142 DESCRIPTIVE GEOLOGY. 



SECTION VI. 

 CRETACEOUS REGION FROM COMO TO SEPARATION. 



BY ARNOLD HAGUE. 



Valley of the Medicine Bow River. — North of the two great Archsean 

 masses, the Medicine Bow and Park Ranges, Cretaceous strata are found 

 over the greater part of the area, extending from the Laramie Plains west- 

 ward to the Hmits of Map I, where they pass beneath the Eocene Tertiary 

 deposits of the Green River basin. 



Over this wide extent of country, the crystaUine schists reach the sur- 

 face at only one point, Rawlings Peak, while Palaeozoic and Mesozoic 

 rocks below the Colorado Cretaceous occupy nearly as restricted limits. 

 The country presents a broken, undulating surface, varying in average alti- 

 tude from 6,500 to 7,000 feet above sea-level. The structural features are 

 somewhat complicated, consisting of a series of anticlinal and synclinal 

 folds, with sharp, prominent ridges of Fox Hill sandstone, and basins and 

 level plains of Laramie beds. 



Between Como and Medicine Bow Stations, the country is overlaid by 

 the Colorado Cretaceous, all the divisions being recognized. The Niobrara 

 yellow marls would appear quite thin, although characteristically developed, 

 and both the upper and lower clays seem highly arenaceous. Just north 

 of the river, at Medicine Bow Station, the beds strike north 65° to 70° 

 west, and dip 16° to 18° southwest. In the bright-colored marls were 

 found Ostrea congesta associated with imperfect fragments of Inocerami. 

 Below the Niobrara beds, in the sandy clays, occur Inoceramus alius, and in 

 a low ridge or bench not far distant, but above the latter, were collected 

 Scaphites Warreni. 



The Medicine Bow River, after leaving the mountains, runs almost 

 exclusively through beds belonging to the Colorado Cretaceous ; its course 

 being guided by the clays and marls, and the overlying Fox Hill sandstone. 

 Beds of the former formation occupy a deep bay, or recess, penetrating the 



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