GLACIAL FEATURES 287 



cover, and a correspondingly wide distribution of the feeble ef- 

 fects of nivation. 



Test cases are presented in Figs. 191, 192, and 193, Cordillera 

 Vilcapampa, for the determination of the fact of the movement 

 of the snow long before it has reached the thickness Matthes or 

 Hobbs believes necessary for a movement of translation to begin. 

 Fig. 191 shows snow masses occupying pockets on the slope of a 

 ridge that was never covered with ice. Past glacial action with 

 its complicating effects is, therefore, excluded and we have to deal 

 with snow action pure and simple. The pre-glacial surface with 

 smoothly contoured slopes is recessed in a noteworthy way from 

 the ridge crest to the snowline of the glacial period at least a thou- 

 sand feet lower. The recesses of the figure are peculiar in that 

 not even the largest of them involve the entire surface from top 

 to bottom; they are of small size and are scattered over the entire 

 slope. This is believed to be due to the fact that they represent 

 the limits of variations of the snowline in short cycles. Below 

 them as far as the snowline of the glacial period are larger re- 

 cesses, some of which are terminated by masses of waste as exten- 

 sive as the neighboring moraines, but disposed in irregular scal- 

 lops along the borders of the ridges or mountain slopes in which 

 the recesses have been found. 



The material accumulated at the lower limit of the snow cover 

 of the glacial period was derived from two sources: (1) from 

 slopes and cliffs overlooking the snow, (2) from beneath the snow 

 by a process akin to ice plucking and abrasion. The first process 

 is well known and resembles the shedding of waste upon a valley 

 glacier or a neve field from the bordering cliffs and slopes. Ma- 

 terial derived in this manner in many places rolls down a long 

 incline of snow and comes to rest at the foot of it as a fringe of 

 talus. The snow is in this case but a substitute for a normal mass 

 of talus. The second process produces its most clearly recogniza- 

 ble effects on slopes exceeding a declivity of 20° ; and upon 30° 

 and 40° slopes its action is as well-defined as true glacial action 

 which it imitates. It appears to operate in its simplest form as 

 if independent of the mass of the snow, small and large snow 



