SPEARFISH FORMATION 387 



taining varying amounts of fine sand admixture. It is generally thin 

 bedded. The gypsum occurs in beds at various horizons, some of the 

 larger beds extending continuously over wide areas. There is also 

 throughout the formation more or less secondary deposition of gypsum 

 in small veins. 



The thickuess of the Spearfish formation can seldom be determined 

 with accuracy, owing to the softness of the material and the predomi- 

 nance of low, variable dips which are difficult to measure. Along the 

 east side of the Black hills the formation appears to have a thickness 

 of from 350 to 400 feet, but the amount increases to the northward to 492 

 feet in the well at Cambria, 695 feet in a well at Sturgis, and at least 650 

 feet in a deep boring at Aladdin. To the southeastward the principal 

 bed of gypsum generally varies from 5 to 15 feet, increasing southward 

 in the vicinity of Hot Springs to the maximum development, in which 

 the principal beds have a thickness of 33? feet, with a 10-foot parting 

 of red shale between. These beds are shown in plate 25. Along the 

 west side of the uplift there is usually a bed of gypsum at a horizon 

 about 150 feet above the base of the formation, and east of Newcastle for 

 some distance there is a 25-foot bed of gypsum at the top of the forma- 

 tion shown in figure 1, plate 26, several thin beds in its center, and at 

 its base a local thin bed of gypsum lying directly on the Minnekahta 

 limestone. Throughout the Black hills the formation is distinctly sep- 

 arated from the underlying Minnekahta limestone by a very abrupt 

 change of material, and from the overlying marine Jurassic deposits 

 and by a well marked erosional unconformity. 



No fossils have been found in the Spearfish formation of the Black 

 hills excepting a few fragments that were indeterminable. Its age has 

 been supposed to be Triassic. While this may be correct, there are 

 evidences in other regions that it may possibly belong to a somewhat 

 earlier period, in which case the Triassic time, as well as earlier Jurassic, 

 is represented by the unconformity at its top. 



SUNDANCE FORMATION 



This name has been given to the marine Jurassic sediments of the 

 Black Hills region from the town in the northern portion of the uplift. 

 The rocks are shales and sandstones in a series, some members of which 

 vary locally. Over a wide area the succession consists of a lower mem- 

 ber of dark gray shales, averaging 50 feet thick ; a prominent ledge of 

 fine grained sandstones of pale buff tint; an intermediate member of 

 sandy shales and sandstones of reddish color, and at the top about 150 

 feet of dark green shale, including thin layers of very fossiliferous lime- 

 stone. A characteristic exposure is shown in plate 26, in which the 



