410 REPORT UNITED STATES GEOLOGICAL SURVEY. 



were spread oyer their upturned edges, and wliicli latter deposits consti- 

 tute tlie existing terraced surface of the valley. At one point the planed- 

 off edges of the beds are curiously turned uj), as though some heavy body 

 had been forced over their tilted edges in the direction of their dij). 



No paleontological evidence was observed by which the age of these 

 latter deposits might be fixed. They differ in so marked degree from 

 anything elsewhere observed in this southwestern section of the district, 

 both in their lithology and position, as to leave little room for doubt as 

 to their comparatively modern date. In both of these respects they 

 offer marked resemblances to deposits occurring in the region of the 

 headwaters of the Missouri, which Dr. Hayden has described under the 

 term "Lake Beds," and which he provisionally referred to Pliocene Ter- 

 tiary age. Yet they may be found to pertain to the great Wasatch Group 

 of Tertiary formations so extensively developed in the region to the 

 south. Their occurrence in the lower valley of the Snake offers fiu-ther 

 phenomena of interest in connection with their relation to the surround- 

 ing and more ancient mountain barriers which deiine the basin in which 

 these sediments were accumulated in horizontal strata. But their 

 present inclined position, by which their upturned edges have been ex- 

 posed to manifestly extensive fluviatile denudation, seems to be traced 

 to late dynamical agencies resident in either one or other of the border- 

 ing mountain areas. Whether the indications point to a late upward 

 movement in the Caribou Eange or a subsidence in the opposed Snake 

 Eiver Eange, which in either case would tilt the deposits that fill the 

 intermediate valley trough into a position hke that actually presented in 

 the present series of beds, is a question which remains for more extended 

 investigation to decide. Also, in regard to the age of these dejjosits, 

 more cannot, in the present state of our knowledge, be inferred than 

 their probable Post-Eocene age. In the Yellowstone expedition of 1872, 

 Dr. Peale describes a series of deposits under the term "Lake Beds" or 

 Pliocene, which occupy a basin between the Bridger and Spring GaQon 

 Eanges, Montana, closely analogous in their occurrence and relations to 

 the older sedimentaries, as here observed. But, to say the least, it 

 would be premature to attempt the positive identification of these widely 

 separated deposits one with the other. The Snake YaUey beds differ in 

 one particular from those above referred to in Montana, viz, in their 

 more varigated, brighter colors ; but this may be merely a local charac- 

 ter, or, as above suggested, they may belong to an older member of the 

 Tertiary formations. 



