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.