SOUTH DAKOTA SCHOOL OF MINES 



43 



nected into irregular sheets. It is to this feature especially 

 that the Pine Kidge escarpment and other prominent topo- 

 graphic features of that part of the country are due. For 

 the manner of development of these concretionary forms, the 

 reader is referred to the discussion of concretions and sand- 

 calcite crystals elsewhere in this paper. 



Figure 11 — Diagramatic section of the Arikaree on the Nebraska- 

 Wyoming line west of Harrison. Osborn, modified from Peter- 

 son, 1906-09. 



The Arikaree has not been carefully denned for all the 

 area where it has been found, and owing to the variable 

 nature of the formation in different localities a number of 

 terms in this connection need to be referred to and denned. 

 Darton in his studies in western Nebraska some years ago, 

 differentiated certain sands and standstones, lying below 

 the Arikaree deposits, as the Gering formation. More re- 

 cent study seems to show that much of this material is little 

 more than non-continuous river sandstones and conglomer- 

 ates that traverse the lower Arikaree clays and occupy in 

 places irregular channels in the partly eroded upper Brule 

 formation, the relation to the Arikaree clays being in such 

 places much as that of the Titanotherium, Metamynodon 

 and Protoceras sandstones to the clays in which they 

 severally occur. The general tendency at present seems to 

 be to consider them as a special depositional phase of the 

 lower part of the Arikaree. According to Hatcher, the 

 Arikaree in Sioux County, Nebraska, and Converse County, 

 Wyoming, is lithologically and faunally divisible into two 

 easily distinguishable horizons, namely, the Monroe Creek 

 beds, below, and the Harrison beds above. 



