Such an increase of volume, if we suppose the aridity to remain as now, could have come about only by an increase of precipitation on the mountain summits.

During the Glacial Epoch, the Rocky Mountain summits were considerably glaciated, 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.