Crevasses. Slight changes of slope in the glacier bed, and the different rates of motion in different parts, produce tensions under which the ice cracks and opens in great fissures called crevasses. At an abrupt descent in the bed the ice is shattered into great fragments, which unite again below the icefall. Crevasses are opened on lines at right angles to the direction of the tension. Transverse crevasses are due to a convexity in the bed which stretches the ice lengthwise ([Fig. 99]). Marginal crevasses are directed upstream and inwards; radial crevasses are found where the ice stream deploys from some narrow valley and spreads upon some more open space. What is the direction of the tension which causes each and to what is it due? (Figs. [100] and [101].
| Fig. 99. Longitudinal Section of a Portion of a Glacier, showing Traverse Crevasses | Fig. 100. Map view of Marginal Crevasses |
Fig. 101. The Rhone Glacier, showing Radial Crevasses, the Alps
Fig. 102. Map View of the Junction of Two Branches of a Glacier