16 The Blackwater Valley, Essex.
The broad gentle slope traversed by these rivers consists of
Boulder Clay, at the surface, from ten to eighty feet thick,
underlaid by sand and gravel, rarely more than thirty feet
thick, and sometimes altogether absent, and this by the
London Clay, the limits of which are near a line through
Thaxted and Bishop Stortford. To the north-west of this
line is a narrow belt of the Lower London Tertiaries, fol-
lowed by the Chalk, the Boulder Clay and subjacent gravels
generally concealing these older beds. On the flanks of the
Ter Valley occur patches of clean brick-earth overlying the
Boulder Clay, but possibly constituting only the upper part
of that formation, and not separated from the remainder by
any important interval.
Between Witham and Feering the Boulder Clay, which
previously only formed the plateau between the Guith and
Blackwater, descends suddenly into the valley, not only cut-
ting off the gravels that originally lay below it, but passing
below the general surface of the London Clay, as shown in
the valley bottoms at Coggeshall, Faulkbourn, &c. The
Boulder Clay only rises a short distance up the flank of the
Tiptree ridge, the sides and crest of which are coated with a
considerable depth of sand and gravel.
A part of Tiptree Heath having been fixed upon as a good
site for the second County Asylum, an artesian well was
resolved upon as the only satisfactory source of water supply.
Calculation from the nearest of previous wells showed that
there was some local disturbance. The Chalk surface
descends from Braintree to Witham at the rate of 21.23 feet
per mile, giving depth at Asylum of 495 feet, whilst it rises
from Maldon and Heybridge at 74 feet per mile, giving
depth at Asylum of 194 feet.
Three possibilities presented themselves :—
1. There might be a gentle roll over of the beds.
2. There might be a powerful undulation or reversal of
the beds.
3. There might be a fault throwing down the beds to the north.
The boring proved the last two of these contingencies to
be combined.