Sir, what we understand from last 4-5 lines.

The mechanics of a glacier flow is not clearly understood, but the velocity of glacier movement can be measured on the glacier surface, by driving a series of stakes and by surveying their displacements Over a certain period of time. On such surveys, the glaciers have been found differing in their movement rates, from a few cm per day to as much as 20 metres per day under extreme conditions. The velocities, deep below the glacier surface, are dificult to be evaluated even with the help of bore holes.lt is however, generally believed that the rate of glacier flow increases with depth, but some minimum movement does OCCur at the base of