78 CONTRIBUTIONS TO PLEISTOCENE GEOLOGY
Tertiary strata. We are in consequence entitled to ask why
the Mardyke should have at that time changed its course below
Stifford from one with a direction to the south to one directing
westwards. We can realize to-day the reason for the existing
bend, for there is an obstacle in the way of the more direct
course, but in High Terrace times there was no such obstruction.
We are not surely to suppose that the change in direction in the
course of the present stream, which is clearly the result of the
obstructing dome of Chalk, should also have occurred in High
Terrace times when that obstruction was buried and conse-
quently inoperative.
It follows, therefore, that, even supposing the sections in the
;large quarry had never been exposed, we should have had good
reason to infer that the present lower course of the Mardyke was
not its original course, and indeed it was on these grounds that
we were first led to the opinion here advocated, viz., that the
Mardyke in High Terrace times continued its course to the
south below Stifford by flowing in the channel now seen in
section as above described.
The High Terrace period, as we have seen, was brought to an
end by a tectonic movement which, lowering the effective base-
level of erosion, re-started the corrasion in the Thames Valley.
Now, corrasion always starts at the mouth of the river or
tributary and works progressively backwards up the stream, the
rapidity of the process depending on all those co-efficients which
go to form what may be termed the strength of the stream on
the one hand, and the resistance offered to it by its bed on the
other. When the influence of acceleration reached Grays it did
not take very long for the Thames to cut through the easily
eroded Thanet Sands to the resistant Chalk, nor did a long period
of time elapse before the Mardyke accomplished the same thing
at the point of junction between it and the Thames. But the
effect of the comparatively great resistance encountered when
the strong river and the weak tributary reached the Chalk was
different upon the two just in proportion to their relative powers.
As we have already seen, the Thames experienced considerable
difficulty in cutting down its channel through the Chalk. There-
fore it is not a surprising fact that the smaller and weaker
Mardyke proved unequal to the occasion, more especially when we
consider that it was working wholly against the dip of the Chalk