146 ON SOME SECTIONS BETWEEN W. THURROCK AND STIFFORD,
Fig, 3. Section along the Grays and Upminster Railway, from West Thurrock to
the northern side of the valley of the Mardyke.
river gravel, have been accomplished without the intervention
of any glacial agency beyond that which we everywhere have
experience of during frosts. For an ancient river running through
a Chalk country would almost necessarily leave somewhere a
similar record of past work. The gravel brought down its
channel and deposited on the chalky bottom at the foot of a cliff
would often receive on its surface masses of chalk of variable size,
the result of the undermining action of the stream below and of
disintegrating influences above. As the cliff receded and the gravel
deposit broadened the same process would go on, till the cliff either
ceased to exist, or the river turned its powers of erosion elsewhere.
The lumps of Chalk mixed with the gravel would almost invariably
be diminished in size, and in many cases would entirely disappear
during the course of ages since their original deposition. But even
after their disappearance they would leave signs of their former
presence in the contorted appearance of the beds in which they had
once existed, and in which their places had been filled by other
material. In short, lumps of Chalk would tend to affect the aspect
of gravel on which they had been deposited, very much as blocks of
ice might do in a glaciated country.
On the other hand it is perfectly possible, and by no means
improbable, that the position of much of the Chalk now found
mixed up with the gravel, and the general confusion at their
junction, may be the result of glacial action. But in that case
nothing more seems to me to be implied than the existence of
blocks of river-ice here and there which have carried down from
elsewhere lumps of chalk and other material. These blocks would
naturally tend to drift in large numbers to particular spots, at which
their weight would give rise to many surface contortions, while the
material resting on the blocks would become irregularly deposited.
Masses of ice quite large enough to produce results of this kind may