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.