534 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



curving to the westward a little above Haul's Hill, ami after crossing a 

 branch of Crow Creek, is seen to turn once more to the northward. The 

 angle of the dip is about 20°. 



ham's fork basin. 



Following Ham's Fork to its head, it is seen to have its sources in a 

 basin of Wahsatch beds lying west of the southern extension of Absaroka 

 Ridges. The extension of this basin northward, as colored on the map, 

 is somewhat indefinite, and the relations of the beds to the older rocks 

 in the hills on its rim are unknown. The river flows south through the 

 basin, receiving lateral branches from both sides. On reaching the north- 

 ern end of the Ham's Fork Plateau the river turns slightly to the east- 

 ward, keeping along its eastern edge. It is evident that the Green Eiver 

 shales once extended over the whole basin but have been eroded away. 

 Ham's Fork Basin represents an old bay of the earlier Tertiary Lake. 

 Whether the ancient lake whose sediments are found in the Bridger 

 deposit, ever extended up this arm it is impossible to tell, as no rem- 

 nants of it are found there now, and even the Green Biver Group is eroded 

 entirely from the greater part of the basin. The bright red and pink 

 Wahsatch beds are seen on the slopes of the Absaroka Bidge, but we 

 Avere not close enough to determine the relations. 



ham's fork plateau. 



This plateau really represents that portion of the ancient arm of the 

 Tertiary Lake just described, from Avhich the Green Biver beds have 

 not been eroded. It is a plateau, cut into mesas by four streams flowing 

 eastward into Ham's Fork, and by the branches of Twin Creek, a tribu- 

 tary of Bear Biver, which flow southward to join the main creek, whose 

 course is westward. It is a tongue extending northward from the west- 

 ern side of what is called 'on the maps of the 40th parallel survey the 

 Aspen Plateau. In the southern extension, however, the capping of 

 Green Biver beds appears to be absent, as there is none of the formation 

 colored on the map. The western boundary of the plateau is an anti- 

 clinal range of Carboniferous rocks, from the sides of which the Green 

 Biver Group dips slightly towards the east, rising a little as Ham's Fork 

 is approached. The upper beds of the group here are fine-grained, com- 

 pact, white limestones. Below are dark shales, that weather white on 

 exposure to the air. In these, near Camp 21, we obtained fish remains. 

 The thickness of the shales and limestone here is about 400 feet. The 

 streams generally cut deep enough to expose the variegated beds, which 

 also show on the slopes of the high hills to the westward. The canons 

 cut in the plateau are marked by almost perpendicular walls, on the sides 

 of which, especially towards the heads, there are numerous springs. 

 Bunning water does not extend far along the courses of the creeks. The 

 canon-heads are just as steep as the sides. They begin abruptly, and 

 are the counterparts, on a smaller scale, of the canons in the Green Biver 

 Group of the Grand Biver Cliffs in Northwestern Colorado, described in 

 the Annual Beport of the Survey for 1876. 



Associated with the fish remains found near Canip 21 (near Sublettes 

 road), I found three leaves, which were sent to Professor Lesquereux for 

 identification. Of them he writes me, " Of your sx>ecimens, I find in Nos. 

 1 and 3 the same kind of leaf of a new species of Myrica; in No. 2, an in- 

 volucre of Ostrya, new species; both are referable to the Upper Green 

 Biver Group, which is by the plants the equivalent of the White Biver 

 Group." 



On Twin Creek two brothers by the name of Bell have been blasting 



