FEATURES OF VALLEY GLACIER EROSION 



719 



rather to direct attention to the phenomena than to present a finished 

 study of the same. 



Features of Valley Glacier Erosion 



ice as an erosive aqent 



During the Glacial epoch the old river valleys were completely filled 

 with ice, the flow of which, notwithstanding its general resemblance to 

 that of water^ differs from the latter with respect to its greater viscosity 

 or plasticity, whichever term describes the case more correctly. Ice is 

 therefore a correspondingly stiffer or less pliable and delicate tool ; it is a 

 more conservative and stubborn erosion agent than water, and shifts its 

 direction less readily. It works on a large scale, filling the entire valley, 

 and tends during its period of rapid cutting to smooth out the little 

 irregularities of the river valley and to truncate and align all projecting 

 and overlapping spurs; in short, to simplify the general shape of the 

 valley and to emphasize its symmetry, with the result that the eye takes 

 in long stretches of the glaciated valley at a glance and at the same time 

 is impressed with the unity and dignity of the whole. Glacial valleys, 

 with their open vistas and majestic curves, possess a charm and an 

 individuality that is unique and characteristic. 



The flow of ice through a valley is controlled and directed by the 

 valley walls and floor ; they prescribe, in a measure, the lines along which 

 the ice must work; and, in turn, they suffer profound modiflcation. 

 These relations between ice-flow and valley shape are so deflnite that it 

 is possible to predict the changes which an eroding ice current or stream 

 would make in the configuration of a given river valley. In an ice cur- 

 rent filling a valley, as in a river^ the rate of flow is generally greatest 

 at a point midstream and near or at the surface, while the points of most 

 intense ice action (abrasion and plucking) are on the valley bottom and 

 the adjacent lower slopes, as there the pressure is greatest and the flow 

 sufficient to carry off the debris from the powerful chiselings of the ice. 

 The erosive action of an ice current is concentrated, therefore, along the 

 valley floor and lower slopes. During the period of maximum ice erosion, 

 the valley bottoms were often cut down far below the baselevel and now 

 appear as basins fllled with glacial rock material. 



THE ICE -FLOOD PERIOD 



The Glacial epoch was a period of ice-flood and of torrential ice action, 

 when prodigious feats of erosion were accomplished, far beyond the 



