d, the amount varying
according to altitude and latitude. The general topography of the
Colorado River was about as it is to-day, and the rainfall in the
valleys probably nearly the same, or at least only a little greater. In
other words, the conditions were those of to-day intensified. In summer,
then, the amount of water seeking outlet by these drainage channels
to the sea was enormously multiplied, and the corrasive power
was correspondingly augmented. When the ice caps finally began to
permanently diminish, the summer floods were doubtless terrific. The
waters of the Colorado now rise in the Grand Canyon, on the melting of
the snows in the distant mountains, from forty to one hundred feet;
the rise must then have amounted to from one hundred to four hundred or
more. The Kanab heads in two very high regions--the Pink Cliffs and the
Kaibab. Though probably not high enough to be heavily glaciated they
were high enough to receive an increased snowfall and to hold it, or a
portion of it, over from one year to another. Thus the canyons having
their origin on these high regions would be given perennial streams,
with torrential floods each summer, compared with which anything that
now comes down the Kanab would be a mere rivulet. The summit of the
Kaibab is covered with peculiar pocket-like basins having no apparent
outlets. These were possibly glacial sinks, 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