380 WILLIAM HERBERT HOBBS 
must in general project above the ice of the glacier. The rock 
surface will also be reached by air and water wherever crevasses 
descend through the ice of the glacier to the bed upon which it 
rests. The conditions essential to the sapping process are a supply 
of water on the rock surface and oscillations of temperature about 
the freezing-point. These conditions are not realized either in the 
case of ice caps or of continental glaciers, save only where nunataks 
emerge from beneath the ice near to the glacial margin. 
When during an advancing hemicycle of glaciation a mountain 
glacier is so amply nourished that the rock walls of its containing 
basins become entirely submerged (ice-cap stage), a profound and 
immediate transformation takes place in the sculpturing processes. 
Sie ee - eS = == SS eee = 7 SS =a ta 
DICED. eB aes Sag Se == ae ase: ness Fe St mae =, 
Fic. 12.—The northern cirque (Kjedel) on Galdhépig in the glaciated surface a 
Norway. (After E. Richter.) 
Up to this time, under the dominating influence of the sapping 
process, the effect of the glacial sculpture has been to sharpen all 
projecting features of the relief as the glacial basins and channels 
are carved deeper and extended outward from each individual 
locus. Now, however, under the plucking and grinding processes 
alone, which have usurped the functions of the frost-weathering, 
the pinnacles and horns within the comb ridges are truncated and 
ground down, with the result that above the shallowed cirques 
and the largely obliterated U-valleys there extends a flatly convex 
surface like that which is fashioned by the same processes beneath 
a continental glacier. The sharp relief which was inherited from 
the period of mountain glaciation is thus gradually ironed out into 
a flatly convex surface which is everywhere ground and polished 
by abrasion. The U-valleys are first effaced, beginning at their 
