But all parts of it do not move with equal velocity.

The center travels quicker than the margins, and the velocity of the surface is greater than that of the bottom.

Naturally the velocity increases with the slope, and when the ice begins to soften in the summer time its rate of motion is increased.
But not only does the ice move.

There have been other forces set in motion as well as that of the ice.

The fierce attacks of the storms, the insidious forces of frost, of expansion and contraction, of lightning, etc., have shattered and loosened vast masses of the mountain summits.