582 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



a function of its necessarily low velocity of flow. The spectacular erosional 

 effects of the larger valley glaciers, in the deepening of rock basins, the truncation 

 of spurs, and the development of hanging valleys, are to be directly referred 

 to the necessarily much higher velocities of such local ice-sheets. 



An example of this contrast may be taken from the Cordillera itself: The 

 vast central ice-cap of British Columbia, though of great average thickness, 

 had relatively small effect in modifying the pre-Glacial forms of mountain and 

 valley. On the other hand, the Chelan valley glacier of Washington, covering 

 in all about 550 square miles, has shaped one of the grandest mountain-troughs 

 in the world, deepening it and truncating the adjacent mountain spurs so as 

 to give a valley form like that of a Norwegian or Alaskan fiord. In the process 

 the rock-basin of Lake Chelan was sunk at least 300 feet beneath the present 

 level of the sea * We must believe that the climatic conditions were here very 

 similar to those which bred the British Columbia ice-cap. The different power 

 of erosion is simply a matter of concentration of flow. .The channel of outflow 

 at maximum glaciation was only about three miles in width at the Narrows 

 of Lake Chelan. The area of ice drained through this trough was that of a 

 circle about ninety miles in circumference. The width of the effluent Chelan 

 glacier at the Narrows corresponds to only twelve degrees of arc measured on 

 that circumference. The outflow of the British Columbia ice-cap must have 

 been on the average much less than one-tenth as much concentrated. The 

 Labrador ice-cap at maximum glaciation was free to move on nearly all radii, 

 so that, if its climatic conditions were like those of the Chelan field, the con- 

 centration of flow was nearly thirty times as great in the Chelan valley as the 

 flow at the average point near the edge of the Labrador ice-cap. Trouton's 

 experiments seem to demonstrate that abrasion or scour on bed-rock is directly 

 proportional to the velocity of the ice. f Hence we may believe that the Chelan 

 glacier on a flat or even a reversed bottom gradient, abraded its floor about thirty 

 times faster than the Labrador ice-cap scoured its larger area. If the Labrador 

 ice-cap lowered its bed on the average fifty feet in solid rock, we can readily agree 

 that the powerful mountain glaciers showing concentration like that at Lake 

 Chelan, could in the same time lower their beds locally for thousands of feet in 

 The living rock. • 



To the present writer, therefore, it appears that there is no real ground 

 for controversy regarding the efficiency of glacial erosion. The principle of 

 concentration of ice flow in mountainous topography has been stated by several 

 writers who are engaged in the controversy. Perhaps because it is so obvious, 

 however, the principle has remained in the background. The purpose of the 

 foregoing note is to point out that the principle is at the essential core of the 

 problem. The glacial erosion actually proved in New York state or in the 

 Midland counties of England, meagre as that erosion has been, is one of the 

 evidences going to show that solid rock thousands of feet in depth has been 

 excavated from Norwegian, Alaska, and British Columbia valleys during the 

 Glacial period. In each case the work accomplished has been a function of the 



* B. Willis, Prof. Paper No. 19, U.S. Geol. Surv., 1903, p. 58 and plate 8. 

 t F. T. Trouton. Proc. Roy. Soc, Vol. 59, 1895, p. 25. 



