18 The Blackwater Valley, Essex.
accumulated waters. The gravel brought down into this
lake by the Guith and Blackwater forms a continuous terrace
from Witham to Braintree, interbedded with shell-marl,
swamp-mud, and brick-earth, as described in the Geological
Survey Memoir on Sheet 47.1 The elevation continued till
the deep channels of the Essex estuaries were formed, when
subsidence took place, readmitting the sea as far as Col-
chester, Maldon, and Battle Bridge. I have elsewhere pub-
lished reasons for believing that a slight subsidence is now
in progress.2
Those who care to investigate the origin of the undulation
described above may be interested in hearing that a parallel
undulation has been noticed in the Chalk ridge above
Royston, with an outward north-westerly dip of 60°, and
that the prolongation of the line of Tiptree Heath coincides,
near Deptford, with a fault bringing up the Chalk through
the Tertiaries, and, in the opposite direction, we have Chalk
coming to the surface in an abnormal way, at Shelly (near
Hadleigh) and at Ipswich, whilst farther away in Suffolk
other points of disturbance have been noticed along a line
nearly coincident with the Yarmouth branch of the Great
Eastern Railway.
I must defer to another occasion my reasons for supposing
that the undulations are confined to the upper 1000 feet of
the earth's surface, and are due to lateral pressure in the
Chalk, and that the subjacent Coal-Measures or other rocks
are not affected thereby.
Whitaker, W., W. H. Penning, W. H. Dalton, and F. J. Bennett.
1The Geology of the N.W. Part of Essex and the N.E. Part of Herts.,
with Parts of Cambridgeshire and Suffolk.'—Geological Survey Memoir.
8vo. London. Pp. vi., 92 ; 19 woodcuts. Price 3s. 6d.
2'Subsidence in East Essex.'—Geol. Mag., dee. ii., vol. iii., pp. 491—
493. (187G).