"CERATOPS BEDS OF WYOMING AND MONTANA 247 



marine invertebrate fauna showing a blending of Pierre and Fox Hills 

 elements. Above that are "soft, arenaceous shales and interstratified 

 beds of clay usually with a capping of sandstone" varying in thickness 

 from 40 to 80 feet. These shales and sandstones have also yielded a 

 mairine fauna and are referred by Brown to the Fox Hills. 



The "Ceratops beds" to which the local name of "Hell Creek 

 beds" has been given are about 300 feet thick, and according to Brown 



may be divided into an upper and a lower member. The former is 

 composed of arenaceous clays alternating with sandstones and car- 

 bonaceous matter. The lower member consists of massive sandstone. 

 . This is the most constant of the series. Itunconform- 

 ably overlies the Fox Hills, as shown near the Cook ranch on Crooked 

 Creek; also on Hell Creek. This unconformity is erosional in charac- 

 ter. 



The unconformity has not been further described, but it may be 

 inferred that the base of the sandstone rests on an uneven surface. 

 Whether such an irregular base is due to an important unconformity 

 or represents a long interval of erosion must be determined by other 

 evidence. Similar phenomena are very common in non-marine 

 deposits such as the typical Fort Union where almost every massive 

 sandstone has an irregular base. A striking example of this may be 

 seen in the Fort Union formation of the Bull Mountains, two miles 

 south of Buckey's, Montana, where the unconformity is so striking 

 that it was supposed to mark the base of the formation until the 

 flora collected beneath it also proved to belong to the Fort Union. 



The vertebrate fauna includes the dinosaur genera Triceratops, 

 Trachodon, Tyrannosaurus and Ornithomimus, together with other 

 reptiles, fishes, and a few small mammals, and is nearly identical with 

 the fauna of the "Ceratops beds" of Converse County except that not 

 so many species have been collected. 



The invertebrate fauna is remarkable for the great development 

 and differentiation of Unios, several of which have not yet been found 

 elsewhere. The list as identified by R. P. Whitfield is as follows: 



Utiio CBSopiformis Whitf. 

 U. corbiculaides Whitf. 

 U. pyramidellus Whitf. 

 U. verrucas if onnis Whitf. 

 *f/. browni Whitf. 



