90 G. M. DAWSON — ROCKY MOUNTAIN REGION IN CANADA 



came renewed denudation or " planation," and this continued until the 

 enormous volcanic extravasations of the Miocene began. 



It is not proposed in this place to recapitulate in detail the physical 

 conditions of the Tertiar}^ period, for it has alread}^ been necessary to refer 

 to these in connection with the description of the beds themselves, which, 

 because they have not been materiall}^ changed since their deposition, 

 really tell their own tale. 



It need only be said that, after the Oligocene lake deposits had been 

 formed, disturbed, and denuded, new series of lakes were from time to 

 time produced at different stages during the Miocene, their beds now 

 generally appearing as intercalations in volcanic deposits of great mass. 

 Both the coast and the interior region appear to have been subject to 

 these conditions, while the Laramide range stood high, with the inland 

 plain of the continent sloping eastward from its base. 



Following the close of, or at least a great reduction in volcanic activity, 

 in the early Pliocene, the interior zone of the Cordillera again assumed 

 a condition of stability for a considerable time, during which wide and 

 " mature " stream valley's were formed. The elevation of the Interior 

 plateau region of British Columbia must then have been about 2,000 feet 

 less than it is at present.''^ Farther north, the yellow Pliocene gravels of 

 Horsefly river, and other places, are attributed to this period, and the 

 southern aspect of their contained fossil plants is such as to indicate 

 that, in the given latitude, the height of that part of the interior can not 

 have been much above the sealevel. 



In the later Pliocene a very marked reelevation of the Cordilleran re- 

 gion evidently occurred, leading to the renewed activity of river erosion, 

 the cutting out of deep valleys and canyons, and the sha})ing of the surface 

 to a form much like that held by it at the present day. This elevation 

 in all probability affected the coast as well as the interior, and it would 

 appear that the rivers for a time extended their courses to the edge of 

 the continental plateau. 



The excavation of the remarkable fiords of British Columbia and the 

 southern part of Alaska must, I think, be chiefly attributed to the later 

 portion of the Pliocene, although it is quite possible that the cutting out 

 of the valleys may have been begun soon after the Laramide upheaval. 

 The antiquity of these valleys is evidenced by the fact that several com- 

 paratively small rivers still flow completely across the Coast ranges in 

 their deep troughs. - The fiords are now essentially the submerged lower 

 parts of these and other drainage valleys of the old land, not very ma- 

 terially affected by the later glacial action, important as this has un- 



* Trans. Royal Soc. Can., vol. viii, sec. iv, p. 18. 



