OF THE THAMES VALLEY.
85
as general. It is with the latter and chief class that we are here
dealing, and such a base-level of erosion depends for its deter-
mination upon the relative altitude of the land when compared
with that of the sea. It is quite obvious that sea-level through-
out any region forms a nearly horizontal plane, and may, there-
fore, in section be represented by a horizontal line. The base-
level of erosion is of a different character, for taken generally
throughout a region, it will be found to coincide with low-water
mark along the sea-board, gradually rising in relation to the land
into the interior of the country, with a concave upper surface,
in fact coinciding in section with the final grade of the trunk
rivers. (Fig. 4). A tectonic movement which raises the
the land lowers the base-level of erosion, and vice-versa, so that a
positive or negative change in the relative altitude of the first
produces a negative or positive change in the value of the other.
The importance of these considerations will be appreciated
in what follows.
In the cases of the Thames, the Lea, and the Mardyke we
have noticed evidence that buried beneath their more recent
deposits, and far below their present beds, lie channels which
they once occupied and excavated, and no doubt when more
facts are available to us we shall find that similar deep channels
exist under all the tributaries which entered into the lower
course of the Thames in late Pleistocene times. This evidence
teaches us that at that time the base-level of erosion was much
lower in relation to the land than at present, and that conse-
quently the land must have stood at a much higher elevation in
relation to sea-level than it does at present. At the present
mouth of the Thames the deep channel reaches a depth which
shows a lowering of the base-level of erosion at that point by
about 80 feet, and to the standard thus afforded we have reduced
our observations and plotted a rough curve (Fig. 5) showing the
relative value of the base-level at that point at the successive stages
the history of the Thames Valley which we have endeavoured to
establish. From the fact, however, that the estuary of the
Thames at the end of the Pleistocene period probably extended
far to the north-east of its present termination, and from the
steep gradient of the old buried channel, it appears to us that
the absolute values given in the diagram are only true as regards
the base-level of erosion at the actual point named ; that is to