OF THE THAMES VALLEY.
65
materials, is not merely permitted, but compelled to deepen its
valley, which it does with great rapidity, with the result that the
deposits in it will be practically confined to the alluvial floor
which it will form on approximating to the effective base-level
of erosion.
In another connection this difference in the structure of the
deposits of the two valleys becomes of supreme importance. In
the event of a region having been subjected to a series of tectonic
movements, the effect of which has been to intermittently elevate
the land above the sea, we should find in the first valley practi-
cally no record of the process, while in the second valley every
stage would be recorded in its erosion and its contained deposits.
For in the last instance every upward movement would be
represented by evidence of erosion and deepening, and every
pause, whether of the nature of quiescence or subsidence, would
be marked by deposits which would bear in themselves the evi-
dence of having once stretched across the valley as alluvial floors.
It is, therefore, obvious that since they hold such a key to the
physical history of a region, such river valleys as those of the
second class must be of far more importance and utility to the
geologist than those of the first class.
It is but rarely, however, that we are fortunate enough to
meet with a valley in which the record of such movements is so
clear as the above theoretical statements would imply. On the
contrary, we find that all sorts of local influences have been at
work counteracting the operation to some extent and obscuring
the record of the greater forces and events. Thus in the case of
the Thames, where, as we submit, such a record of tectonic
movements does exist, we find that the little outcrops of the
comparatively resistant Chalk have tended to complicate the
drifts somewhat, and by interfering in times past with the regu-
lar and rapid transmission of the accelerating influences have
prevented the record from assuming so clear an appearance as
might otherwise have been the case. Thus it is that, while we
find that the Thames drifts generally occur at fairly well-defined
altitudes, by which they may be classed as belonging to the High
Terrace, Middle Terrace, etc., we also have many local deposits
which, occupying intermediate positions, cannot be classed with
either.
Applying these principles, we will endeavour to read the
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