366
PLEISTOCENE GEOLOGY OF THE THAMES VALLEY.
tracing the beds to the south across their outcrop this northerly
dip gradually ceases and the strata become horizontal. Soon
after the Thames is reached, however, the beds become again
inclined, but this time to the south, so that in the quarries at
Greenhithe in Kent the Chalk is seen dipping in this direction
at an angle of from 2° to 3° (Fig. 5, p. 367).
Disturbance of a similar, if not the same, date has resulted
in faulting the Cretaceous and Tertiary rocks a little up the
Thames Valley from the Grays district. One of these faults has
been traced as far east as Erith where it is lost under the
Alluvium. The tension resulting from the crumpling has not
been great enough to produce faults in the district under con-
sideration so far as is known at present, but it has been great
enough to open fissures in the Chalk. These fissures are most
abundant in that portion of the Chalk which forms the crest of
the little anticline, the tension being naturally greatest there,
and a good instance of their development in this part of the
district was mentioned by Mr. F. C. J. Spurrell in the Report of
an Excursion of the Geologists' Association to Grays (64)
The next point to be considered does not at first sight have
much connection with the subject under discussion, namely,
the principle governing the formation of terraces. Assume
that we have a country whose elevation above the sea may be
represented by the symbol A, and that the rivers have adjusted
their courses to the base level of our suppositious region, i.e.,
they are graded. It is likely that the rivers are depositing
sediments at this stage ; if slight subsidence of the area goes on
they will certainly do so. Suppose that the country has its
height above the sea A augmented by a further rise of 50 feet.
What happens ? The rivers at once cut through their old beds
and commence to excavate the valley 50 feet deeper with the
object of keeping their channels graded. The elevation of the
land and the erosion of the valleys by the rivers goes on
simultaneously. The ultimate result of this is that the deposits
laid down by the river when the land was at A, form, when the
succeeding period of elevation has ceased, a "terrace" 50 feet
above the surface of the river. This is the only way in which
river terraces could be formed at the present day, and there is
certainly nothing illogical in applying the same principle to the
terraces and raised beaches of Pleistocene age. The average