CREVASSES. Although the ice of glaciers can suffer some distortion without breaking, if the rate of distortion is too great the ice will crack and great crevasses form. Crevasses can be di vided into distinct classes: Marginal crevasses, which occur on the sides of glaciers and point up stream at an angle of about 45°; they are the result of increasing velocity from the sides to the centre of the glacier. There must also be a tendency to the formation of crevasses at the bottom of the glacier pointing up-stream, but it is extremely probable that the weight of the ice is sufficient to prevent their forming except occasionally very near the end of the glacier. Transverse crevasses are formed when the slope of the bed increases. Longitudinal crevasses form near the end of the glacier, especially when the ice spreads out on a plain; they are due to the pressure of the ice behind and are usually arranged radially. Irregular crevasses may be formed as the result of some irregularity in the bed of the glacier. There is usually a very
large crevasse, called the bergschrund, at the up per margin of the reservoir; it is due to the more rapid motion of ice of the reservoir pulling it away from the ice clinging to the mountain slopes. In the dissipator the crevasses are in full view, but in the reservoir they are frequently covered with snow; this makes traveling above the snow-line very dangerous, except for parties .of several persons properly roped together. When crevasses first form they are mere cracks, which afterwards widen out as the result of the motion of the ice and the melting of their sides, until they sometimes are fifty or even a hundred feet wide. They may be half a mile or more in length, but the great depths which they are supposed to reach are exaggerated; they are rare ly so much as 200 feet deep, and probably never as deep as 300 feet.