746 GEOLOGY 



The Cretaceous beds of the Gulf coast (Alabama) have been 

 disturbed rather more than the corresponding beds along those 

 parts of the Atlantic coast where the system has been carefully 

 studied. They have been bent into low anticlines and syncliries in 

 some places, and even faulted to a slight extent. 



The Western Gulf Region 1 



The general stratigraphic relations of the system here are the 

 same as farther east, but deposition seems to have been well under 

 way in Texas before the oldest beds of the system farther east were 

 laid down. Alternating beds of sand, shale, limestone, and marl, 

 most of which are of marine origin, make up the system. They 

 attain a maximum thickness of about 4,000 feet. Three principal 

 subdivisions are recognized: 2 (1) The Dakota; (2) the Colorado; 

 and (3) the Montana. 



The Dakota formation, 600 feet and less thick, is largely of 

 sandstone, with some lignite, and is, for the most part, of non- 

 marine origin. The Colorado series contains much limestone (or 

 chalk) of marine origin. Its thickness is about 1,000 feet. The 

 Montana series is more largely clastic, and from it the oil of the 

 Corsicana oil field of Texas is derived. Locally, the system is much 

 faulted. From Texas it is continued northward into Arkansas, and 

 westward into New Mexico. 



The Cretaceous of the western Gulf region differs from the cor- 

 responding system farther east, in its greater thickness, and in its 

 greater proportion of calcareous matter, largely in the condition of 

 chalk. Of limestone or chalk, the Cretaceous of the Atlantic coa-t 

 contains little, that of the eastern Gulf region (Alabama and Mis- 

 sissippi) more, and that of Texas much; nor is the chalk confined 

 to Texas, as will be seen. 



The Western Interior 



The Cretaceous system of the western interior consists of the 

 following subdivisions, commencing at the bottom: 1. Dakota: 

 2. Colorado (including the Benton and the Niobrara formations) ; : : 

 Montana (including Ft. Pierre and Fox Hills); and 4. L(inuni>. 



1 Hill and Vaughan, 18th Ann. Kept., U.S.Geol.Surv., Pt. 1 1. pp. 238 2 U 



2 Hill, 21st Ann. Kept., U.S. Geol. S,,,\ ., IM. VII. p. 114. 



