HELL CREEK FORMATION 357 



Creek and Crooked Creek calcareous concretions similar to those of the 

 Pierre are found in stratified planes, and the following invertebrates 

 were taken from a concretion on Hell Creek about 20 feet below the 

 overlying massive basal sandstone: Cardium suhquadratum E. and S., 

 Xucnla cancellata M. and H., TelHna scitula M. and H., Lunatia con- 

 ( inna M. and H., Scaphites ronradi Morton, and Bacidites ovcdus Say, 

 all of wliicli testify the marine character of the sediments at this point. 



On tlie east fork of Crooked Creek near the old Cook ranch, on the 

 west fork of Crooked Creek near the Cus Colin claim, and on the east 

 fork of Hell Creek near the EE cattle camp these marine beds have been 

 eroded in places, sometimes to a depth of 10 feet, before the succeeding 

 massive sandstones of the fresli-water "Lance" were deposited. The 

 strata are, however, in all cases parallel to the bedding plane of the 

 succeeding sandstones, and the break is evidently of local erosional 

 character. 



It was observation of these local erosional breaks in the vicinity of 

 Hell Creek that led to the statement that the beds did not represent a 

 continuous sedimentation from the marine Fox Hills. In view of later 

 work, this statement must be modified. 



In the eastern exposures of these beds on tributaries of Big Dry, 

 Prairie Elk, and the small streams emptying directly into the Missouri 

 River the sandstones of the Fox Hills are finer grained, harder, and 

 more compact. Along the eastern outcrop for a distance of nearly 30 

 miles and on the north side of the Missouri River these marine^ and 

 brackish-water sandstones grade into the massive sandstones above with- 

 out any sign of discordance. 



A typical section may be seen on the east side of the Big Dry opposite 

 the ranch-house of Mr. John Willis, 38 miles south of Glasgow, Montana. 

 At this point there is a prominent mass of shells, Corhula cf. subtri- 

 gonalis, forming a layer 4 to 8 inches thick, which was traced for a quarter 

 of a mile. 



Above this shell layer there are 20 feet of rust-red sandstones that 

 are arbitrarily chosen as the close of the Fox Hills. These are laminated 

 shaly sandstones, usually capped by a thin layer of flattened limonite- 

 covered concretions that mark the point of contact with the overlying 

 fresh-water sandstones. At another point this upper stratum of Fox 

 Hills is a thin layer of red sandstone marked off in irregular squares, 

 which seem to indicate a drying up of ponds, where the cracks were 

 later filled with sand. The shaly sandstones of these upper strata are 

 frequently spotted with radiations of a removed crystallized mineral, 

 presumably some soluble alkaline salt, as the stone reacts for sulphates. 



