770 INDEX TO THE STEATIGEAPHY OF NORTH AMEEICA. 



one, although, as now well known, there were confused under the name of Loup Fork a number 

 of separate formations rangmg ha age from Miocene to early Pleistocene. 



The White River group has been studied mainly in the Big Badlands, lying southeast of 

 the Black HiUs, where three principal subdivisions were defined, the lower one as the Titano- 

 therium beds, the middle as the Oreodon beds, and the upper as a series consisting of the Pro- 

 toceras sandstone and some overlying clays. This classification is distinctly set forth by -Dr. 

 Wortman." In the Oreodon clays there is often a marked sandstone horizon known as the 

 Metamynodon beds. 



In mapping the geology of western Nebraska, in 1897, 1 had need to subdivide the formations 

 in the valley of Platte River and Pine Ridge, where I found that the White River group con- 

 sisted of the usual basal series, the Titanotherium beds, which I designated the Chadron for- 

 mation, and the usual overlying series of flesh-colored sandy clays, the Oreodon beds, to which 

 the name Brule clays was apphed. 



The Chadron formation [consists mainly of] sandy clays and greenish-gray sandstones, 

 mostly of soft texture, in which characteristic Titanotherium remains are of frequent occurrence. 

 The upper limit to the formation is placed arbitrarily just below some pink clays lying below a 

 thin bed of limestone, which is believed to be at the same horizon as one in the Big Badlands, 

 where it immediately overlies the last of the beds of Chadron character. The thickness of the 

 formation appears to be about 200 feet at most; but, as it Hes on a very uneven surface, the 

 amount varies greatly. The sandstones, which occur at various horizons, appear to he in narrow 

 belts, doubtless indicating channels of deposition. The lineal character of some of these channels 

 is very distinctly exposed in the western portion of Goshen Hole. 



The Brule clay consists mainly of a hard sandy clay of pale-pink color and massive structure, 

 having near the base a thin layer of limestone. Locally some portions are sandy and contain 

 beds of sandstone. * * * jjj ^^^^ northern face of Scotts Bluff, * * * from the base of 

 the overlying Gering beds to the surface of the river, there is a vertical interval of 500 feet 

 continuous outcrop. The formation also has a small additional thickness below the level of 

 the river. * * * ^he badlands topography is a characteristic feature of most exposures 

 of the Brule clay. * * * The basal portion of the formation generally includes a thin bed 

 of limestone, and at various horizons there are occasional irregular lens-shaped masses of sand- 

 stone. * * * [The limestone] is a very thin bed of compact cream-colored rock lying on a 

 series of pinkish and greenish clays which are regarded as the base of the formation. 



Beds of volcanic ash occur in the Brule clay, some of them of wide extent and apparently 

 at constant horizons. One bed conspicuous in many outcrops hes at 60 to 70 feet below the 

 top of the formation in the district south and southeast of Gering. It is about 150 feet below 

 the top at Scotts Bluff, a position which it holds for some distance to the west. Another higher 

 bed often occurs. Fossil bones of various mammals and turtles characteristic of the Oreodon 

 fauna of the Oligocene occur in the Brule clay. 



Osborn "^^'^ assigns the Chadron formation to the lower Oligocene and gives its 

 homotaxis and synonymy. The lower part of the Brule Osborn places in the middle 

 Oligocene and the upper part in the upper Oligocene; the former comprising the 

 Oreodon zone and "Metamynodon sandstones," the latter containing the Lep- 

 tauchenia zone and " Protoceras sandstones." He also gives the homotaxis and 

 synonymy of the lower and upper parts of the Brule.^^^' 



" Wortman, J. L., On the divisions of tte White River or lower Miocene of Dakota: BuU. Am. Mus. Nat. Hist, 

 vol. 5, 1893, pp. 95-105. 



