42 The Dad land Formations of the Black Hills Region 



Osborn, modified from Peterson, shows the relations on the Ne- 

 braska- Wyoming line west of Harrison.* 



The Rosebud Beds. The Arikaree has been studied with 

 much care near Porcupine Butte and farther east on White 

 river by representatives of the American Museum of Natural 

 History. Matthew and Gidley, who first collected fossils there, 

 designated the series of strata as the Rosebud beds. These beds 

 are believed to be approximately equivalent to the Arikaree 

 formation as the latter is now coming to be understood, but 

 exact relations have not yet been fully determined over any very 

 large section of the country. Matthew describes the beds in 

 their typical eastern locality as follows : "The western part of 

 the formation attains a thickness estimated at 500 feet on Por- 

 cupine creek, a southern tributary of White river. The base 

 is taken at a heavy white stratum which appears to be identical 

 with the stratum capping the White River formation on Sheep 

 Mountain in the Big Badlands. This stratum can be seen ex- 

 tending interruptedly across the river to Sheep Mountain, 

 about twenty miles distant, capping several intervening buttes 

 and projecting points of the underlying formation. The Rose- 

 bud beds at the bottom approximate the rather hard clays of 

 the upper Leptauchenia beds, but become progressively softer 

 and sandier towards the top, and are capped at Porcupine Butte 

 by a layer of hard quarzitic sandstone. Several white flinty, 

 calcareous layers cover the beds, one of which, about half way 

 up, was used to divide them into Upper and Lower. The strati- 

 fication is very variable and inconstant, lenses and beds of soft 

 fine-grained sandstone and harder and softer clayey layers alter- 

 nating with frequent channels filled with sandstones and mud- 

 conglomerates, all very irregular and of limited extent. The hard 

 calcareous layers are more constant. A bed of volcanic ash lies 

 near the top of the formation, and there may be a considerable 

 percentage of volcanic material in some of the layers further 

 down . These volcanic ash beds should in theory be of wide 

 extent, and may be of considerable use in the correlation of the 

 scattered exposures on the heads of the different creeks — a very 

 difficult matter without their aid. 



The beds form the upper part of the series of bluffs south 

 of White river on the Pine Ridge and Rosebud Reserva- 



*U. S. Geol. Surv., Bull,- No. 361, 1909, p. 73. 



