EASTERN FOOT-HILLS. 49 



liei-e southward to the main valley, a clistanco of 3 miles, all the character- 

 istic beds, from the Palaeozoic to the top of the Colorado group, are well 

 represented. The most striking feature, however, is the bold, abrupt bluffs 

 of the Paljeozoic rocks, which rise above the top of the Triassic beds from 

 50>) to 600 feet, exposing- almost sheer cliffs of Carboniforous limestone to 

 the eastward, the strata standing at an angle of 70°. Three streams from 

 the range nearly equally divide the limestone into three somewhat isolated 

 ridges with curiously eroded summits, whose trend is a few degrees west 

 of north. Numerous forms of the genus Productus are found here, but the 

 only one that could be determined was P. semireticulatus. 



The Triassic beds are, in general, fine-grained, somewhat shaly and 

 argillaceous, with both deep red and reddish-yellow beds. The Jurassic 

 occupies a shallow depression at the base of the Dakota sandstone, which, 

 in many localities, appears to form a hard caj)ping to the more easily eroded 

 marls below. 



Overlying the Dakota occur somewhat thin exposures of the lower dark 

 clays, in turn overlaid by yellow and blue marls carrying immense numbers 

 of the genus Ostrea, which, in places, almost makes up the rock. The upper 

 dark plastic clays which carry numbers of BacuUtes ovatus would appear to 

 be well developed, cropping out in low ridges and in occasional mounds 

 and hillocks. 



Not far north of Horse Creel?;, the dark clays of the Fort Pierre division 

 of the Colorado group are overlaid by a series of beds, which crop out in 

 long narrow lines, rising only from 12 to 15 inches above the plain. They 

 consist of yellowish-brown sandstones, with seams of arenaceous clay, and 

 soft friable sandstone with ferruginous concretions, and some vegetable 

 remains. On the geological map, these beds have been referred to the Fox 

 Hill group, but they may possibly belong to the upper members of the 

 Fort Pierre. The onl}^ fossils found were a number of species of the genus 

 Ostrea ; but none of them have been specifically determined, and may not 

 be characteristic of either formation. The beds strike considerably west of 

 north, and dip 25° to 30° to the east. They occupy a very small area, 

 deriving their chief interest from being, so far as observed, the highest 

 members of the conformable series of pre-Tertiary strata east of the Lara- 



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