68 CONTRIBUTIONS TO PLEISTOCENE GEOLOGY
entered the main stream. It is evident that where a tributary
enters the main stream its current must neutralize, at the point of
junction, the current of the main stream to an extent dependent
upon the volume and velocity of each and the angle of their
junction. So that, although lower down the effect of the
tributary is to augment the velocity of the main stream and to
increase its transporting capacity, at the point of junction the
velocity is checked, and there is produced a local diminution of
transport power. Therefore, in the case of the river building up
its bed, at these points finer material would be deposited than
elsewhere. Now in all cases seen by us, as for example
at Greenhithe in Kent, at the ancient mouth of the Roding,
etc., it is evident that during the High Terrace stage the
Thames was carrying only the finest gravel, together with
large quantities of sand and brickearth. There is evidence that
the river had its velocity and volume at times increased by
floods, in the little secondary channels and bleached and buried
surfaces which occur in these deposits, examples of which have
been described in Part I., in the papers on the Ilford and
Wanstead district, and by many other authors. In the loss of
velocity indicated above and shown to be general throughout
the lower Thames, thereby rendering it unable to transport its
coarser detritus, we think we have abundant proof that the river
had corraded its valley as far as was then possible, and that it
had reached its effective base-level of erosion.
The conditions just described seem to have endured for a
very considerable period of time, after the elapse of which
tectonic movements succeeded in lowering the effective base-
level of erosion. This is shown in the resumed general
deepening of the lower Thames Valley subsequently to High
Terrace times. The extent to which the base-level of erosion
was lowered in the valley below London appears to have been
about 100 feet, for it is at nearly the present Ordnance Datum line
that we meet with evidence of the next pause in the deepening
process.
A study of the corrosion, consequent upon the lowering of
the base-level of erosion at the end of the High Terrace stage,
which took place in the Thames Valley, affords us with an
admirable illustration of the influence which outcrops of com-
paratively resistant strata have in retarding the erosion of the