110 GLACIERS OF THE CANADIAN ROCKIES AND SELKIRKS. 



across the gradually rising ridges of the mountains and the lesser folds of the 

 foot-hills. In many cases, the troughs, lying between parallel ridges, or the 

 gaping crevasses in the rock strata parallel with them, captured the drainage 

 and a system of longitudinal river courses was developed, much younger than 

 the transverse system. As a result of these great erogenic movements, combined 

 with the atmospheric and aqueous agencies operating since, we have an uplifted 

 and dissected peneplain. 



b. Pre-pleistocene erosion. With the completion of the mountains at the 

 close of the Mesozoic,the more or less sluggish streams of the ancient peneplain ac- 

 quired velocity and renewed their activity, cutting deeply into their former beds. 

 The newly born longitudinal streams incised still further the channels provided 

 for them and there were developed two systems of V-shaped valleys more or less 

 intimately connected. The agencies of weathering broadened the valleys 

 above and delivered the rock fragments to the stream below, by which tools 

 the water still further deepened its beds. This action went along slowly from 

 the beginning of Cenozoic time to the beginning of the Pleistocene, during which 

 time the roughly angular blocks were carved into jagged peaks and many 

 of the divides into sharp-crested ridges. The outline of the old peneplain is to 

 be recognized only when one ascends until his eye is on a level with its uplifted 

 surface, when peaks and ridges all blend into the common level that cuts the 

 sky at the limit of vision. See plates n, xvi, and xxxn. 



c. Pleistocene erosion. The opening of the Pleistocene and the advent of 

 the glaciers introduced a new geological agent into the region. A reduction 

 in the mean annual temperature, combined with an increase in precipitation, 

 allowed the snow to accumulate about the higher peaks and ridges more rapidly 

 than it could melt away during the warmer season. Year after year the snow 

 banks thickened, sent their avalanches into the valleys faster than they could melt 

 away, and thus the mountains became enveloped in snow and ice. Glaciers 

 moved down from the more elevated valleys, joined forces with their neighbors, 

 grew in volume and power, took possession of the river valleys, and sent massive 

 tongues of ice far beyond the limits of the mountains. The valleys were filled 

 to depths of 4,000 feet from their floors, in certain cases, the actual elevation 

 rarely falling below 7,000 feet above sea-level. These ice streams exercised a 

 powerful effect upon the rock strata over which they passed; in general, rounding 

 and smoothing their outlines, cutting down prominences, and truncating moun- 

 tain spurs. In some cases where plucking was most active the rocks were made 

 still more jagged and irregular than the ice had found them. The lower half 

 of the valleys, which had been invaded by the ice, had their floors broadened 

 and their sides correspondingly steepened, giving this portion of the valley a U- 

 shaped cross-section. The upper portion, under less pressure of ice, still retains 

 more or less of its pre- pleistocene V-shaped form, the sides being simply smoothed 

 and fluted. The extension of these V-slopes until they intersect in the valley 

 may be assumed to mark the level, approximately, from which the glaciers began 

 deepening their beds. In the floors of the valleys at certain places rock-basins 



