Alaska by James Albert Michener — Read

AT VARIOUS TIMES IN THE ANCIENT PAST, FOR A COMPLEX SET of reasons which have yet to be untangled, ice began to collect at the poles in vast quantities, growing ever thicker and more extensive, until it created immense ice sheets which encroached on surrounding continents. Snow fell at such a rate that it could not melt as it would have done under ordinary circumstances. Instead, it piled to unprecedented heights, and the pressure from that on top was so considerable that the snow at the bottom was turned into ice, and as snow continued to fall, ice continued to form, until it stood in certain places more than a mile and a half thick. The weight was so oppressive and inescapable that certain parts of the earth's surface, heavily encumbered, began to sink perceptibly, so that land which once stood well above the surface of the oceans was now depressed to sea level or below.

If in any given region this enormous accumulation of ice rested on a flat plateau, a huge and quietly spreading ice cap resulted, but since the surface of the earth, because of the violent way in which it had been formed, was irregular, with mountains and valleys predominating, the ice which found itself on a slope, as most of it did, began slowly to move, under the force of gravity, to lower elevations, and as it did, its weight was so great that it dragged along with it a mass of rubble composed of sand, gravel, rocks and, occasionally, boulders of gigantic size. This lateral transport of material occurred wherever the ice field was in motion, but when a snowfield accumulated on some high plateau and began to send glaciers down valleys which might have steep gradients, the consequences could be dramatic, for then the ice formed a moving glacier which routed out the bottom of the valley and scored the sides with streaks so pronounced that they would still be visible eons later.