely the varying depths at which the
London-clay, was found in the puddle-trenches, my object being
simply to give the distribution of the depths which are within
normal limits and of those which are beyond them.
In the first place, it seems desirable for the sake of com-
parison, to note at what depths the London-clay has been
touched beneath the river deposits of the Lea Valley outside the
new reservoirs but within a short distance of them. On con-
sulting Mr. Whitaker's Geological Survey Memoir on The Geology
of London and of Part of the Thames Valley (vol.2) we learn that
at Waltham Abbey the East London Water Company in their
well close to Waltham Lock, found 18 ft. of river deposits above
the London-clay, at Walthamstow Marsh 171/2 ft., at Chingford
Mill 10 ft., and at Longwater Pumping Station, Tottenham,
20 ft. at one spot and 16 ft. 100 yards away. And the records
of borings and sinkings through the somewhat older river
deposits of Tottenham and Edmonton nowhere show them to be
more than 21 ft. thick. But as they are few and scattered over
a large area, they do not necessarily preclude the existence of
spots between them which might give a different result.
Dr. Henry Woodward, F. R. S., in his paper on "The
Ancient Fauna of Essex" (Trans. Essex Field Club, vol. iii.,
pp. 1-29, 1882-3) gives an account of the sections exposed during
the construction of the reservoirs of the East London Water
1 T. V. Holmes "Geological Notes on the New Reservoirs in the valley of the Lea, near
Walthamstow,"ante pp. 1-16.