OF THE THAMES VALLEY.
67
physical history of the lower Thames valley from the time of
the formation of the first definite paired terrace of which we
have evidence, to the end of the Pleistocene period. The first
definite paired terrace, or that which attains the greatest eleva-
tion above the river, is the one we term the High Terrace6.
Other deposits of fluviatile origin occur at superior elevations on
the sides of the valley7, but their relationship to the existing
course of the valley is not so obvious, and their continuity is
broken to a much greater extent. They may be regarded as
representing an older series of events than that marked by the
High Terrace; they may, in fact, be the remnants of more ancient
paired terraces devastated throughout the course of ages by
denudation ; or they may be ancient minor terraces or those
which are fully explained by the winding of the river and its
consequent erosion. In either case we are not specially con-
cerned with them here, and may dismiss them from the
discussion.
We will commence by capitulating the evidence, as
concisely as possible, for regarding the High Terrace as repre-
senting a stage in the history of the Thames when its effective
base-level of erosion stood higher than it does at the present day.
The High Terrace is continuous on both banks of the river,
except where cut through by side valleys, and such other breaks
as do occur can usually be satisfactorily accounted for by taking
the various degrading influences, which have been at work in
those localities since the date of its formation, into consideration ;
it attains an altitude above the present river which is practically
constant; and its lithological constitution is of a remarkably
uniform character. These characters, and more particularly its
continuity and regular height, suffice to show that it once formed
the whole floor of the lower courses of the valley. The coarse
gravel of which it is so uniformly composed proves that at the
time of its deposition the river had lost the power of transport
which it previously had possessed when using this very material
to scour out its valley. We have an indication of what the
velocity of the river was during High Terrace times, i.e., after
the loss of transporting capacity just mentioned, in the material
deposited at the points of local check or where old tributaries
6 See figure 2, "l."
7 "a" in figure 2.