TEETIAEY PLAINS OF WYOMING. 65 



region of the South Platte, and on the low bluffs between Evans and 

 Greeley, and, indeed, in all the shallow basins worn out of the Cretaceous 

 rocks. They lie scattered over the plains in irregular detached beds, and 

 are not represented on the geological maps. 



Tertiary Plains of Wyoming. — Between 5 and 6 miles south of the 

 forty-first parallel, the Fox Hill and Laramie Cretaceous pass beneath the 

 nearly horizontal beds of Tertiary strata. Abundant evidence has been 

 furnished that these deposits are, without doubt, the sediments, filling the 

 basins of the ancient fresh- water lakes, which formed so characteristic a fea- 

 ture of the Rocky Mountains through Tertiary times. East of the 'Rocky 

 Mountains, these formations cover a very large area. From Chalk Bluifs 

 they extend completely across the Territory of Wyoming, having been 

 traced far northward into Dakota. To the eastward, they stretch far out 

 upon the Nebraska Plains, their boundary in that direction having never 

 been carefully determined ; while westward the Colorado Range formed 

 an eff*ectual barrier. 



Within the area of our survey occur both Miocene and Pliocene strata, 

 with palseontological evidence of their age. To the beds of the Miocene 

 lake-basin, the name. White River, first used by Dr. Hayden, has been 

 applied, while, for the overlying Pliocene strata, the name Niobrara, first 

 suggested by Prof. O. C. Marsh, has been retained. 



White River Miocene.— As the AVhite River formation is overlaid by 

 the Niobrara, the outcrops of the former are necessarily somewhat limited, 

 and the difiiculty of tracing the outlines of the ancient Miocene lake is greatly 

 enhanced. Within the area of our map, the only opportunity for studying the 

 White River strata is to be found along the south and east face of Chalk Bluffs, 

 where they protrude from beneath Pliocene beds. They rest unconformably 

 upon the Laramie sandstones, and although the Cretaceous lies essentially 

 horizontal, it has undergone considerable erosion before the deposition of the 

 Tertiary, as the latter is found filling the uneven surfaces of the former. 

 The Chalk Bluffs are but the abrupt terminations to the south and east of 

 the Tertiary strata, which rise from the Cretaceous Plains by receding 

 benches and terraces some 700 feet in height. From Cheyenne southward, 

 the surface of the country is nearly level until within a short distance of the 

 5 D a 



