156 DESCmPTIVE GEOLOGY. 



SECTION VII. 

 WEST OF NORTH PLATTE RIVER. 



BY S. F. EMMONS. 



Bridgee's Pass Ee&ion. — The valley of Sage Creek is an open, shallow 

 basin, wliose bottom is covered by a considerable accumulation of alluvial 

 deposits; the few exposures along the benches bordering the stream show- 

 ing only decomposed clay beds of the Colorado Cretaceous. At a re-en- 

 tering angle of the northeastern face of the Savory Plateau are found, at 

 the base of the bluffs, beds of coarse gravelly sandstone, striking little east 

 of north, and dipping at an angle of 55° to the east. In the valley, these 

 beds curve rapidly to the westward, shallowing in dip, while in Bridger's 

 Pass they are found with a northeast strike and a northwest dip, showing 

 that the Tertiary beds of the plateau cover a quaquaversal uplift in the 

 Lower Cretaceous rocks. A section taken in a northeasterly direction from 

 the northern point of the Savory Plateau, diagonally across 'Sage Creek 

 Valley, shows, above the gravelly sandstones, which represent the Dakota 

 group, beds of blue clay-shales overlaid by thin-bedded sandstones and 

 interstratified clays; these again succeeded by yellowish-brown sandstone, 

 containing spherical concretions of more calcareous material. Above are 

 about 100 feet of white and blue clays, containing thin calcareous seams 

 full of Ostrea congesta. Above this is a seam of only a few inches in thick- 

 ness of white shaly limestone, also containing abundant remains of Ostrea 

 congesta, associated with thin seams of arragonite and selenite. These beds 

 are exposed only on the upper slopes near the bluffs; their angle of dip 

 decreasing with their distance from the bluffs. Through the middle of 

 the valley, over an extent of nearly 4 miles, are found no exposures; but 

 the character of the soil shows the underlying rocks to be made up mainly 

 of the yellowish clays and marls of the Colorado Cretaceous. On the north- 

 east border of the valley rises a bluff of massive white sandstones belonging 



