54 DESCEIPTIVE GEOLOGY. 



South of Box Elder Creek, the Triassic beds extend westward for 3 or 

 4 miles, in a broad, level table, occupying a depression in the Archaean 

 body, overlapping it in a nearly horizontal position, dipping not more than 

 2° toward the southeast. Lying next the granites occurs a bed of coarse, 

 reddish-gray conglomerate, which underlies the bright red sandstone. It 

 was not recognized to the north, but it is probably only a conglomerate bed 

 in the Triassic, which here, owing to its position next the granite, appears 

 somewhat coarser and more largely developed East of the Triassic table, 

 the Upper Mesozoic formations dip with varying angles toward the Great 

 Plains, showing considerable disturbance and displacement of strata. Where 

 the stage-road enters the mountains, 1) or 4 miles north of Park's Station, 

 the Cretaceous and Jurassic beds are seen occupying a small re-entering 

 bay or depression. On the inside of this bay, the Jurassic and Dakota 

 Cretaceous curve around, so as to form nearly a semicircle, while the softer 

 beds of the Colorado group fill the central portion. On the north side, the 

 Colorado beds strike north 45° to 50° east, with a dip 25° to 30° to the 

 southwest; while, on the south side, they present an equally steep dip to the 

 north, forming a sharp synclinal, with the strata contorted and pressed 

 together in a ver}^ marked manner; the yellow and cream-colored marls 

 forming so conspicuous a feature of the Cretaceous that the structure is 

 easily made out. The light-colored chalky marls are here characterized by 

 a great abundance of the genus Ostrea, associated with gypsum. From 

 Park's Station southward to the Cache la Poudre, the ridges present a very 

 uniform appearance, with a strike approximate!)^ north and south, and a dip 

 varying from 15° to 25° east; the Dakota Cretaceous forming an almost 

 unbroken wall of hard reddish-brown quartzite. 



Along the base of this wall, all the subdivisions of the Colorado group 

 may be recognized, especially as the Niobrara beds are well developed, 

 although it would seem impossible to define either their lower or upper 

 limits, as the calcareous beds, which are their characteristic lithological fea- 

 ture, pass gradually into both the Fort Benton and Fort Pierre divisions. 

 The Fort Benton and Niobrara beds at Park's Station dip from 16° to 18° 

 east, while the Fort Pierre clays to the eastward rapidly flatten out, indi- 



