324 ELIOT BLACKW ELDER 



because the vegetation serves to entrap the dust from the western 

 basins. 



This prelude to the general discussion of Quaternary glaciation 

 is given in order to show that the satisfactory correlation of the 

 different drift deposits and other glacial phenomena of the district 

 is a difficult matter, calling for critical interpretation. In order to 

 eliminate the effect of variable conditions, it is necessary so far as 

 possible to compare only cases where the environment has been 

 generally similar ; and I may add that a large amount of additional 

 information from the field is needed. 



In many canyons glacial drift of two distinct ages can be recog- 

 nized easily, and it seems to me there is sufficient evidence that the 

 later stage of glaciation is divisible into two quite distinct episodes. 

 There are a few facts which hint at a still older or fourth stage, but 

 they may be otherwise interpreted. Without attempting an actual 

 correlation, the three sets of deposits may be compared in general 

 state of preservation to the late and early Wisconsin, and the 

 Illinoian or the Kansas drift sheets in the Mississippi Valley region. 

 In explanation of this view I shall describe the characteristics of the 

 drift and other glacial features of the various stages here suggested, 

 beginning with the most recent. 



Pinedale stage : The youngest moraines, which for convenience 

 may be called the Pinedale drift, are conspicuous in the vicinity of 

 Torrey Lake and Bull Lake in the Wind River valley, Osborn ranch 

 on Green River, Jenny Lake and Taggart Lake in Jackson Hole, 

 around each of the large lakes near Pinedale on the southwest side 

 of the Wind River Range, and also in many other valleys of the 

 district. They are very rough and fresh in appearance, being 

 covered with bowlders which themselves show but little effect of 

 weathering (Fig. 42). Many of the bowlders of hard, fine-grained 

 rocks show polished or scratched surfaces, even though fully 

 exposed to the weather; and in the cirques above the moraines such 

 surfaces, bare or now covered only with moss, are abundant. The 

 terminal moraines are generally intact and the lateral moraines con- 

 tinuous, save where unusual conditions have subjected them to 

 abnormally rapid erosion. Lakes and ponds which contain but 

 little post-glacial filling are characteristic features of these newest 



