[camsell] great canon OF FRASER RIVER 57 



the drainage lines of the region could have been changed until the 

 revolution in Miocene times and its accompanying extravasations of 

 lava. It is quite possible and, indeed, has been shown by Dawson 

 that these lava floods caused the Fraser to cut new coiu-ses in that 

 portion of its course above the canon; but in the carion itself there 

 are no extrusive rocks to be found and it is unlikely that either the 

 floods of lava or the mountain building processes themselves had any 

 influence in blocking the drainage or diverting the course of the stream. 



Since no Miocene beds are found covering the Eocene of the 

 delta it is probable that the Miocene Fraser river flowed across the 

 Eocene beds and discharged farther to the west in the Straits of Juan 

 de Fuca or even beyond. Patches of Miocene on the southern shores 

 of the western parts of the straits may represent deposits by such a 

 stream. 



During early Pliocene times no disturbances of sufïïcient magni- 

 tude took place to affect the Fraser river. Towards the close of that 

 period, however, a broad uplift occurred which resulted in the eleva- 

 tion of the Interior Plateau region and the upwarping of the Coast 

 and Cascade mountain ranges along an axis striking northwest and 

 southeast directly across that part of the valley now occupied by the 

 canon. This uplift caused a revival of drainage giving the streams 

 power to deepen their valleys; but it was s^o slow and gradual 

 that the erosive power of the Fraser river was such that it was 

 able to deepen its valley at a rate equal to the uplift, so that no diver- 

 sion of its course took place. Relative to the Coast and Cascade 

 ranges then, the Fraser river, like the Skeena to the north and the 

 Columbia to the south, is an antecedent stream. This uplift was 

 really the cause of the development of the Fraser canon as it is today, 

 and the result, before it became modified by glacial action, would 

 have been a deep V-shaped carion commencing at the head of the delta 

 and extending on through the axis of uplift into the Interior plateau. 

 Such a valley would have been constricted, as it is now, in the gorge 

 where it cuts through the massive igneous rocks of the Coast Range 

 batholith and more flaring in the softer rocks above and below the 

 gorge. Rock benches about 1,500 feet above the present valley 

 bottoms in both the Fraser and other valleys in the adjacent plateau 

 region are believed to represent the old pre-Pliocene valley slopes and 

 indicate the amount of Pliocene deepening of these valleys. 



The Pliocene uplift and the consequent revival of the Fraser 

 drainage probably led to changes in the direction of drainage of other 

 streams in the uplifted region. It is quite possible that the Fraser 

 drainage at this time captured some of the Columbia drainage going 



