PALEOGEOGRAPHIC MAPS 

 LATE MESOZOIC 



BAILEY WILLIS 

 U. S. Geological Survey 



II AND 12. LOWER CRETACEOUS' (COMANCHEAn) AND UPPER 



CRETACEOUS^ 



In passing from the Jurassic to the Lower Cretaceous North Amer- 

 ica underwent but little change along the Atlantic border and through- 

 out the east. It remained a low land and the coastal plain was some- 

 what more deeply submerged. But on the Pacific coast, on the con- 

 trary, there was pronounced movement, particularly in the Coast 

 Range of California. A bold peninsula developed from Oregon south 

 to Santa Barbara and, being eroded, yielded the thick sediments of the 

 Shasta group, which were deposited in marine water east of it, in part. 



In Alaska the Shastan sea appears to have invaded the Jurassic 

 land widely, but the details are not yet known. 



On the east of the Cordillera, from British Columbia to Wyoming, 

 coal-bearing continental deposits (Kootenie) accumulated in a deepen- 

 ing trough. In Wyoming, Dakota, and Nebraska a deposit of sand 

 (Lakota) was spread upon the plain. South of this occurs the much 

 older Morrison formation, which is regarded as probably Jurassic by 

 Stanton and which is overlapped by the marine Comanchean strata of 

 the gulf. The Kootenie, Lakota, and Morrison are comprised in the 

 area mapped as continental deposits. 



The striking feature of Comanchean geography is the expansion 

 of the Gulf of Mexico toward the west and northwest and the deep 

 subsidence of its floor, upon which accumulated a remarkable thick- 

 ness of limestone. The unusual calcareous deposit of organic remains 

 indicates the rich life of an equatorial ocean current. 



The fauna of the Gulf of Mexico in Comanchean time is entirely 

 unlike that of the Pacific coast. No adequate explanation of this fact 



1 Map prepared in collaboration with Dr. T. W. Stanton. 



2 Map prepared in collaboration with Dr. T. W. Stanton. 



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