conducting away some of the surplus water from the melting snow and
ice by subterranean channels. It seems probable, therefore, that
glacial flood-waters were an important factor in the formation of the
canyons of the Colorado. If this supposition is correct it would
account, at least in a measure, for that distinct impression of
arrested activity one receives from the present conditions obtaining
there.*

* Some canyon floors, where there is no permanent large stream,
appear to have altogether ceased descending. Dutton says of those
which drain the Terrace Plateaus: "Many of them are actually filling
up, the floods being unable to carry away all the sand and clay which
the infrequent rains wash into them."--Tertiary History, p. 50. See
also pp. 196 and 228 Ib.


The drainage at the edges of most canyons is back and away from the
gorge itself. The reason is that the rains cannot flow evenly over a
canyon brink, owing to irregularities of surface, and once an
irregular drainage is established, the water seeks the easiest road.
A side canyon is formed, draining a certain area. Another is formed
elsewhere, and another, and so on till all drainage is through these
tributaries and away from the brink, by more or less circuitous
channels to the main stream. This backward drainage leaves the
immediate brink, or "rim," till the last, in its work of erosion and
corrasion, and the rim consequently is left higher than the region
away from it. This effect of a backward drainage is very plain on
both sides of the Grand Canyon, though it is somewhat assisted, on
the north at least, by the backward dip of the strata. It may be
modified by other conditions, so that it would not always be the case.