534 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



curving to tlie westward a little above Ham's Hill, and after crossing a 

 brancli of Crow Creek, is seen to turn once more to tlie northward. The 

 angle of the dip is about 20°. 



ham's fokk basin. 



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

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

 Eidges. 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 sMghtly 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 earher 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 Eiver Group is eroded 

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

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

 were not close enough to determine the relations. 



ham's foek plateau. 



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

 Tertiary Lake just described, from which the Green Eiver 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 Eiver, 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 Eiver 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 

 Eiver Group dix)s 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 hmestones. Below are dark shales, that weather white on 

 exposure to the au\ 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. 

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

 canon-heads are just as steep as the sides. They begin abrux)tly, and 

 are the connterx^arts, on a smaller scale, of the canons in the Green Eiver 

 Group of the Grand Eiver Cliffs in ^STorthwestern Colorado, described in 

 the Annual Eeport of the Survey for 1876. 



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

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

 identification. Of them he writes me, " Of your specimens, I find in ISTos. 

 1 and 3 the same kind of leaf of a new species of Myrica; in l^o. 2, an in- 

 volucre of Ostrija, new species 5 both are referable to the Upper Green 

 Eiver Group, which is by the i3lants the equivalent of the White Eiver 

 Group." 

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



