588 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



ice-filled. The main Purcell Trench glacier then occupied the site of the 

 present delta of the Kootenay and furnished the western shore of the postulated 

 temporary lake. The lacustrine clay now exposed in the Porthill bench may 

 in part have been derived from the debris washed out of the trench glacier, but 

 the topographic relations suggest that the late-Glacial Goat river delivered 

 most of the clay to the temporary lake. East of Copeland the clay bench is 

 bounded on the east by a strong moraine which extends southward from the 

 mountain-spur separating the Moyie river valley from the Purcell Trench. 

 This moraine is homologous to those winged-out from the Clarke range into 

 the Flathead valley and shows the place of meeting between the Purcell Trench 

 glacier and the Moyie glacier. The Porthill clay is a later deposit formed after 

 a pronounced shrinkage of the trench glacier, which had then retreated to the 

 western half of the trench. The bed of that diminished ice-sheet was, on the 

 final disappearance of the ice, filled with the waters of Kootenay lake. The 

 building of the Kootenay river delta from Porthill to Kootenay Landing, a 

 distance of twenty miles, is the work of post-Glacial time. 



Selkirk Mountain System. 



The summit ridge of the Nelson range located a notable and abrupt change 

 in level of the ice-cap at its maximum strength. From Mt. Ripple eastward 

 this upper limit occurred near the present 7,300-foot contour. On the western 

 side of the divide the surface of the cap declined rapidly so as to reach little, 

 if any, higher than the 6,800-foot contour. At the Columbia it appears to have 

 been as low as the 6,500-foot contour. Since the mountains west of the high 

 ridge of the Nelson range were completely buried by the ice-cap, the latter 

 figures have not been based on direct observations. The values have been 

 obtained by interpolating on the flat curve joining observed points in the Nelson 

 range and in the Kossland mountains. Notwithstanding this sharp change of 

 level for the ice-surface, it seems best to regard the ice-sheets on each slope of 

 the Nelson range as part of the one ice-cap. The summit ridge was simply 

 a long nunatak within the great sheet. 



The observations on the stria? occurring on the ridges and peaks showed 

 that the general movement of the ice on the eastern slope of the Nelson range 

 was in the direction S. 30° E. On the western slope, as over the Bennington 

 range, the ice moved, on the average, about S. 10° E. 



Cirques are common along the belt of the Nelson range nunatak, but are 

 rare to the westward, where for a distance of forty miles all the peaks of the 

 ten-mile belt were covered by the ice. (Plate 54.) The generation of the 

 cirques has here, as usual in the Cordillera, caused the residual peaks and ridges 

 to show systematic slopes. The slopes facing southwest, south, and southeast 

 are, as a rule, much less steep than those facing northeast, north, and north- 

 west. The reason is obviously due to the varying strength of the glaciers as 

 they and their respective snow-fields thickened in the shadows of the northerly 

 slopes or thinned under the direct solar rays beating on more southerly slopes. 

 An illustration of the resulting asymmetry of the peaks is given in Plate 19. 



