REPORT OF THE CHIEF ASTRONOMER 589 



SESSIONAL PAPER No. 25a 



The cap was 4,000 feet deep over the Salmon river, and at least 5,200 

 feet deep pver the Columbia river. 



The evidences of greatly increasing abrasion and plucking power as the 

 depth of ice increased are very striking as one descends from the heights to 

 east or west of the Columbia river, down to the floor of the valley. Yet, volume 

 for volume, even this thick part of the ice-cap was vastly inferior in quarrying 

 efficiency to the relatively insignificant cirque-glaciers at the summits. The 

 average depth for the whole Selkirk system in the ten-mile belt was about 2,500 

 feet. 



Drift deposits are not abundant on the eastern slope of the range. They 

 become thicker and more important as the Columbia river is approached. The 

 grinding up of the auriferous rock along the Pend D' Oreille, followed by the 

 washing of large quantities of the rock-flour and sand into and along that 

 valley, has led to the local concentration of gold-bearing gravels. (Plates 55 

 and 56.) 



The well-known terrace sands and gravels of the Columbia valley were 

 accumulated during the slow retreat of the ice-cap and local glaciers. At the 

 Forty-ninth Parallel the surface of the main terrace is about 80 feet above 

 the river. (Plate 73, Fig. A.) Four other terraces occurring on the valley 

 slopes about five miles southwest of Waneta, were barometrically determined to 

 be 350, 400, 525, and 725 feet higher, but these are probably of quite local origin 

 and do not represent a corresponding amount of excavation by the Columbia 

 in the gravel-filling of its own valley. 



Columbia Mountain System and the Interior Plateaus. 



From the Columbia river to the Similkameen river, a distance of 100 miles, 

 the mountains crossed by the Boundary are at only two places high enough 

 to show the maximum height of the ice-cap. The one locality is Record 

 mountain ridge and its northern continuation toward Old Glory mountain. 

 The other favourable locality is at Mt. St. Thomas and the ridge running south- 

 ward from it. The usual criteria for both ridge? showed that the general cap 

 did not submerge any slopes higher than the present 6,600-foot contour. Obser- 

 vations made on Mt. Chopaka just west of the Similkameen river, showed that 

 the upper limit of the ice was there at about the 7,200-foot contour. The surface 

 of the cap thus slowly declined from the Okanagan range to the Columbia river 

 at an average rate of six feet to the mile. 



The ice-cap was about 4,500 feet deep over Sheep creek valley, Christina 

 lake, and the Kettle river valley. The maximum thicknesses in the Boundary 

 belt, about 6,300 feet, were to be found over the Osoyoos lake and Similkameen 

 river valleys. The average thickness throughout the hundred miles was about 

 3,000 feet. 



The average directions of ice-movement across summits were, for the Ross- 

 land, Christina, aaid Midway mountains, about S. 20° E. At several elevated 

 points in the plateau-like Anarchist and Kruger mountains, well-marked striae 



