LOWEE AUBREY UPPEE AUBEEY. 149 



ity in a narrow zone to the western border of the region under discussion. 

 These beds outcrop again in Split Mountain Canon in a deep gulch running 

 westward from its head, and again on the Yampa Plateau. Here some of 

 its harder limestones are occasionally found standing in low peaks. 



UPPER AUBREY GROUP. 



In the Uinta Mountains the Upper Aubrey is composed of two mem- 

 bers, a massive, homogeneous, light gray sandstone at the base, which I 

 have called the Yampa Sandstone, having a thickness of a thousand feet or 

 more; and above, a cherty limestone from 150 to 200 feet in thickness, which 

 I have -called the Bellerophon Limestone. These beds from, their indura- 

 tion and homogeneity are well adapted to the formation of ridges. 



The group is seen outcropping in the Flaming Gorge district, and in 

 a zone stretching westward from Horseshoe Canon for many miles. A little 

 patch has been caught in the fault on the west side of Quartz Mountain and 

 thrust between strata of Cretaceous Age, and its area is marked on the map. 

 The monoclinal ridge on the east side of Po Canon is of this age. It is seen 

 on all sides of Junction Mountain. Yampa Canon is carved through this 

 sandstone. Here the rocks dip to the south and the homogeneous Yampa 

 Sandstone on the north side of the canon is cut by a multitude of little can- 

 ons, the channels of rainy day brooks. The entire slope is minutely carved 

 in this manner and the spaces between the meandering canons in many 

 places are but narrow walls, and sometimes these walls are broken where 

 the channels approach too closely, and by this process buttes with narrow 

 bases, and towers and pinnacles are formed. Along the courses of these 

 intermittent streams great numbers of pot-holes are found. This topography 

 is too minute to be represented on the map. On the eastern end of the 

 Yampa Plateau this sandstone forms the slope of the plateau where it de- 

 scends into the valley, and is in like manner carved with innumerable 

 gulches whose courses are interrupted by pot-holes. 



This group has an extensive outcrop in the Yampa Plateau, and finally 

 it has been traced from Whirlpool Canon westward beyond the limits of the 

 map. 



