NO. 4 PRE-DEVONIAN SEDIMENTATION 165 



in open connection with the seaways to the north and the connecting 

 seaways on the south. 



BEAVERFOOT TROUGH 

 Plate 25 and fig. 21 



On the western side of the Cordilleran GeosyncHne there was a 

 narrow trough that connected on the north with the Glacier Lake 

 Trough (B on map), and terminated on the south, as far as we now 

 know, at about what is now the southern end of the Stanford Range. 

 That this trough connected with seaways extending north to the Arctic 

 Ocean and far to the south, is indicated by the presence of similar 

 genera in the faunules of Cambrian, Ozarkian and Silurian forma- 

 tions in Alaska, British Columbia, Idaho, Ctah, and Nevada. In this 

 trough, south of the Kicking Horse Canyon, there were deposited in 

 Upper Cambrian time the following formations : Lyell limestones, 860 

 feet (262.1 m.),' and the Sabine shales, 750 feet (228.6 m.) ; in-Ozark- 

 ian time, the Mons limestones and shales, 3,400 feet ( 1,036.3 m.) ; 

 in Ordovician (Canadian) time, the Sarbj.ch limestone, 200+ feet 

 (60.9+ m.), ajid the Glenogle shales, 2,160 feet (658.4 m.) ; and in 

 Silurian time, the Beaverfoot and Brisco formations, 1,600 feet 

 (487.7 m. ) ; making a total thickness of over 8,700 feet (2,651.7 m.) 

 of sediments deposited prior to the incursion of the Devonian sea. 

 There was also a considerable thickness of arenaceous and siliceous 

 material deposited in Lower Cambrian time that now forms the 

 Lower Cambrian sandstones and shales on the eastern slopes of 

 the pre-Canibrian on the western side of the Columbia River Valley. 

 These Lower Cambrian sands and muds were deposited in the Bow, 

 Glacier Lake, and Beaverfoot Troughs, and in the depressed areas 

 of the Purcell and Selkirk Mountains. This is evidenced by the pres- 

 ence of the same character of sandstone, containing a similar late 

 Lower Cambrian fauna, wherever the outcrops now occur from Ghost 

 River on the eastern side across the area of the Bow Trough and on 

 the western side of the Columbia River Valley. 



The extension of the Beaverfoot Trough has not been traced north 

 of the Kicking Horse River, but at Glacier Lake, about 50 miles 

 (80.5 km.) north of the Kicking Horse River, the thick-bedded 

 Copper Cambrian Lyell limestones are overlain by calcareous shales 

 and thin layers of interbedded limestone of the Sabine formation, and 



^ Walker reports a maximum thickness of about 2,000 feet (609.6 m.) 

 south of Fairmount Springs, where he confuses it with the Ottertail forma- 

 tion. Geol. Surv. Canada, Memoir 148, 1926, p. 21. 



