90 THE ROYAL SOCIETY OF CANADA 



10. The Peace River which rises on the eastern flank of the Coast 

 range pursues an unbroken course across the Rock}'' Mountain trench 

 and the Rocky Mountains. 



Cretaceous History 



In order to understand the present drainage system, it is neces- 

 sary to trace the geological history of British Columbia from at least 

 the close of the Jurassic period to the present. 



The fundamental factor in this history is the delineation of the 

 areas of erosion and sedimentation at the close of the Jurassic period. 



The orogenic movements which took place at the close of the 

 Jurassic period compressed the stratified rocks of the Jurassic and 

 Carboniferous periods into folds trending northwest-southeast and 

 permitted the intrusion of vast quantities of granitic material, over 

 an area which roughly extended from the western edge of the Rocky 

 Mountain trench, westwards to the Pacific Ocean whose shore may 

 have been situated some distance west of Vancouver and the Queen 

 Charlotte Islands. From the distribution of the Cretaceous sediments 

 which occur in three main belts separated by highlands of pre-Mesozoic 

 rocks, it can be seen that present distribution of the mountains of 

 British Columbia (except the Rocky Mountains) was already out- 

 lined at the close of the Jurassic period. The western highland 

 occupied the site of the Rocky Mountains. Separating these two 

 main highlands was a less elevated tract or interior basin now occupied 

 in great part by the Interior Plateaus. After the building of these 

 Jurassic mountains they were subjected to erosion, the products of 

 which may be seen in the Cretaceous rocks which occur along the 

 Pacific coast on Vancouver and Queen Charlotte Islands, in the 

 Interior Plateau region, and in the Great Plains and the Rocky 

 Mountain region. The granite pebbles which occur in the Cretaceous 

 formations in the three main regions mentioned above prove that the 

 two main highland areas were subjected to erosion during the Cretace- 

 ous and that the granitic cores of these areas at that time were un- 

 roofed. It is probable that the drainage at this time would be towards 

 the Cretaceous basins which occur on both sides of the two elevated 

 highlands. Erosion during the whole of the Cretaceous reduced these 

 highlands to a state of old age or peneplanation.'^ The products of 

 this erosion of the eastern highland were carried by rivers and in part 

 deposited in the great géosynclinal which occupied the site of the 

 present Rocky Mountains and the Great Plains where they may be 



1 Schofield, S. J., Geol. Surv.,Can., Mem. 76, 1915; Trans. Roy. Soc. Can., 

 vol. 13, 1919, p. 23. 



