86 CONTRIBUTIONS TO PLEISTOCENE GEOLOGY
say, the actual amount of vertical rise or fall in the relative
altitude of the land was probably much greater than is indicated
by the diagram. A great deal of further study will have to be
devoted to this very difficult branch of the subject before we
have really satisfactory data at our disposal touching the
absolute values of the variations.
There is a subsidiary point connected with classification,
which we had better notice at this juncture. In the table it
will be noticed that, following the Third Terrace stage, we give a
tectonic movement which, placing the land at a higher relative
altitude, lowered the base-level of erosion and caused the
excavation of the deep buried channels of the Thames and its
tributaries. The gravel which fills the bottoms of these
channels we class as earliest Holocene, and for this reason—
its deposition marks a change in physical conditions. Instead of
the elevation which produced the excavation, there was a
movement of subsidence which carried on with minor inter-
ruptions, produced the conditions obtaining at present in the
Lower Thames basin. Some may prefer to call this gravel
latest Pleistocene, but it is a matter of indifference, and we have
merely preferred to take as our dividing line cause rather than
result.
The last question with which we have to deal is in some
respects the most interesting. It cannot, in our opinion, be too
strongly insisted upon that the denudation which sculptured the
South-East of England, and made its hills and vales as we now
know them, has taken place to a very large extent subsequently
to High Terrace times. This, we think, stands sufficiently
established by the Mardyke evidence so far as the Grays
district is concerned, but independently of this evidence it is also
established by the present course of the Thames in that area.
It is obvious that if the valley of the Thames in this district
were filled up level with the surface of the High Terrace,
together with the low-lying grounds to the north of that terrace,
and the Thames then started to excavate its valley, it would not
choose the path which it now occupies, but would seek a much
easier course parallel to it, but more northerly in position, so as
to escape having to cut through the Chalk of the anticline.
Therefore it follows that in High Terrace times the area to the
north of the anticline could not have been planed down to the