444 J. L. Rich — Large Bowlders in Gravel Deposits. 



The large bowlders under discussion lie at or near the 

 northern edge of the gravel plain (at X and Y, fig. 1) and in 

 the upper part of the gravels, 100 feet and more above the 

 level of the streams which dissect them. 



The bowlders, which so far as observed are rhyolites like 

 those forming the escarpments to the east and the base of that 

 to the north, occur in groups or nests of many individuals, 

 lying close together. Three such groups, two of which appear 

 in tig. 2, were noted. Doubtless others might be found. 



One of the bowlders in place in the gravels measured 15 

 feet in length and 4 feet in thickness for the exposed part. 

 How much thicker or how wide the bowlder was could not be 

 determined. Twenty feet away, but not exactly in place lay 

 another (fig. 3), which measured roughly 15 feet in diameter 

 and from 6 to 9 feet in thickness. Others of smaller size lay 

 nearby. 



The cattle shown in fig. 2 indicate by comparison the size of 

 the bowlders there exposed. 



The matrix in which they lie is a desert conglomerate of 

 medium to fine texture, the components ranging for the most 

 part from 1-8 inch to one foot in diameter — the finer material 

 predominating. The nearest highland from which the bowl- 

 ders could have come is, the escarpment to the east, 2 or 3 

 miles away and separated from the gravel plain by the half- 

 mile wide valley of Whitewater Creek. Another possible 

 source is the escarpment to the north 6 to 7 miles away, across 

 the intervening lowland. 



It might be argued, and possibly correctly, that the bowlders 

 were carried to their present position by torrential streams 

 before the development of the lowland, and when the. escarp- 

 ment stood nearer ; or it might be argued that the gravels 

 must be of glacial origin. The latter, however, clearly 

 is not the correct explanation of this case, for there are no 

 evidences of former glaciation in the region. 



The possible explanation of the phenomenon which the 

 writer would propose, and which seems to him the most satis- 

 factory, is that the bowlders may have rolled down to their 

 present position as landslides, or rockslides, from one or 

 another of the escarpments (probably that to the east) when it 

 stood near where the bowlders now lie. The retreat of escarp- 

 ments by sapping and landsliding is too well known to require 

 explanation. These escarpments are now retreating, and they 

 must have retreated in the past. So far as it is known the 

 geological evidence is in accord with the hypothesis that the 

 scarp-forming lavas once covered the area now occupied by the 

 lowland and by the part of the gravel plain where the bowl- 

 ders lie. 



