OF THE THAMES VALLEY. 87
level marked by the High Terrace, but must have on the
contrary towered above it to a considerable extent. In other
words the pre-High Terrace Thames must have flowed along a
line in this district nearly coincident in direction and latitude
with its present course, and along this line it excavated a deep
valley in the bottom of which there was eventually deposited the
High Terrace gravels. The north side of this valley may have
attained at this time an altitude equal to that of the present
Brentwood and Warley hills, which, being reduced to the effec-
tive base-level of erosion of the period, equals a height of 200
feet above the stream. The whole of this material has gone, and
its removal was accomplished for the most part by the time the
Middle Terrace deposits were formed.
The development of a Middle Terrace in the valley of the
Mardyke proves that before the close of that period that valley
had assumed in embryo the features which it presents to us
to-day. The final touches so far as erosion was concerned were
put to the picture between Middle Terrace times and the end of
the Pleistocene period. At this latter date the whole of the
Lower Thames basin must have presented a most remarkable
appearance. Instead of the sluggish canal-like stream
meandering through the monotonous alluvial marshes of the
broad open valley of to-day, with its similar little tributary
canals, there was the great river flowing with torrential velocity
through a narrow ravine-like valley, the latter ever-deepening
seawards, and receiving on either hand as tributaries smaller
torrents flowing each through a smaller but equally deep ravine.
The geologist, impressed with his study of the past, and deeply
imbued with the picture of the Thames as it was in the latest
of Pleistocene times, finds it at first hard to realise that in the
present river he has the same Thames, merely enfeebled by the
degeneration consequent upon the changes which have taken
place in its physical environment, and the contrast is not the
less striking to him when he reflects upon the briefness of the
space of geologic time which such vast changes have required
for their operation.