J. L. Rich — Large Boivlders in Gravel Deposits. 445 



The clue to the explanation outlined above presented itself 

 when it was observed that huge bowlders, similar in size to 

 those in the gravels, lie either singly or in groups at the base of 

 the present eastern escarpment where they have rolled or slid 

 down from the cliffs above. It is conceivable that under an 

 aggradational regimen of Whitewater Creek these bowlders 

 might become buried in gravels as the others seem to have 

 been, and further retreat of the escarpment might leave them 

 buried far out on a gravel plain. 



Bowlders rolled or slid into position in this way might in 

 some cases exhibit striation, and thus lead still further astray 

 the geologist inclined to look upon them as of glacial origin. 



If one may judge from the published descriptions, without 

 having been on the ground, some of the so-called older drift of 

 the San Juan region described by Cross and Howe and others* 

 may be susceptible of an alternative interpretation similar to 

 that outlined above. 



Several of the deposits in question occur in or at the mar- 

 gins of desert gravel plains ; they are marked by enormous 

 bowlders, in many cases without undoubted glacial striations; 

 they are so situated with respect to high scarps of flat-lying 

 lavas as to be within the area probably once covered by those 

 lavas and therefore within the areas of possible landsliding 

 from the scarps ; and finally, at the base of the present scarps 

 are extensive masses of landslide debris, in many respects re- 

 sembling the so-called old drift, which may on further retreat 

 of the scarps come to occupy a similar position. 



In view of these considerations, it seems hazardous to assign 

 to a bowlder-bearing deposit a glacial origin until the alter- 

 native outlined above has been specifically considered and 

 positively eliminated. 



University of Illinois, Urbana, 111. 



*U. S. Geol. Survey, Folio 153, Eico, Colo. Cross and Howe, Bull. Geol. 

 Soc. Am., xvii, 251-74, 1906. Hole, A. D., Jour. Geol., xx, 710-37, 1912. 

 Atwood, W. W., Jour. Geol., xx, 385-409, 1912. 



