OF THE THAMES VALLEY.
83
result of erosion We feel confident that the deep channel of
the Lea simply represents in that tributary the deep channel in
the Thames Valley, and therefore that its excavation took place
in late Pleistocene times. If, as Mr. Holmes believes, the
excavation of the Lea Channel was Pre-Glacial or early Glacial,
then he must also show that the excavation of the valley of the
Thames was accomplished first at a corresponding period ; then
that it in common with the Lea Valley was filled up with glacial
detritus; and lastly that it was re-excavated and filled again
with later Pleistocene and Holocene deposits. Of this there is
certainly no evidence whatever, and on the contrary, in our
opinion there is direct evidence against any such view. We
may state in conclusion, that the deep channel of the Lea was
exactly what we expected, and indeed believed to occur, long
before its discovery was actually made, by the consideration of
the evidence of the Thames Valley.
IV. CONCLUSIONS.
The manner in which the evidence has been treated in the
preceding pages renders any attempt at setting forth our
conclusions in extenso unnecessary. There still remain, however,
three points on which we have to touch.
The first of these is the subject of the stratigraphy of the
deposits, and as we have summarised our views with regard to
the deposits of the Thames and the Mardyke in the annexed
table, we may dismiss it from the discussion.
We have endeavoured to show that the deposits of the
Lower Thames, taken as a whole, form terraces, which, from
their structure, give clear evidence of their formation under those
conditions which would result from a series of changes in the
relative value of the base-level of erosion of the region, and we
have indicated that such changes in the base-level of erosion
have probably been occasioned by tectonic movements which
altered the relations between land and sea. It remains for us to
give, as far as the evidence noted in this paper carries us, an
idea as to what those changes were at the various stages, that is,
as to whether they were of a positive or a negative character.
In doing so it is necessary to ascertain precisely what the base-
level of erosion of a region is. We have already defined its
function and we have shown that base-levels may be local as well