64 CONTRIBUTIONS TO PLEISTOCENE GEOLOGY
if it be liable to floods, it now tends to widen its valley, since
it cannot deepen it. If tectonic movements occur of a kind such
as to produce the effect of subsidence in the basin of the river,
the line representing the base-level of erosion in the valley is
heightened in relation to the valley, and the river at once begins
to build up its bed by aggregation. In any case the arrival of a
river at its effective base-level of erosion stops further corrosion
and the production of alluvial plains on the valley floor is the
consequence.
Now, any lowering of the effective base-level of erosion, such
as an increment to the altitude of the land in relation to the sea-
level, whether it be caused by terrestrial elevation or marine
subsidence, or as the breaching of some resistant barrier which may
have determined for a time the effective base-level of erosion of a
region, has the effect of reviving the rivers and so restarting the
corrosion of their valleys. This is true in all cases, but the
extent to which the accelerating influence so produced operates in
any given river valley depends entirely upon the structure of
that valley. Take the first case for example, that of an
increment to the relative altitude of the land. Suppose one of
the river valleys has a band of hard and resistant strata stretch-
ing across it near its mouth5; the influence of acceleration cannot
be transmitted into the courses above the band until this
obstruction is breached, and this in turn may require a vast
period of time to effect. Another valley may be carved in soft
clays or other easily eroded strata, and throughout this valley the
acceleration is experienced very rapidly. In the first of the two
valleys corrosion will be more tardy, and it will take in propor-
tion more time to reach the effective base-level of erosion than
will be required in the second valley. Now, this difference of
degree produces an essential difference in the structure of the
deposits formed by the two rivers in their respective valleys.
In the first valley, that carved in the resistant strata, we shall
find fluviatile terraces, strictly speaking, developed par excellence,
for the river has received but little aid from the tectonic
movement in accomplishing its work of deepening its valley.
Indeed, the aid received may be likened to a permission and
nothing more. In the second case matters are very different,
for this river, flowing in a valley eroded in non-resistant
5 See figure 1.