The Blackwater Valley, Essex.
17
The base of the London Clay was met with at 295 feet, in
the shaft, with a westerly dip of 18 in 68. Boring soon after-
ward commenced in the Pleading Beds, and at 343 feet a fault
was passed, and the London Clay reappeared. Its base was
again reached at 383, the Thanet Sands at 422, and the
Chalk at 477 feet. The Reading and Thanet Beds must be
inclined at high and varying angles, as at Witham (only two
miles off) they are only 27 and 24 feet thick respectively.
We have here, therefore, a great wave, broken along the
crest, of the earth's crust, and, in a way that is most unusual,
determining approximately the form of the surface. In hard
rocks such a structure would most likely be along a valley,
with beds dipping into the hill on both sides. Even if a
stream had commenced a channel vertically over the present
course of the Blackwater, landslips would have perpetually
occurred from the south-east bank, till the stream was
shifted to the centre of the geological ridge.
But in this case the surface consists chiefly of gravel,
without any bedding to produce landslips, and the under-
lying clay is a homogeneous mass, more prone to slipping
along its joint faces than the slightly-marked bedding-
planes, so that internal structure does not much affect the
physical features.
But the coincidence of a very marked ridge with an excep-
tional undulation of the beds is suggestive of cause and
effect, and the draping of the hill with Glacial gravels, usually
only present at lower levels, and the absence from the crest
of any trace of Boulder Clay, which mantles round the foot
of the ridge to the N.E., seem to point to an elevation during
the Glacial period, whereby the crest of the ridge was raised
above the berg-covered sea, and a current produced at its
foot, which scoured away the gravel and dug into the London
Clay, leaving a channel to be afterwards occupied by the
Boulder Clay.
On the subsequent emergence of the entire country, the
slope of the clay-bed determined the general trend of the
streams down to the N.W. foot of the ridge, and during the
cutting through of the estuary a lake was formed by the
C