OF THE THAMES VALLEY.
71
a step between them. Therefore we have come to the conclu-
sions, firstly, upon the evidence of the sections, that the gravel
is newer than the uppermost part of the Middle Terrace
deposits ; secondly, that it forms a distinct feature ; and these
conclusions, taken in conjunction with the lower level of its base,
led us to finally conclude that it formed a distinct Third Terrace
in this district.
This terrace does not attain a greater height, calculated to its
surface, than 20 feet above the river ; it is found at various
places forming the surface of the ground on either bank of the
river throughout its lower course, and it is present continuously
under a portion of the Holocene Alluvium throughout the entire
lower valley. Perhaps the most interesting sections in it are
those which have been described from time to time in connection
with excavations in the Westminster district. From these
remains of numerous animals and plants have been obtained.
In this district the basal level of the terrace is about 10 feet
below O.D.
The pause marked by this Third Terrace does not seem to
have endured for a very lengthened period of time. Further
movement occurred, restarting corrasion in the Thames Valley,
and the river scoured out its channel to a depth estimated at its
present mouth of at least 80 feet below O.D. The numerous
borings, excavations for the piers of bridges and so forth, which
have been made along the river afford ample evidence of this,
but at the same time it is not certain that at any point the
deepest part of this buried valley has been touched. Since also
at this last stage of Pleistocene time the valley of the Thames
must have extended to the north-east far out into what is now
the North Sea, it is tolerably certain that the mouth of the buried
valley or gorge must occur to-day in the North Sea at a far
greater depth than 80 feet below O.D.
In the fact that the Thames between the time marked by the
lower paired terrace and the commencement of the Holocene
period was able to scour out the lower courses of its channel to
so great a depth, we have a clear indication of what the relative
level of the land was during that time as compared with its
present relative altitude above the sea. It is obvious that, for
the river to have excavated any portion of its channel so far
below the present low-water mark, the land must have stood at