GEOLOGICAL SUEVEY OF THE TERRITORIES. 75 



trend of the ridges is about northeast and southwest. The branch of 

 the Muddy, which rises in the " pass," seems to form the line of separa- 

 tion between the coal-beds and those of well-known cretaceous age. 

 A bed of massive sandstone forms a wall on the north side nearly to the 

 head of the Muddy, where the softer clays of the cretaceous seem to rise 

 from beneath it. On the south side of the road is a high, rugged group 

 of hills, which weather into rounded forms, which are plainly upper 

 cretaceous. Since leaving Washakie there has been a decided im- 

 provement in the vegetation ; grass grows quite abundantly on the hills 

 and in the valleys, and the shrubs and trees have the healthy growth 

 peculiar to the foot-hills of the mountains. 



Between Bridger's Pass and Pine Grove the wall on the north side 

 continues much the same, but the cretaceous clays seem to underlie the 

 sandstone ; but on the south side the cretaceous beds rise up abruptly 

 to the height of about eight hundred feet above the road ; and on the 

 summit is a broad plateau, covered with a deposit of drift material, and 

 well grassed over. Here and there we see beautiful groves of aspen 

 and pine. The belt of cretaceous beds is very wide. In the vicinity of 

 Bridger's Pass there is an enormous thickness of the coal strata, which I 

 have estimated at three thousand to five thousand feet. The inclina- 

 tion is 10° to 15°. The cretaceous clays are also very largely developed 

 fifteen hundred to two thousand feet. Near Aspen Grove I found Bacu- 

 lites ovatus and some undetermined marine shells. At Pine Grove the wall 

 on the north side extends across the country, towards Rawlings' Springs, 

 in one of the most handsome and symmetrical anticlinals I have ob- 

 served in the West. The valley is about four miles wide between the 

 outcropping walls, and forms a sort of rolling prairie, which shows the 

 style of surface weathering when underlaid by the soft, yielding, creta- 

 ceous clays. The trend of the valley is northeast and southwest, and 

 forms an extension of the axis of elevation of some range of moun- 

 tains near Bear River. 



About northeast of Pine Grove there is a small lake in this valley, 

 half a mile long and nearly the same width, the shores of which are 

 covered with an alkaline efflorescence. This anticlinal valley is of the 

 same character as the one that forms the extension of the axis of the 

 Uinta range at Salt Wells, on the Union Pacific Railroad. Here we see 

 the same smooth, rounded appearance to the surface from Rock Springs to 

 a point about two miles east of Salt Wells, where the outcropping 

 edges of the coal-bearing rock appear. Passing up the South Fork of 

 Bitter Creek, we have the more modern tertiary beds inclining at a 

 small angle. Near La Clede Statiou is a high ridge, extending across 

 the country like a low range of mountains, composed of the somber indu- 

 rated, sandy clays of the upper miocene, of which Haystack Butte forms 

 a part. Continuing our way eastward, we descend again across the edges 

 of the same beds we saw on Bitter Creek, and gradually passing through 

 a tremendous thickness of eocene coal strata, reach the soft clays of 

 the cretaceous group. The exact line of separation between the true cre- 

 taceous and tertiary beds in this region I cannot positively determine. 

 We know that fossils, brackish and sometimes purely fresh-water, char- 

 acterize the upper eocene above the thickest coal beds ; that at Hallville, 

 in the dark clays or slates, above one of the most valuable coal beds, there 

 is a profusion of Cyrenas and other brackishj-water fossils; that, as we de- 

 scend in the coal strata, beds of several species of Ostrea are found ; 

 showing clearly that the great lake at this time had access to the salt sea. 

 I am not able to draw the exact line of separation between the tertiary and 

 the cretaceous beds in this region, and I am inclined to believe that there 



