OF THE THAMES VALLEY. 63
Powell long ago distinguished between the excavation
or deepening of the channel of a river and the erosion of the
banks produced by the shifting of its course. The former
process he called "corrasion," a term which has been widely
used in America. Gilbert in his memoir on the Henry
Mountains includes a chapter dealing with "Land Sculpture,"
in which he states with exemplary clearness the laws which
govern the exercise of the corrosive function of rivers.20 He
says :—
" Where a stream has all the load of a given degree of comminution, which
it is capable of carrying, the entire energy of the descending water and load is
consumed in the translation of the water and load, and there is none applied to
corrosion. If it has an excess of load, its velocity is thereby diminished so as to
lessen its competence, and a portion is dropped. If it has less than a full load, it
is in condition to receive more, and it corrodes its bottom. A fully-loaded stream
is on the verge between corrosion and deposition. . . . The work of trans-
portation may thus monopolize a stream to the exclusion of corrosion, or the two
works may be carried forward at the same time."
" A stream which has a supply of debris equal to its capacity tends to build
up the gentler slopes of its bed and cut away the steeper."
It must be borne in mind, when applying this to rivers like
the Thames, which never seem to have had a very considerable
fall, that the expressions deposition and corrosion in the passage
quoted apply only to the building up or erosion of the bottom of
a river, and not to its lateral movements.
Suppose we have a country of low relief such as Eastern
England is to-day, and but slightly elevated above sea-level.
Erosion or denudation in such an area is carried on at a
minimum rate. The rivers have cut down their channels so
nearly to sea-level that the declivity is so small as to give the
streams barely sufficient power to transport the detritus which is
brought into them by rain and their tributaries. They cannot
corrode their channels any further, their whole sphere of action
consists of transportation and the slow modification of their
banks by their lateral movements. In other words their
destructive energies are limited to aiding the other atmospheric
forces to reduce the country to the condition of a peneplain, and
this state of affairs may be summed up in Powell's happy
phrase—"the country is at a Base-level of Erosion."
A river at base-level, therefore, since it cannot corrode its
channel, is confined to wandering about over its valley floor
planing it down and forming a broad expanse of Alluvium and