DEVELOPMENT OF THE CORDILLERAN GEOSYNCLINE 189 



It extended from Oregon south to the end of Lower California. During 

 Mesozoic time it subsided not less than 27^000 feet^ and to this must 

 be added the Paleozoic strata, amounting to something like 8,000 feet 

 more. Toward the close of the Cretaceous large parts of the trough 

 were folded, roughly in what is now the Great Valley of California and 

 the Coast Eanges; then during the Cenozoic the trough continued to 

 receive marine and fresh-water sediments that on the average will prob- 

 ably exceed 30,000 feet in depth. The stratal history of Lower Cal- 

 ifornia is too little known to go profitably into its detail except to say 

 that it is best known since early Upper Cretaceous time. 



The orogeny of the late Cretaceous does not appear to have been of a 

 very marked character, since in places the Mesozoic and Cenozoic deposits 

 are conformable to one another. Orogeny was more pronounced at the 

 close of the Oligocene, strongly so at the end of the Middle Miocene 

 (Monterey), moderately at the close of the Miocene, and most pro- 

 nounced of all at the close of the Pliocene. 



In the evidence recited we see that northeastern California subsided 

 during the Paleozoic and up to the close of the Jurassic something like 

 22,000 feet, and following the rise of the Cordilleran Intermontane 

 geanticline central California sank during late Mesozoic and all of 

 Cenozoic time probably as much as 45,000 feet. 



ROCKY MOUNTAIN SEQUENT GEOSYNCLINE 

 (See Maps, Figures 14 to 17) 



We have seen that early in Lower Cretaceous time there came into 

 full existence the Cordilleran Intermontane geanticline. To the east 

 •of this long arch, that continued to rise throughout Cretaceous time, 

 there gradually developed the Eocky Mountain geosyncline that in full 

 development extended from Siberia into the Caribbean mediterranean. 

 This vast subsiding realm is readily divisible by its structural features 

 into three areas. In the north (1) is the Arctic-Mackenzie region, (2) 

 south of the Liard Eiver into northern New Mexico is the very wide 

 Eocky Mountain or Dakotan sea, and (3) from southern New Mexico 

 and Arizona south is the Mexican area. 



Let us take up first the Mexican geosyncline, since the marine trans- 

 gression begins earliest here. Nearly all of Mexico was land throughout 

 most of the Paleozoic, and the only parts frequently under water were 

 the narrow southern Tehuantepec region and the greater Sonoran em- 

 bayment across Sonora, Arizona, Chihuahua, New Mexico, and western 

 Texas. As yet, it can not be said with certainty that Mexico has the 

 characteristics of an ancient shield, and that it was a low land throughout 



