590 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



and grooves showed that the ice moved nearly due east, evidently flowing from 

 the high Okanagan range toward the lower ground of the Interior Plateaus. 

 In the middle of the 100-mile section R. W. Brock has made a study of the 

 glacial phenomena in the area of about 220 square miles included in the 

 Boundary Creek district. A brief statement of his results regarding the direc- 

 tion of ice flow may be quoted; it corroborates the present writer's conclusions 

 regarding this area. 



' The direction of the movement as shown by the striation on the 

 polished surfaces of the rocks, is influenced by the local topography, the 

 ice having a tendency to move in the direction of the principal valleys. 

 On the summits of ridges and mountains, it shows greater independence. 

 It varies from S. 15° W. to S. 41° E. An average of a great number of 

 readings gives S. 18° E. as the general direction of flowage.'* 

 The drift mantle in the Midway mountains and Belt of Interior Plateaus 

 is thicker and more continuous than in any other part of the whole transmon- 

 tane section. The erosion of the bed-rock is much less conspicuous than in the 

 ranges farther east or in the Cascade ranges. Well characterized roches mouton- 

 nees are not common. The bed-rock is often weathered more deeply than is the 

 rule in other parts of the bed of the ice-cap. The facts show that in this part of 

 the Cordillera, the ice-cap, thick as it was, performed relatively little erosion. Its 

 activities were largely spent in transporting and depositing the abundant drift 

 material won from the Interior Plateaus and that brought to the ice-cap by the 

 feeding glaciers which drained the neve of the Cascade range. The explanation 

 of the feeble erosive power is to be found partly in the fact that the front of 

 the ice-cap during its maximum extension lay not far south of the Eorty-ninth 

 Parallel; yet still more clearly in the fact that the average speed of ice-move- 

 ment must have been low in the whole ice-cap area. The character of the drift 

 is highly variable; it includes much boulder-clay, as well as washed drift. 



As the general ice-cap wasted away, the uplands were uncovered and, during 

 a considerable time, the Okanagan, Similkameen, and probably other valleys 

 were occupied by local glaciers of great size. These were responsible for the 

 intense erosion of the valley bottoms and sides, which are therefore character- 

 ized by abundant polished and grooved ledges of fresh rock. The increase 

 in the erosive effects is very noticeable as one descends the 4,000 feet into the 

 Osoyoos lake trough. 



The bed-rock sides of the trough are at many points covered with local 

 and discontinuous terraces composed of sand or of roughly stratified gravel. 

 The highest observed deposit of the kind was found at the 3,700-foot contour 

 on the east side of Osoyoos lake. Others at a dozen or more different levels 

 occur on both slopes. (Eor locality see Plate 63). These are almost without ques- 

 tion deposits of rock-debris which were washed into the valley and lodged between 

 the valley wall and the Okanagan glacier. As the ice-sheet diminished these 

 lateral terraces were formed at lower and lower levels. The resulting step-like 

 forms are not, therefore, stream-cut terraces but are little-altered constructional 



E. W. Brock, Ann. Rep., Canadian Geol. Survey, Vol. 15, 1902, pp. 94 and 



96. 



