822 



INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Approximate correlations oftJie Arilcaree formation. 



a Estimated. 



Darton^'*'^ described the Arikaree and Ogalalla as follows: 



A large portion of tiie higlier lands of western Nebraska and southeast Wyoming extending 

 south from Pine Ridge is capped by sands and soft sandstones which have been designated the 

 "Arikaree formation." It is the principal component of the series formerly termed "Loup 

 Fork." Generally it lies unconf ormably on the Brule clay but overlaps the margins of that forma- 

 tion in some portions of the area. It attaias a thickness of over 800 feet in southeast Wyoming 

 and, judging from the occurrence of outliers, formerly extended far up the slopes of the mountains 

 to the north and west. 



The predominant material is sand loosely cemented into a soft sandstone. The colors vary 

 from white to light gray. Characteristic, partly calcareous concretions occur throughout the 

 formation and greatly predominate in its lower members. They are mostly of elongate 

 cylindrical form, usually occurring in layers connected to a greater or less extent into irregular 

 sheets. * * * From observations by Profs. Barbour and Fisher it has been found that these 

 concretions are due to the growth of calcite crystals in certain portions and layers of the sandstone. 



In studying the geology of North Platte Valley in west Nebraska it was discovered that the 

 Arikaree deposits were overlain unconformably by deposits of sand, gravel, and calcareous 

 grits, which, on being traced south, were found to be the north extension of the Tertiary grit 

 and mortar beds of the Kansas and east Colorado region. They were given the local designation 

 Ogalalla formation. 



The materials of the Ogalalla formation are mainly sands, merging into gravels, and gravelly 

 sands more or less cemented by carbonate of lime into a grit rock which often has the appearance 

 of rough mortar, from which the name "mortar beds" is derived.. Sometimes the lime rock 

 contains but little sand, and usually it varies from grit to conglomerate or conglomeratic grit. 



The original descriptions of the Monroe Creek and Harrison are by Hatcher, *^^ 

 as follows : 



Returning to the Arikaree formation, I have alreadj^ remarked that in Sioux County, 

 Nebr., and Converse County, Wyo., it is lithologically and faunally divisible into two easily 

 distinguishable horizons. Commenciag below, these may be named and characterized as 

 follows : 



1. The Monroe Creelc ieds. — These are well shown in the northern face of Pine Ridge, at 

 the mouth of the Monroe Creek canyon, 5 miles north of Harrison, Nebr., where they overlie the 

 Gering sandstones and are composed of some 300 feet of very light-colored, fine-grained, not 

 very hard, but firm and massive sandstones. On account of their usually barren nature they 

 have been neglected by collectors, and very little is known concerning their fauna beyond the 

 fact that toward the top they contain Promerycochcerus. They decrease in thickness very 

 rapidly to the eastward and increase to the westward. 



