66
THE ESSEX NATURALIST.
not the result of the uplift of a former ridge within an antecedent
drainage system), it would appear that the upper Pant first
captured the upper Cressing, and was itself captured by the
middle Cressing; a stream which in turn was captured by the
Brain (fig. 5). This produced the present river plan within
the Thurstable and Lexden Hundreds and must have occurred
before the glaciation, since the glacial deposits capping the
Tiptree ridge show no signs of wind-gaps. As previously
shown, wind gaps must have been present, but they are,
apparently, buried under the glacial material.
In the same map the beginnings of the Holland Brook and
the Colne are shown. Their waters must have been melt-waters
from the ice and must also have formed the large glacial outwash-
fans in their vicinities. The absence of any such material in the
Dengie and Rayleigh Hills gives further support to this theory
of a mid-Essex river—its oblique position across the County
preventing glacial outwash penetrating to its south-east bank.
Before the period of the glaciation a further episode of river
capture must have occurred. The Blackwater effected the
capture of the upper Chelmer and initiated the development
of the Sandon Brook as an obsequent stream, whilst the beheaded
trunk—the Crouch—became an obvious misfit for its broad
valley. The Crouch continued south-eastwards, but its de-
pleted volume could only permit of a narrower trench to be
produced within the former wide valley. Professor Gregory
has argued that the absence of glacial material in the valley of the
pirate River Blackwater denotes that this capture took place
after the glacial period, the Blackwater having removed the
glacial material.2 The presence of Boulder Clay, however,
occurring in the wind-gap and at an altitude of 150 ft. appears
to demonstrate that the capture definitely occurred before the
glaciation.
With the close of the Glacial period the physiography of
Essex must have approached the present pattern. The middle
broad valley was in existence, and numerous rivers, small in
width, but rapidly lowering their valleys by means of the plentiful
supply of ice-water, had become prominent. Capture had,
moreover, produced the most westerly of the three broad valleys
and developed it as a recognisable feature occupied by a subse-
2 Gregory, J. W. The Evolution of the Essex Rivers.