580 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



square miles. The quadrangle is bounded on the north by the Forty-ninth 

 Parallel and includes part of each mountain range. Considerably more than 

 one hundred cirques, about sixty mountain-tarns, and scores of typical, U-shaped 

 glacial troughs are shown in this one sheet. The vast precipices, horns, and 

 knife-edge ridges which dominate the magnificent scenery of the Rockies are 

 faithfully represented. For all of these features there can be no question as to 

 glacial origin. The glacierlets of the present day are, on a small scale, con- 

 tinuing the work of the hundreds of heavy ice-sheets which like double batteries, 

 assailed the eastern and western slopes of each mountain range. 



The drastic change in topographic quality induced by the erosion of the 

 cirque glaciers is well illustrated in the photograph of Plate 51. This is a view 

 of an 8,100-foot summit situated one mile northwest of the Boundary monument 

 at the Great Divide in the Clarke range. The smooth, domical slope covered 

 with a fine-textured felsenmeer of Kintla argillite represents a type of pre- 

 Glacial slope. The nearly vertical head-wall of the cirque represents the quarry- 

 ing of a small, north-flowing glacier. From the summit to the tarn at the 

 bottom of the cirque is a drop of 1,500 feet. At least two-thirds of that depth 

 is due to the erosion of one of the smallest of these Pleistocene valley glaciers.* 



The ' over-steepening '■ of slopes and the development of fiord-like profiles 

 are illustrated in Plates 7, 50 and 72, A. Where a pre-Glacial ridge suffered 

 attack by glaciers on both slopes a razor-back form was often produced. A 

 continuance of this double head-wall attack led to the isolation of sharp peaks 

 or horns, like Mt. Thompson and its neighbours (Plate 9). 



Nature and Extent of Glacial Erosion. — During the Boundary survey the 

 writer's opportunities for quantitative studies of these vanished glaciers were 

 limited. The studies actually made referred especially to the sheets which, 

 during the maximum of glaciation, occupied Starvation creek and Kintla creek 

 valleys. In each case the usual signs of direct ice-erosion were not found above 

 the 6,100-foot contour along the middle and lower part of the valley. In that 

 part, the Starvation glacier was about 1,000 feet thick, while the Kintla glacier 

 was about 2,000 feet thick. The bottom gradients for each trunk glacier were, 

 respectively, 100 feet and 50 feet to the mile. The surface gradients for the 

 trunks of the glaciers were of the same order but those for the high level 

 branches must have been much steeper, from 200 feet to 1,500 feet or more to 

 the mile. The thrusts exerted by these high-grade affluent streams must have 

 played an important part in developing erosive power in the trunk glaciers. 



Yet more important than bottom or surface gradients in causing the pro- 

 digious erosion of the mountains, was the bergschrund or master crevasse 

 which, as usual with valley glaciers of all kinds, was kept open between ice and 

 rock. It is generally conceded by glacialists that the conditions for glacial 

 quarrying are most favourable in the depths of the bergschrund. Alternate 

 thawing and freezing in that crevasse loosens the rock which, block after block, 

 is carried away by the ice. Since the bergschrund is developed on the sides 



* See similar photographs by W. W. Attwood, published since this paragraph 

 was written. Prof. Paper No. 61. U.S. Geol. Survey, 1909, Plates 2 and 8. 



