By watching the changing position of marked objects placed in the ice, it has been proved that, in a valley glacier, the top moves faster than the bottom; the middle moves faster than the sides; the rate of motion increases with thickness of ice, slope of floor over which it moves, and temperature.
Ice, like molasses candy, tends to crack when subjected to a relatively sudden force, and where the ice rides over a salient on the bed of the glacier, transverse cracks or fissures often develop. Due to more rapid motion of the central part of a valley glacier, stresses and strains are set up and crevasses are formed, usually pointing obliquely upstream. Where the ice tends to spread laterally in a broad portion of a valley, longitudinal cracks may develop. Crevasses vary in size up to several feet in width and hundreds of feet in depth. Owing to the forward motion of the ice, old fissures tend to close up and new ones form, and, aided by uneven melting, the surface of a glacier is generally very rough.
Like running water, ice may have considerable erosive power when it is properly supplied with tools. The total erosive effect which has been, and is now being, accomplished by ice compared with that of running water is, however, slight. One of the main processes by which ice erosion is accomplished is "corrasion" due to the rubbing or grinding action of hard rock fragments frozen into the bottom and sides of the glacier. Thick ice, shod with hard rock fragments and flowing through a deep, narrow valley of soft rock, is especially powerful as an erosive agent because the abrasive tools are supplied; the work to be done is easy; and the deep ice causes great pressure on the bottom and lower sides of the valley. Rock surfaces which have been thus subjected to ice erosion are characteristically smoothed and more or less scratched, striated, or ground due to the corrosive effects of small and large rock fragments. This affords one of the best means of proving the former presence of a glacier over a region or in a valley. A typical V-shaped stream cut (eroded) valley is changed into one with a U-shaped profile or cross section by glacier erosion (Plate 5).
Another important process of ice erosion is "plucking," which consists in pushing among already more or less loosened joint blocks by the pressure of the moving ice. The pressure thus exerted, especially by a deep valley glacier, may be enormous. This process was an important factor in the development of the famous Yosemite Valley, a very brief account of whose history it will now be instructive to give.