OF THE THAMES VALLEY.
77
High Terrace gravel which caps the higher ground to the south.
But since, as we have shown in the preceding section, the High
Terrace gravel of the Thames represents the alluvial plain which
the Thames formed when it had cut down its channel to the
then base-level of erosion, it follows that not only was this inter-
Stifford and Brentwood Valley non-existent at this time, but
that its place was occupied by the high ground forming the
ancient north bank or side of the Thames Valley. To sum
this up in simple language, it is perfectly clear that denudation
since High Terrace times has here resulted in completely
altering the configuration of the ground ; it has made a hill from
what was once the floor of the Thames Valley, and has planed
down the high ground which once rose above this ancient valley
floor, and carved a vale into its foundations.
Simultaneously with noticing this denudation, we recognize
the agents by which it was accomplished. These are the little
rivulets which, ramifying through the valley to-day, join together
to form the Mardyke. Thus there, are no less than four
of these branches in the western portion of the inter-Stifford and
Brentwood valley, three of which, having their sources in the
hilly country to the north, have been in existence since the
erosion first commenced, the fourth rising from the drift
covered area to the south, and therefore not started until the
excavation of the valley had succeeded in tapping the district.
If we examine the present course of the Mardyke, the reason
for the sudden bend round to the west which the stream makes
at Stifford is quite clear. To the south of Stifford the Lower
London Tertiaries and the Chalk are brought up to the surface by
an anticlinal fold. The resistant barrier of Chalk thus formed
constitutes a very serious obstacle to the Mardyke in continuing
its course to the south, and it finds an easier path by flowing
round and skirting the anticline. Its course is here, by reason
of the bend, wholly, with the exception of a low mass of Chalk at
Aveley, through soft Tertiary strata.
Now, during High Terrace times the Thames even had not
laid bare the Chalk of the anticline, at least on the Essex sider
for its deposits here rest upon the Thanet Sand, and nowhere
upon the chalk directly. It is evident, therefore, that the
course of the Mardyke at that time, whether it flowed southwards
or westwards below Stifford, was wholly in the non-resistant