THE CRETACEOUS GROUPS. 155 



usually friable arenaceous and argillaceous shales, while in the Aspen 

 Mountain district they are arenaceous shales and thinly bedded sandstones, 

 itiid the greater induration of some of the beds in the last mentioned dis- 

 trict causes the valley spaces, which are characteristic of this group, to be 

 diversified with low ridges and cliffs. 



POINT OF ROCKS GROUP. 



The beds of this group can usually be divided into three somewhat 

 well defined members, the Upper Hogback, Middle Hogback, and Golden 

 Wall Sandstones, which are usually separated by a few feet of shaly sand- 

 stones, and these stratigraphic characteristics under conditions of upheaval 

 and erosion usually result in the production of three ridges, which are the 

 topographic features giving names to the several members. The upper 

 sandstone is usually massive and of light gray color weathering unequally 

 on exposed surfaces, which inequality is not determined by lines of stratifi- 

 cation. Here an irregular mass tumbles down and a cave is formed ; 

 there an irregular mass is more indurated than the general body to which 

 it belongs and stands in relief, often in some fantastic form; and such 

 weathering gives" the cliffs which are usually found along the outcrop of 

 these beds a strange and weird appearance. The general structure is sub- 

 concretionary, and true concretions sometimes weather out. The same 

 weathering is sometimes found in the Middle Hogback Sandstone. 



The Golden Wall Sandstone is often homogeneous and of a bright 

 yellow color in the Uinta region, and often stands as a sheer wall ; hence 

 its name ; but in the Aspen Mountain district these yellow sandstones are 

 broken into strata, and light gray sandstones and shales are intercalated. 

 The ridge like topography characteristic of this group of beds so preva- 

 lent everywhere, renders it easy to trace every outcrop, and the peculiar 

 and persistent characteristics of the upper member greatly facilitate the 

 study of the relation between the Cretaceous below and Tertiary groups 

 above. In the Uinta Mountains the unconformity at this horizon is every- 

 where apparent. The difference in dip is from two to fifteen degrees, and 

 the evidence of intervening erosion is apparent ; but in the Aspen Mountain 



