368 PLEISTOCENE GEOLOGY OF THE THAMES VALLEY.
height of the High Terrace is about 100 feet higher than that
of the river of to-day, a relation which is exhibited from the
eastern portion of Essex up the valley as far as Reading if not
further. Very commonly also we have a Raised Beach at the
same relative level occurring on different parts of the coast
which we would correlate with the High Terrace.
It follows from this that in High Terrace times the land
stood at an elevation 100 feet lower in relation to the level of the
sea than at the present time. This being the case it means a
corresponding general rise of the saturation plane in High
Terrace times since it is evident that strata now 100 feet above
that plane were then just beneath it.
Prestwich objected to the view that fissures favourably
influenced the dissolution of the Chalk on the ground that acidic
water would pass so freely through them that very little solvent
action would be carried on upon their sides and edges. But this
difficulty is removed when we consider the physical conditions of
the High Terrace stage in the light of recent scientific advance. It
is now more than probable that the view expressed above as to the
formation of terraces is correct and this being so it is clear that
the fissures in the Chalk of Grays were in High Terrace times
completely immersed below the saturation plane. It is evident
that such fissures, granted favourable conditions, will always
form the vulnerable portions of a rock which will most readily
yield to denuding agents whether they be of a mechanical or a
chemical nature. They constitute, together with joints, natural
lines of weakness and greatly assist in the disintegration of
rock-masses. Mr. Rutley's experiments in the dissolution of
blocks of chalk with weak acids, described by him in his
paper on "The Dwindling and Disappearance of Limestones"
(66), fully corroborate this view as applied in the present
connection.
The solvent will most readily attack the edges afforded by
the junction of the walls of the fissures with the upper surface of
the Chalk, because there it will have the greatest superficial area
to act upon, while the strains, of which the fissures are resultant,
will have increased this large working surface by the partial
shattering or splitting of the surrounding rock. The dissolution
will, therefore, tend to widen the upper part of the fissure by
gradually rounding off the edges in such a manner that a shallow