136 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



down into the lower country and appear as lost strangers amid tlie 

 recent Tertiary deposits. They extend for some distance eastward. Dr. 

 Hayden says of them: "The immense quantities of granite bowlders, 

 red and gray, * * * must have been swept down from the Wind 

 River Mountains. Some of these granite masses are 10 to 15 feet in 

 diameter; others are sunk so deep in the earth that they appear to be 

 in place." If they be regarded as glacial, I should consider them due 

 to the effects of the extensive ice-field which left its ground moraines 

 near Camp Stambaugh. It passed downward from the southern end of 

 the range, and may have extended across the gulfs which now separate 

 the erratic bowlders and the well-defined moraines. As the former ap- 

 pear only sporadically, not following any definite horizontal arrangement, 

 I do not class them among moraines, but consider them as " scattered 

 glacial drift." Probably they were carried southward to a region where 

 the glaciers had lost the greater portion of their terminal moraines, and 

 found no material for the formation of others. Thiis, then, do these 

 rocks indicate the extreme sourthern limits which were reached by the 

 moving ice-fields of the Wind River Range. 



LAKE DEPOSITS. 



From the structure of this southern area, and from the features ex- 

 hibited by post-Tertiary erosion, we may expect to find a large quantity 

 of lake-beds. In the Shoshone Basin the majority of them are located. 

 We observe there a number of lakes which contain water throughout 

 the entire year, although receiving supplies during only a few months. 

 In almost every instance it was seen that, within comparatively recent 

 times, their horizontal extent was much greater than it is now. Slight 

 rises, separating perhaps a dry bed from a lake, were then overflowed, 

 and large bodies of water appeared on the surface where now we find 

 only smaller ones or none at all. On the surface the lake deposits ap- 

 pear perfectly level or slightly concave accumulations of fine arenaceous 

 mud containing strata of sand. In case any strata showing bright col- 

 ors were cut by the drainage which fed the former lakes, we find the 

 fact indicated by layers of clay and sand exhibiting red, brown, white, 

 and yellow bands. White efflorescences of alkali cover those deposits 

 which contain water during a portion of the year. Sodium sulphate and 

 sodium chloride compose the main bulk of this "alkali." Generally the 

 lake deposits occupy depressions of oval form and jut directly against 

 the surrounding strata. Water entered these old lakes from several 

 directions usually, the prevailing one usually being indicated by the 

 longer dimensions of the deposits. 



In case no water has stood in the depressions for a long term of years, 

 the efflorescences wfll have disappeared entirely, and nothing remains 

 but the sterile waste exposing fine mud intermixed with some sand. An 

 instance of this kind is presented by Red Desert. For centuries, per- 

 haps, no water has been standing on the old lake bottom. It is hard 

 and firm, baked by the hot sun of many successive summers. Southeast 

 of Mount Essex we noticed a number of such beds. South of the Sem- 

 inole Hills two extensive valleys show deposits of the same nature. One 

 of them, lying between the exposure of Colorado shales and the bluff 

 formed by the Fox Hills series, is of considerable extent. The other one, 

 to the westward, is smaller. 



Dependent, to a certain extent, upon the area of the deposits, is their 

 thickness. From temporary creeks which have cut narrow gullies into 

 the soft material, we find that it exceeds 20 to 30 feet in many places. 



