During the past century a great number of ice-movement experiments have been made in various parts of the world. Some investigators have concentrated on a physical and chemical analysis of the behavior of glacier ice under stress. Others have focused their interest upon observations of glacier motion, advance and retreat over monthly, yearly, or seasonal periods. A host of others merely have made notes on the visible changes taking place from time to time in the dimensions of present-day glaciers. Many photographs, both on the ground and from the air, have been taken to record permanently these variations.
Forbes, Agassiz, and Tyndall were particularly intent upon studying the great seasonal and monthly variations of the glaciers of the Alps. Tarr, Martin, Chamberlin, Von Engeln, Hess, and many others have more recently been conducting field work bearing upon the physical nature of ice movement and on the shearing, thrusting, and . . .