20 



During Triassic, Jurassic and Cretaceous, sedimenta- 

 tion was continuous in the middle and eastern part of the 

 Rocky Mountain geosyncline with a probable increase 

 in the area of sedimentation of at least the Cretaceous 

 beyond the Rocky Mountain geosyncline proper. This 

 period of sedimentation was brought to a close by the 

 Laramide Revolution (Eocene), whose effects are seen in 

 the folding and overthrust faulting so characteristic of the 

 structure of the Rocky Mountain system. Since that time, 

 this belt has been subject to denudation, the detritus of 

 which is seen in the Tertiary and superficial deposits of the 

 piedmont belt of the Great Plains. 



Passing to the western or Pacific geosynclinal, which 

 lies between the Purcell Trench and Pacific ocean, the 

 earliest record is the important Pre-Cambrian (Archean) 

 sedimentation, leading to the formation of the Shuswap 

 limestones, schists and gneisses, the latter at least partly 

 of igneous origin. From this time until the Mississipian 

 period, the western geosyncline was an area of erosion, 

 which supplied the material for the formation of the Rocky 

 Mountain geosynclinal. At or near the close of the Missis- 

 sippian, the western geosyncline area was submerged and 

 received a great load of Pennsylvanian sediments and 

 accompanying lava floods. The record for the Jurassic 

 is meagre, indicating that an upheaval of the Triassic sea 

 bottom had begun as an early preparation for the Jurassic 

 revolution. This was closely followed by the intrusion of 

 many large batholiths of granodiorite and related rocks. 

 Erosion of these Jurassic mountains produced the material 

 for the smaller Cretaceous geosynclinals at various points 

 in the Main Pacific Geosynclinal. Orogenic movements, 

 called the Laramide Revolution, and batholithic intrusion 

 followed. 



During the Tertiary, erosion was dominant in this 

 belt with accompanying deposition in isolated basins. 

 Sedimentation was interrupted by local folding in late 

 Miocene and Oligocene. Vulcanism was prevalent 

 throughout the Tertiary while batholithic intrusion was 

 confined to the Miocene. 



GLACIATION. 



In that portion of the North American Cordillera, 

 which is embraced by Southern British Columbia and 



