PLEISTOCENE GEOLOGY OF THE THAMES VALLEY. 369
groove of considerable width is formed and into this groove the
superincumbent and elastic strata slowly descend, and thus a
valley is gradually formed which marks the line of fissure.
It has been previously said that the Chalk inlier of Grays
Thurrock really forms part of a small anticline. On the summit
of the dome the fissures are probably widest, but they have not
given origin to the biggest valleys. The little valley seen to the
west of Belmont Castle mentioned above is a case in point and
the explanation of this apparent anomaly is not far to seek.
From their elevation it is evident that the dissolution of the
Chalk and the consequent widening of the fissures occurring
there could only have been carried on for a short time after
elevation of the land had set in at the close of the High Terrace
stage as the upper portions of the fissures would soon be placed
above the level of saturation. On the flanks of the little anti-
cline, however, the Chalk strata are carried lower, the Tertiaries
being consequently in greater thickness. Here the valleys are
much larger, as we should expect, seeing that the fissures, of
which they form the surface indication, have always been below
the plane of saturation since High Terrace times from which
date the erosion of the Chalk by acidic water has been continu-
ally carried on. (Fig. 6, p. 367).
There is a question as to the date of the movement bringing
up the Chalk at Grays and so forming an anticline, the con-
sideration of which we do not wish to enter into in this part of
our paper. It will, therefore, suffice to say that the evidence so
far as we have examined it at present tends to show that the
movement which caused the crumpling of the Chalk and
Tertiary strata was also the movement which elevated the land
at the close of the High Terrace stage.
VII. CONCLUSIONS.
The conclusions arrived at in the papers on the Ilford
District require a little modification, principally with regard to
the High Terrace Drift. In the first of the former papers it
was argued that this portion of the Pleistocene period was
characterized by a climate of considerable rigour. Undoubtedly
there is evidence of the action of river-ice in the deposits of
this terrace, but since these deposits have been yielding to us a
fauna as rich as that of the brickearths at a certain locality in