OF THE THAMES VALLEY.
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showed several feet of similar gravel. The composition of this
gravel, as one would expect, is of a more varied character than
is that of the gravel filling the lower part of the old channel seen
in the quarry. Tertiary pebbles are equally abundant, but there
is a larger admixture of subangular flints, and what may be
termed Thames debris than in the latter case.
From its position, direction of slope, and the general relation-
ship which it bears to the conformation of the surrounding
country, it is evident that this gravel terrace cannot be referred
to the deposits of the Thames, but that on the contrary it belongs
to the Mardyke. Its structure at once suggests the final floor of
a small river valley such as is formed when the river has exca-
vated down to its effective base-level of erosion, and we regard
it as the equivalent of the Middle Terrace of the Thames valley,
i.e., it was formed at the same time and under the same set of
conditions as was the Middle Terrace of the Thames. At the
same time it affords an illustration of those physical conditions
operating under different circumstances, for in the small
tributary valley the final gradient is of necessity steeper than is
that of the main valley.
As the base-level of erosion was lowered at the close of the
Middle Terrace stage, corrasion was restarted in the Thames,
and this in due course had its effect upon the Mardyke. Its
lower course at this time, though a westerly one, as described
above, was, we venture to think, further to the south than its
present one, but parallel to the latter. Space will not permit of
us entering into any detail regarding the evidence upon which
we take this view, but with the restarting of the corrosion the
Mardyke here cut down to the Chalk, and in our opinion it was
this corrosion which excavated the now dry east and west valley
mentioned above, i.e., this valley was the channel of the Mardyke
in the period between Middle Terrace and Lower Terrace times.
This view seems to be corroborated to some degree by the fact
that the floor of the valley is covered with drift material, though
whether in the form of a regular deposit or not we are unable to
say in the absence of sections.
After the slight pause marked by the Lower Terrace in the
Thames Valley corrosion restarted on a large scale. Once again
the Mardyke sought an easier path for its lower course. This
easier path it now occupies. As we have seen, the Thames cut