242 HISTORICAL GEOLOGY 



marls, of marine origin, reaching an estimated thickness in this 

 region of 250 to 350 feet." x 



From the above descriptions, the Atlantic and eastern Gulf 

 Coastal Plain Cretaceous deposits are seen to be largely uncon- 

 solidated and only slightly tilted sediments. 



Texas. In the Texas region the Trinity formation consists 

 mostly of light colored sands, with some alternating marls, clays, 

 and limestones. Its lower portion is of continental origin, while 

 its upper portion is marine. The whole formation attains a maxi- 

 mum thickness of about 2000 feet. The Fredericksburg formation 

 is typically almost entirely chalky limestone of marine origin from 

 1000 to 5000 feet thick. It covers wide areas in Mexico and Texas. 

 The Washita formation 2 comprises chiefly alternations of light and 

 dark colored marly clays, limestones, and sandy limestones whose 

 thickness is from 200 to 400 feet. This formation extends across 

 much of Mexico, Texas, and northward into Oklahoma, southeast- 

 ern Kansas, eastern Colorado, and possibly into Wyoming. 



The Woodbine formation, according to Hill, 3 is made up of 

 ferruginous sands and clays (600 i feet thick) ; the Eagle Ford 

 formation is essentially bituminous clay with some limestone 

 (600 i feet thick) ; the Austin formation is largely impure chalk 

 with some softer beds of marl (600 i feet thick) ; the Taylor for- 

 mation is calcareous clay marl (several hundred feet thick) ; and 

 the Navarro formation is mostly made up of sands, chalks, and 

 clays with some glauconite (thickness?). 



Western Interior. In the western interior region mostly just 

 east of the main axis of the Rocky Mountains from Colorado 

 northward, there occur certain formations — Morrison, Kootenai, 

 etc. — of rather doubtful age and origin. They consist mostly of 

 shales, sandstones, and limestones, and, in part at least, they are 

 Lower Cretaceous deposits of continental origin. 



The Dakota formation is chiefly sandstone, mostly of marine 

 origin, and up to 100 feet thick. The Colorado formation is very 

 largely of marine origin and comprises mostly clastic sediments 

 but with considerable chalk. The Montana formation comprises 

 mostly clastic sediments of marine origin, though with some 

 continental deposits, as, for example, local coal beds. It shows a 



1 L. W. Stephenson, U. S. G. S., Professional Paper 81, pp. 20-21. 



2 Recent work has shown the Washita to be partly Upper Cretaceous. 



3 U. S. G. S., Professional Paper 71, pp. 20-21. 



