WHITE.] SUEFACE GEOLOGY ^DEIFT. 57 



It is upon the upland borders of Axial Basin, between Yampa and 

 Junction Mountains, from 400 to 800 feet above the level of Yampa 

 Eiver, and so high that it could never have been reached by its waters, 

 that we find the mixture of eastern and western drift-pebbles. No west- 

 ern drift-pebbles have been found east of Yampa Mountain, and no peb- 

 bles of eastern origin have been found west of Junction Mountain, ex- 

 cept that which the currents of the rivers have transported as before 

 mentioned. 



Junction and Yampa Mountains seem to have stood respectively as 

 barriers, or, at least, as obstacles in the way of the transportation of the 

 drift-pebbles in the direction of radiating lines from the two mount- 

 ain ranges. For example, in that i^ortion of the valley of the Yamp.i 

 which lies north of an east and west line through the north base of 

 Yampa Mountain, I found eastern drift-pebbles plentiful, with only a 

 very slight admixture of those of western origin, and even this small 

 proportion is not certainly known to be of more western origin than 

 Yampa Mountain, because it may have been derived from that mountain 

 itself, which has the same lithological composition that the eastern por- 

 tion of the Uinta Eange has. 



Furthermore, I found very few eastern drift-pebbles upon the surface 

 immediately west of Yampa Mountain, and also very little western drift 

 upon the surface immediately east of Junction Mountain. Western drift- 

 pebbles, however, exist in great abundance upon all the surface east of 

 the Uinta Uplift and south of an east and west line through the south 

 base of Junction Mountain, and also extending nearly as far eastward 

 as Yampa Mountain. 



All these facts are at least suggestive of the glacial distribution of 

 this drift from both mountain-ranges, but it is proper to say that no 

 glacial strisB, either upon the pebbles or upon the rocks in situ, were 

 observed in this district, although it is not certain that they do not 

 exist there. Also, that the character of the underlying rocks of all that 

 region is seldom such as to have preserved the strise if they had ever 

 been made upon them. As to the origin of the pebbles, as such, of 

 whick the drift is so largely comj)osed, the phenomena observable in 

 this district afford no satisfactory information ; but their origin as 

 pebbles was probably anterior to, and the result of, other causes than 

 that which produced their distribution. 



As to the time when this distribution took place, some important 

 data were obtained. The unconformity of the Uinta group upon all 

 the other Tertiary groups, as well as upon those of older date, has been 

 mentioned, which shows that group to be of much later Tertiary age 

 than the Bridger Group. The great outflow of basaltic trap that has 

 been before mentioned, is known to have taken place long after the 

 deposition of the Uinta Group, because the trap is found resting upon 

 a considerably eroded surface of that group at Fortification Butte, five 

 or six miles north of the northeast corner of this district. 



A large percentage of the pebbles of the eastern drift of this district 

 consists of fragments of this trap, which fact proves both the origin of 

 the pebbles and the distribution of the drift to have been subsequent to 

 the trap outflow. Nevertheless, this drift is known to be of great age, 

 because there is much evidence that extensive erosion has taken place 

 since its distribution ; but still there is evidence that the great general 

 features of the region as they now exist were produced by erosion 

 before the distribution of the drift. So far as my observation has 

 extended, there appears to be no reason why we may not regard the 

 distribution of this drift of the Park range and Uinta Mountains as 

 contemporaneous with that of the great northern glacial drift. 



