US Geological Survey


Date of this Version



The Journal of Geology, Vol. 17, No. 5 (Jul. - Aug., 1909), pp. 424-425


In passing from the Jurassic to the Lower Cretaceous North America underwent but little change along the Atlantic border and throughout the east. It remained a low land and the coastal plain was somewhat more deeply submerged. But on the Pacific coast, on the contrary, 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 deepening 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 thickness 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 has been suggested except that a land mass diverted the ocean current. The position of the supposed land was southwest of Mexico and is indicated by the dotted area.