PLEISTOCENE VULCAN ISM OF BRITISH COLUMBIA 261 



2. Main valley of the Cheakamous, which has a total depth 

 approaching 4,000 feet, and consists of well-marked inner and outer 

 valleys. Both of these are glaciated parallel to their length. The 

 outer valley is represented by a terrace at least a mile wide on the 

 eastern side, and some 3,000 feet below the rim of the valley. 



3. On reaching the upper edge of the valley one emerges at 

 about 5,100 feet elevation, on a plateau, which has been dissected 

 into broad, flat-topped spurs by the streams tributary to the 

 Cheakamous. The surface of the plateau slopes upward to the 

 east at an angle of from 7° to 10°, which increases toward the east, 

 and an extension of the plateau surface probably forms the summit 

 level of the range, at an altitude of about 8,000 feet. Incised in its 

 surface are the remains of one or two older valleys which once ran 

 parallel to the Cheakamous, but have since been cut across by the 

 tributaries of that stream, and appear as notches crossing the 

 intervening spurs. The age of this plateau is indicated by the fact 

 that two ridges rise above it whose summits consist of remnants of 

 andesitic lava flows, probably of early Miocene age. These flows 

 originally probably filled valleys. The floor upon which the flow 

 rested is now in one case, the Black Tusk Ridge, at least 1,200 feet 

 above the plateau-surface below, while in another case, near Table 

 Mountain, the columnar andesite itself forms a part of that surface. 

 The end of the erosion-cycle which was responsible for the plateau 

 can therefore hardly be earlier than Pliocene or late Miocene. On 

 the other hand, both the plateau-surface and that of the ridges above 

 it bear striae of the earliest glaciation of which record remains. 

 The ice during this time covered the range up to a height of at least 

 6,500 feet as a sheet, whose general movement in this locality was 

 S.S.W., or parallel to the trend of the main valleys. The plateau 

 may therefore be assumed to be pre-Pleistocene. It is composed of 

 rocks of Palaeozoic (probably Devono-Carboniferous)^ age, of the 

 subsequently intruded (Upper Jurassic) granites of the Coast 

 batholith, and, as already explained, in some parts of the Miocene 

 andesites. The tributary valleys which dissect it end in cirques in 

 the upper part of the range, which are for the most part still occupied 

 by glaciers. These valleys are of considerable depth, in one case 



' See O. E. LeRoy, G.S.C., Publication No. 996, 1908. 



