DEVELOPMEXT OF THE C0RDILLP:RAX GEOSYXrLINE 191 



io have been oscillatory during Lower Cretaceous time, just as were the 

 Comanchian (Washita) seas spreading from Mexico northward to make 

 the Dakotan sea. Finally, in early Upper Cretaceous (Benton) time, 

 the Rocky Mountain geosyncline was at its height of flood, making the 

 largest inland sea that ever lay upon the Xorth American continent. It 

 then extended continuously from the Arctic Ocean into the Caribbean 

 mediterranean, and from British Columbia, Idaho, and Utah eastward 

 into Manitoba, Minnesota, Iowa, Kansas, and eastern Texas. Along the 

 western side of the trough from Wyoming north into Alberta, from 

 10,000 to 20,000 and locally even 25,000 feet of coarse elastics were laid 

 down. Toward the east all the formations thin away rapidly to a few 

 thousand feet in depth. However, soon after the fullness of this great 

 sea the northern end of the geosyncline began, in Xiobrara time, accord- 

 ing to Dowling,-^^ to warp above sealevel. The higher arching continued 

 to spread southward during the remainder of Cretaceous time, causing 

 the wide Coloradoan sea to change into a continually southwardly shrink- 

 ing gulf, and the last of its marine waters (Cannonball) were blotted 

 out just before Fort Union time.'^^ Fresh-water formations, however, 

 continued to accumulate for a time (Fort Union, Puerco, Torrejon), 

 and finally, late in the Paleocene or, more exactly, just before Wasatch 

 time, came the second and most marked time of late Cretaceous orogenies, 

 transforming the Eocky Mountain geosyncline into the mighty Cordillera 

 of western Xorth America. The folding and thrusting was away from 

 the geanticline and toward the east. On the other hand, as the eastern 

 mountains of the Pacific system appear to be folded and thrusted away 

 from the Intermontane geanticline, we see in parts of the polygenetic 

 Cordilleras of western Xorth America another example of bilaterally 

 symmetrical mountains. 



We must return once more to the sediments of the liocky Mountain 

 geosyncline to bring out the striking fact that in the area of greatest 

 subsidence — that is, in Idaho, western Montana, southeastern British 

 Columbia, and southwestern Alberta — there had been no orogeny from 

 early Proterozoic time until the close of the Cretaceous. During this 

 vast time there was laid down in this area about 20,000 feet of Mesozoi(3 

 strata, resting conformably upon 26,000 feet of Paleozoic formations, 

 and these in turn lie conformably upon about 30,000 feet of but little 

 metamorphosed Proterozoic rocks. It is the longest accessible geological 

 section know^n anywhere and attests the striking fact that the earth's crust 

 may subside at least 14 miles before it becomes folded into mountains. 



»« D. B. DowUng : Summ. Rept., (ieol. Survey Canada. 1021. 1922, pp- T0B-90B. 

 ^' T. W. Stanton : The Cannonball marine meml>er of the Lance formation. U. 

 •Geol. Survey, Prof. Paper 128. 1920. 



