throwing off the water that falls upon it into the main stream.* Thus
the foundations of these walls are not assailed from BEHIND, which is
their weakest point. If the land surface is broken up, permitting the
rains to soak in and saturate the clay or earth, the whole mass
becomes softened and will speedily fall and slide out into the
canyon.** The sides of all canyons in an arid region are more or less
protected in the same way. That is, the rains fall suddenly, rarely
continuously for any length of time, and are collected and conducted
away immediately, not having a chance to enter the ground.
Homogeneous sandstone preserves its perpendicularity better than
other rocks, one reason being that it does not invite percolation,
and usually offers, for a considerable distance on each side of the
canyon, barren and impervious surfaces to the rains. Where strata
rest on exposed softer beds, these are undermined from the front, and
in this way recession is brought about.

* Just as wheat flour getting wet on the surface protects the portion
below from dampness. The rainfall is often so slight, also, that a
surface is unchanged for years. I once saw some wagon tracks that
were made by our party three years before. From peculiar
circumstances I was able to identify them.

** Robert Brewster Stanton explained this very clearly in his
investigations for the Canadian Pacific Railway into the causes of
land-slides on that line.


In the basin of the Colorado are found in perfection all the
extraordinary conditions that are needed to bring forth mammoth
canyons. The headwaters of all the important tributaries are