LATER TERTIARY (MIOCENE AND PLIOCENE). 821 



recent fresh-water moUusks, the genera at least being chiefly the equivalents of existing types. 

 These beds along the western base of the Wasatch are approximately horizontal. Three conr 

 siderabie depressions east of the main ridge of the Wasatch — ^Morgan, Cache, and Ogden val- 

 leys — which unquestionably represent bays formerly connected with the main Phocene lake 

 west of the Wasatch, have been receptacles of Pliocene sediments very similar to the frag- 

 ments of horizontal Pliocene terraces on the west base of the Wasatch. They are all char- 

 acterized by recent genera of fresh- water moUusks. The height of the Tertiary in all these 

 valleys reaches a full thousand feet above the level of Salt Lake. 



With the exception of terrace masses along the western base of the Wasatch, which for 

 the most part are deeply covered by Quaternary deposits, the valley of Salt Lake carries a 

 sheet of Quaternary, through which rise masses of Paleozoic and volcanic rocks. The northern 

 boundary of this great basin is beyond the limits of our map but has been crossed by us in 

 several places, and the members of the exploration have been unanimous in referring to the 

 Phocene period a considerable series of horizontal rocks which occupy a divide between the 

 waters of the Utah Basin and those of Snake Valley. These rocks are composed chiefly of 

 friable gray, wlaite, and drab sandstones and marly limestones, for the most part horizontal 

 but in places uplifted at low angles. At the northwest boundary of the Salt Lake Basin, near 

 the one hundred and fourteenth meridian, at latitude 40°, are further exposures of horizontal 

 Pliocene rocks, which rise to altitudes of 1,000 to 1,800 feet above the level of the basin. 



K 13-14. NORTHERN GREAT PLAINS, WYOMING, NEBRASKA, AND SOUTH DAKOTA. 



The lower Miocene of the northern Great Plains in the United States is the 

 Arikaree formation, including locally the sediments to which the names Gering, 

 Monroe Creek, and Harrison have been applied. "^^'^ The Arikaree rests on the 

 Brule clay (Oligocene), in some places unconformably, and is unconformably over- 

 lain by the later Miocene Ogalalla formation. 



Darton ^^^' ^^^ originally described the Gering as a distinct formation, rather than 

 as part of the Arikaree. The deposit is locally developed and consists of coarse sands 

 and soft sandstones which contrast with the finer sediments of the Arikaree. It is 

 now ^^^ interpreted as a river deposit laid down in the channels of early and possibly 

 somewhat later Miocene streams. Osborn**^^' says: 



The typical Gering formation of Darton, 1899, is at Scotts Bluff, western Nebraska- the 

 broad extension by Darton of this formation to other localities is somewhat doubtful. The name 

 Gering formation as used by Darton, Hatcher, and Peterson probably applies to noncontinuous 

 river sandstones and conglomerates (maximum 200 feet), which are in a manner analogous to 

 the "Titanotherium," "Metamynodon," and "Protoceras sandstones" that traverse the lower 

 Arikaree clays or finer beds and partly erode irregular channels in the upper Brule clay (Leptau- 

 chenia zone) . This formation is thus probably of the same age as the lower parts of the Arikaree 

 Monroe Creek, and Rosebud. Its known fauna is very limited. The so-called Gering of Hatcher 

 and Peterson is in southeastern Wyoming and northwestern Nebraska; in their section it is said 

 to be lithologically similar to the overlying Monroe Creek. 



The typical Arikaree formation of Darton, 1899, is at Pine Ridge Bluffs, in South-Dakota- 

 whether or not this extends to southeastern Wyoming rests on future paleontological correlation. 

 The Arikaree as described and mapped by Darton would broadly include the whole of the 

 Rosebud formation of Matthew, as well as the Monroe Creek and Harrison, and broadly cover the 

 whole of the Miocene. The entire Arikaree formation of Darton consists of finer materials 

 whitish or light-buff sandstones, more continuous and widespread, lying either on the Gering 

 formation or on the Brule clay. 



