DEVELOPMENT OF THE CORDILLERAN GEOSYNCLINE 187 



LATE CORDILLERAN GEOSYNCLINE 

 (See Maps, Figxires 12 to 17) 



We now return to a further consideration of the Cordilleran geosyn- 

 cline, and more especially to its changes after Devonian time. It has 

 been shown that this trough had a uniform development throughout the 

 Proterozoic and early Paleozoic. For the first time crustal deformation 

 set in late in the Devonian, but only locally in northern California 

 (Shastan channel) ; and a little later, in the Mississippian, upwarping 

 took place over a very wide region in the Mackenzie Eiver area, com- 

 pletely blotting out here the Cordilleran geosyncline. With these move- 

 ments we see Cascadia transgressed more definitely by the Shastan chan- 

 nel and the Alexandrian embayment, and taking on northwest-southeast 

 trends that bring about the further delimitation of its parts into the 

 borderlands, California and Charlotte. In consequence the inland seas 

 of Carboniferous times also took on northwest-southeast trends. This 

 netv alignment of lands and seas is seen even better in late Pennsyl- 

 vanian time, but here the areal expanse of the seas is still very extensive 

 and much like the conditions earlier in the Cordilleran geosyncline. 



RISE OF THE CORDILLERAN INTERMONTANE GEANTICLINE 



With the continued rising of the x\ncestral Eocky Mountains, the 

 Cordilleran sea of Middle and Upper Triassic time was pushed westward, 

 blotting out gradually the whole of the medial portion of this old 

 geosyncline. Pinally, early in the Jurassic there began farther west the 

 rising of the Cordilleran Intermontane geanticline, which during the 

 Cretaceous separated two independently evolving geosynclines. This 

 greater arch, however, was not completed until after the close of the 

 Jurassic, for the Logan sea of early Upper Jurassic time was still in 

 wide marine connection with both the North Pacific and the Arctic 

 oceans. Finally, toward the close of the Jurassic, followed the Sierra 

 Nevada orogeny — a time of marked mountain-making throughout the 

 length of western North America, but chiefly in the area of the Cor- 

 dilleran Intermontane geanticline. This further rising of the geanti- 

 cline extended it unbroken as a marine barrier from Siberia into Central 

 America and brought into existence two new and wholly distinct sequent 

 geosynclines — sequent because of their formation out of the gTeater 

 and older Cordilleran geosyncline — the Eocky Mountain one on the east 

 and the Pacific trough to the west of this geanticline. This arched land 

 exists today as the Northern Interior, Columbia, and Nevada-Sonoran 

 high plateaus of Eansome.^^ 



^» F. L. Ransome : The Tertiary orogeny of the North American Cordillera and its 

 problems. In "Problems of American geology," 1915, pp. 287-376, 



