44 
The Colorado River 
pends on the volume and continuity of the flowing stream, on 
the aridity of the country through which they are cut, and 
on the rock-formation. A fierce and continuous torrent, where 
the rainfall is at the minimum, will so speedily outrival the 
forces of erosion that the canyon will have vertical walls. An 
example is seen in those frequent “mud ’’ canyons found in 
arid regions, where some brook, having its source in highlands, 
cuts a channel through clay or dry earth with vertical sides, 
that stand for years. As long as the surface of the adjacent 
lands is undisturbed, it acts like a roof, throwing off the water 
that falls upon it into the main stream. ‘ Thus the founda¬ 
tions of these walls are not assailed from hehmd^ 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 sud¬ 
denly, rarely continuously for any length of time, and are col¬ 
lected and conducted away immediately, not having a chance to 
enter the ground. Homogeneous sandstone preserves its per¬ 
pendicularity better than other rocks, one reason being that it 
does not invite percolation, and usually offers, for a consider¬ 
able distance on each side of the canyon, barren and imper¬ 
vious 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. 
In the basin of the Colorado are found in perfection all the 
extraordinary conditions that are needed to bring forth mam¬ 
moth canyons. The headwaters of all the important tribu¬ 
taries are invariably in the highest regions and at a long distance 
from their mouths, so that the flood waters have many miles 
of opportunity to run a race with the comparatively feeble 
‘ 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. 
2 Robert Brewster Stanton explained this very clearly in his investigations for 
the Canadian Pacific Railway into the causes of land-slides on that line. 
