THE WORK OF SNOW AND ICE 



283 



Within the zone of shearing, the gliding planes of the crystals 

 may come into effective function. It is thought that the combined 

 effect of the vertical pressure, the forward thrust, and the basal 

 drag of the ice, may be to increase the number of granules whose 

 gliding planes are parallel to the glacier's bottom. At any rate, 

 such an arrangement in the basal portion of the Greenland glaciers 

 at their borders is said to exist. It is conceived that where strong 

 thrusts are brought to bear upon such a mass of granules, those 



Fig. 239. Portion of the east face of Bowdoin Glacier, North Greenland, 

 showing oblique upward thrust, with shear. 



whose gliding planes are parallel to the direction of thrust are 

 strained with sufficient intensity to cause the plates to slide over 

 each other, while those which are not parallel to the direction of 

 thrust are either rotated into parallelism when they also yield 

 or are pressed aside out of the plane of shear. As previously 

 noted, shearing is observed to occur chiefly where the ice below the 



I plane of shearing is protected more or less from the force of the 

 thrust, as in the lee of a hill or mass of debris. It perhaps also 

 occurs at the top of the basal zone of ice so loaded with debris that 

 it is incapable of ready movement. 



