LAEAMIE PLAINS. 81 



Near where the North Park road leaves the plains, going over to the 

 Upper Laramie River, the Triassic and Jurassic formations jut up against 

 the Archsean, which here occurs as a low, narroAv ridge, only 300 or 400 

 feet in height, forming the east side of the Laramie Valley. On the oppo- 

 site side of this ridge, in the broad river- valley, occurs an isolated body of 

 Red Beds, the sandstones which formerly covered the ridge having been 

 entirely eroded. The exposure of Red Beds, however, is limited, as a 

 short distance below, where the road crosses the valley, the river runs 

 through a narrow gorge in the Archsean rocks, west of Sheep Mountain. 

 Under the characteristic red sandstones is found a narrow layer of blue 

 limestone, only a few feet in thickness, below which occur white sandstones. 

 Returning to the Plains again, we find that east of Sheep Mountain the 

 beds of the Colorado group are the oldest formations found lying upon the 

 Archsean body. The older rock-mass presents a steep slope toward the 

 plains, around which curve the Cretaceous clays and marls, with a very 

 varying strike and a somewhat varying dip. The line of junction between 

 the two formations is very much obscured by detrital accumulations from 

 the mountain, the clays presenting few well-defined outcrops. Approach- 

 ing the mountain by the North Park road, the first outcrops that are seen, 

 after leaving the nearly level plain and commencing to ascend the gentle 

 grade, are a series of dark brown and black, thinly-bedded shales and clays, 

 with interstratified layers of impure limestone, more or less mixed with coarse 

 sand. In the limestone occur some fragments of organic remains, but too 

 poor for specific determination, while the purer black clays carry thin 

 seams of carbonaceous material, with occasional coatings and incrustations 

 of gypsum. Underlying these dark clays, which have been referred to the 

 Fort Pierre division, occur yellow and blue marls, which crop out in low, 

 rounded banks, rising but a few inches above the level plain, but which 

 mark very clearly the horizon of the Niobrara beds. They are character- 

 ized by the presence of large numbers of the species Ostrea congesta. Below 

 the light-colored marls, the beds pass down into slate-colored mud-rocks, 

 becoming more arid more argillaceous, and apparently losing the calcareous 

 character of the overlying strata ; these clay -beds, in turn, are underlaid by 



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