This is really what takes place when a hard snow-ball is made by pressure in the hand. So, by subjecting fragments of ice to pressure it is first crumbled to powder, and then, as the particles are pressed together in close contact, it resumes the nature of ice again, though in a different form, taking now the shape of the mould in which it has been pressed.
Thus it is supposed that, when the temperature of ice is near the melting-point, the pressure of the superincumbent mass may produce at certain points insensible disintegration, while, upon the removal of the pressure by change of position, regulation instantly takes place, and thus the phenomena which simulate plasticity are produced. As the freezing-point of water is, within a narrow range, determined by the amount of pressure to which it is subjected, it is not difficult to see how these changes may occur. Pressure slightly lowers the freezing-point, and so would liquefy the portions of ice subjected to greatest pressure, wherever that might be in the mass of the glacier, and thus permit a momentary movement of the particles, until they should recongeal in adjusting themselves to spaces of less pressure.[AR] This is the theory by which Professor James Thompson would account for the apparent plasticity of glacial ice.
[AR] Forms of Water, p. 168.
SIGNS OF PAST GLACIATION.
The facts from which we draw the inference that vast areas of the earth’s surface which are now free from glaciers were, at a comparatively recent time, covered with them, are fourfold, and are everywhere open to inspection. These facts are: 1. Scratches upon the rocks. 2. Extensive unstratified deposits of clay and sand intermingled with scratched stones and loose fragments of rock. 3. Transported boulders left in such positions and of such size as to preclude the sufficiency of water-carriage to account for them. 4. Extensive gravel terraces bordering the valleys which emerge from the glaciated areas. We will consider these in their order:
1. The scratches upon the rocks.
Almost anywhere in the region designated as having been covered with ice during the Glacial period, the surface of the rocks when freshly uncovered will be found to be peculiarly marked by grooves and scratches more or less fine, and such as could not be produced by the action of water. But, when we consider the nature of a glacier, these marks seem to be just what would be produced by the pushing or dragging along of boulders, pebbles, gravel, and particles of sand underneath a moving mass of ice.