142 WORK OF SNOW AND ICE 



beyond. In coherence, these aggregations vary from the neve stage, where the 

 grains are small and spheroidal, to the ice stage, where the cohesion is strong 

 through the interlocking growths of the large granules. 



MOTION OF GLACIER ICE > 



Rotation and sliding of granules. There seems to be no escape from the con- 

 clusion that the primal cause of glacier motion is one which may operate even 

 under the relatively low temperatures, the relatively dry conditions, and the 

 relatively granular textures at the heads of glaciers. These considerations lead to 

 the view that movement there takes place by the movements of the grains upon 

 one another. While they are in the spheroidal form, as in the neve, this would 

 not seem to be difficult. They may rotate and slide over each other as the weight 

 of the neve increases, and the motion between the granules might be comparable 

 to that between shot in great quantities in similar positions. The amount of 

 motion required of an individual granule is surprisingly small. In order to account 

 for a movement of three feet per day in a glacier six miles long, the mean motion 

 of the average granule relative to its neighbor would be roundly, TOOOO of its own 

 diameter per day; in other words, it should change its relations to its neighbors to 

 the extent of its diameter once in about thirty years. A change of such slowness 

 under the conditions of granular alteration can scarcely be thought improbable. 



Melting and freezing. After the granules become interlocked, as in the body 

 of the glacier below the neve field, rotation and sliding must be more difficult. 

 Then, if not earlier, the movement between granules is supposed to be effected 

 chiefly by the temporary passage of minute portions of the granules into the fluid 

 form at points of greatest compression, the transfer of the water thus produced to 

 adjoining points, and its resolidification. The points of greatest compression are 

 obviously those whose yielding most promotes motion, and the successive yielding 

 of points which come in succession to oppose motion most (and thus to receive the 

 greatest stresses) , permits continuous motion. It is only necessary to assume that 

 the gravity of the accumulated mass is sufficient to produce a little temporary 

 liquefaction at the points of greatest stress, the result being accomplished not so 

 much by the lowering of the melting-point as by the development of heat by 

 pressure. This is believed to be the largest single element in glacier motion. 



This conception of glacial movement involves the momentary liquefaction of 

 minute portions of the ice, while the mass as a whole remains rigid, as its crystalline 

 nature requires. Instead of assigning a slow viscous fluidity like that of asphalt 

 to the whole mass, which seems inconsistent with its crystalline character, it assigns 

 a free fluidity, momentarily, to a succession of particles that form only a minute 

 fraction of the whole at any instant. This conception is consistent with the reten- 

 tion of the granular condition of the ice, with its rigidity and brittleness, and with 

 its strictly crystalline character, a character which a viscous liquid does not possess, 

 however much its high viscosity may make it resemble a rigid body. 



Accumulated motion in terminal part of glacier. However slight the relative 

 motion of one granule on its neighbor, the granules in any part of a glacier partake 

 in the accumulated motion of all parts nearer the source, and hence all except those 

 at the head are thrust forward. Herein appears to lie the distinctive nature of 

 glacial movement. Each part of a stream of water feels (i) the hydrostatic pres- 



1 For fuller discussion, see the authors' Geologic Processes, pp. 308-321. 



