182 THE ROYAL SOCIETY OF CANADA 



The Younger, Eastern Division of the Rocky Mountains 



The subsequent history of the basin to the east of the earlier 

 formed mountains as disclosed in the character of the later sedi- 

 ments is: first, a narrowing of the sea, probably largely confined 

 to the northern half of the Canadian area; then a widespread sub- 

 sidence of the southern half; and, lastly, a final retreat of the sea from 

 the north accomplished in two stages as registered in the shore de- 

 posits. In general, after the subsidence, there was an extended 

 period of tranquillity closed by an uplift, along the western margin, 

 consisting of short periods of uprise and settlement during which 

 the Belly River series was deposited, and then followed a partial 

 advance of the sea before its final retreat during which the Edmonton 

 formation was laid down. The movements during this period pro- 

 bably indicate alternating compression and tension in the crust and 

 no doubt some deformation was accomplished in the land mass to 

 the west. In the north the later Cretaceous sediments were probably 

 land deposit, but as they have largely been removed we can only sur- 

 mise that the compressive strains which, in the north, caused defor- 

 mation before the deposition of the Tertiary measures, were con- 

 temporaneous with the general uplift marking the disappearance of 

 the Cretaceous sea. 



The early Tertiary sediments, generally freshwater deposits, 

 indicate a period of continued uplift made apparent by the mass of 

 sands and clays deposited in the western part of the area and denoting 

 an increase in the grade at the western margin of the region, while 

 the central part to the east was undisturbed and became a brackish 

 lake. Certain refractory clays distributed during an early stage of 

 this lake basin are said to be derived from the Archaean surface to 

 the east. The material is also given an eastern source because of its 

 purity in the east and its admixture with less refractory clay in the 

 west. The present uptilt in the west is thus a later movement and 

 one more nearly connected with the general deformation of the outer 

 ranges. 



The coarse sediments, mainly heavy conglomerates, which cap 

 the Cypress Hills and are unconformably above the Eocene beds, 

 are generally attributed to the denudation of the then newly risen 

 mountains. The age of the beds as determined by vertebrate remains 

 is Oligocène. The last mountain building and the deformation of 

 the Tertiary and Cretaceous measures, especially near the moun- 

 tains, is thus placed at the close of the Eocene and is probably con- 

 temporaneous with the period of vulcanism in the interior of British 



