CLASSIFICATION, COMPOSITION, AND NOMENCLATURE 



387 



Age. 



Lower Cretaceous (or Jurassic?) . 



Formation. 



Lakota. . . 

 Beulah . . . 



Jurassic Unkpapa . 



Sundance. 



Triassic Spearfish . 



Principal components. 



Coarse buff sandstones with fire- 

 clays and local coal beds. 



"Atlantosaurus beds;" gray to 

 gi-een shales. 



Fine-grained massive sandstones, 

 pink, white, buff, or purple. 



Green shales and thin bedded sand- 

 stones. 



Eed beds. 



In the following table are given the thickness of the three Jurassic 

 formations at localities where typical sections were studied : 



Locality 



Cascade Springs 



Southeast of Hot Springs 

 East of Hot Springs . . . . , 



Buffalo Gap 



Fuson creek . 



Lame Johnny creek . . . 



French creek 



Dry creek 



Spring creek. 



Belle Foui'che valle}\ . . . . 



Cambria . . 



Minnekahta 



Sundance 

 formation. 



400 

 220 

 242 

 230 

 .S50 

 284 

 150 

 60-80 

 220 



300 



Unkpapa 

 sandstone. 



50+ 

 225 

 160 

 120 

 100 



80 

 100 



90 



40 

 Absent 



Beulah 

 shales. 



Absent 



100 



Yoo+ 



50 



Total. 



450 

 445 

 402 

 350 

 450 

 364 

 250 



175r 



360 

 550 

 380- 

 350 



Sundance Formation 



G EN ERA L CHA RAC TERIS TICS 



This member of the Jurassic extends continuously around the Black 

 Hills uplift, and throughout its course presents characteristics by which 

 it can be easil}^ recognized. It carries abundant marine fossils, not 

 only to the north, as stated b}^ Newton, but in profusion also around 

 the zone of outcrops to the south. The formation comprises shales and 

 sandstones, in greater part in alternating series which vary somewhat 

 in relation in different portions of the region. The shales are mainly 

 dark green and the sandstones pale buff in color, but there is an inter- 

 mediate member of sandy shales and sandstones of reddish color, and 

 often a local basal member of red sandstone. The shales usually in- 

 clude thin layers of limestone which are always highly fossiliferous. 



