NO. 4 I'RK-DKVONIAN SEDIMENTATION I51 



This enormous mass of sediments accumulated in a trough (Sel- 

 kirk ) , the western margin of which was formed by a pre-Beltian 

 land area west of the present Alberta Canyon on the Canadian Pacific 

 Railway, or it may have been as far west as Revelstoke in the Colum- 

 bia River valley. This distance is now only 40 to 60 miles ( 64 to 

 97 km.), as it has been shortened by compression, folding, and more 

 or less faulting of the strata. 



To the eastward of the Purcell Range, the pre-Cambrian ( Beltian) 

 is concealed by Cambrian and later formations all the way to the 

 Bow Valley ' where a series of fine, impure sandstones and shales 

 of pre-Cambrian age are exposed that indicate the a])proach to an 

 eastern shore line of relatively low relief. On this eastern side of the 

 geosyncline there are only about 3.470 feet (1,057.6 m.) of Beltian 

 sediments exposed. These accumulated in a broad, shallow sea that 

 preceded the Bow Trough. 



To the south in Montana, Beltian sediments similar to those of the 

 Bow A'alley area accumulated to a depth of 36,000 feet ( 10,972.8 m.)' 

 or more prior to the advent of the Cambrian sea. 



The close Df the period of deposition of Beltian sediments was 

 followed by a slight diastrophic movement that in the Canadian area 

 resulted in low undulations and minor faulting of the strata and the 

 formation of the Bow Trough, so that when the Lower Cambrian 

 sea advanced from the south it had an unobstructed seaway. This 

 trough deepened as sediments were deposited until over 2,000 feet 

 (609.6 m.) in depth of sands and siliceous muds accumulated in it. 

 On the western side of the geosyncline the Purcell Trough deepened 

 and a great thickness of sands referred by Daly to the Lower Cam- 

 brian were deposited. These include : 



Feet. Meters 



Sir Donald Quartzite 5,000 4- 1,524.0-!- 



Ross Quartzite 2,750 838.2 



7,750 4- 2,362.2 + 



The upper part of the Ross Quartzite is referred by Daly to the 

 Lower Cambrian and the lower 2,500 feet ( 762 m. ) to the Beltian. 

 He states that " The lower part of this formation is of pre-Cambrian 

 age ; the upper part is probably to be assigned to the Lower Cam- 



^ Smithsonian Misc. Coll., Vol. 53, No. 7, 1910, pp. 423-431. 

 ^ Bull. Geol. Soc. America, Vol. 17, 1906, pp. 7, 15. Bull. U. S. Geol. Surv,, 

 No. 384, 1909, p. 41. 



