THE WORK OF SNOW AND ICE. 317 



There is, so far as we know, no experimental or natural evidence that 

 any typical viscous body in flowing over a rugose bottom detaches and 

 picks up fragments and holds them as graving tools in its base so fixedly 

 as to cut deep, long^ straight grooves in the hard bottom over which it 

 flows. It would seem that competency to do this pecuhar class of work, 

 which is distinctive oi glaciers, should be demonstrated before the vis- 

 cous theory of glacial movement is accepted as even a good working 

 hypothesis. Somewhat in contrast with viscous movement, it is con- 

 ceived that a glacier is thrust forward rigidly by internal elongation, 

 shears forcibly over its sides and bottoms, and leaves its distinctive 

 marks upon them. 



Auxiliary Elements. 



Shearing. — In the lower portion of a glacier where normally the 

 thrusts are greatest, the granules fewest, and their interlocking most 

 intimate, shearing takes place within the ice itself. This is illustrated 

 by the accompanying Figs., 292-295. The shearing results in the foli- 

 ation of the ice and in the forcing of debris between the sheared layers. 

 Thus the ice becomes loaded in a special englacial or baso-englacial 

 fashion, as previously mentioned and illustrated in Fig. 268. 



Within the zone of shearing, it is probable that the gliding planes 

 of the crystals come into effective function. It is thought that the com- 

 bined 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, Drygalski 

 reports ^ that there is a tendency to such an arrangement in the basal 

 portion of the Greenland glaciers at their borders. It is conceived 

 that where strong thrusts are brought to bear upon such a mass of 

 granules, those 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, shear- 



^ Gronland-Expedition der Gesellschaft fiir Erdkunde zu Berlin, 1891-93, Bd. I, 

 p. 491 et seq. 



