VALLEY OF THE LITTLE SNASB RIVER. 189 



covered by the remains of a coarse conglomerate, formed of large, rounded 

 pebbles of quartzite, lying perfectly horizontal, and unconformable to the 

 underlying sandstones, which probably represents the Wj^oming conglom- 

 erate. This formation, which is regarded of Pliocene age, is the latest de- 

 posit of the Tertiary waters in the Rocky Mountain region. It is a structure- 

 less conglomerate, made up of coarse pebbles of the more resisting rocks of 

 the ranges near which it is found, generally loosely compacted by a gravelly 

 or limy cement. The pebbles generally decrease in size with their dis- 

 tance from the mountains from whose rocks they are derived. Only rarely 

 is any considerable thickness of this conglomerate found ; but the larger 

 pebbles resulting from its decomposition cover the higher flat-topped peaks 

 and ridges wherever they have resisted the effects of erosion sufficiently to 

 preserve the original surface of this latest Tertiary deposit. That it was 

 in a measure a littoral or beach deposit is shown by the fact that it is 

 more frequently found, and in greater development, in close proximity to 

 the higher mountain-ranges; but there are evidences, particularly in the 

 Green River Basin, which seem to show that it extended, though perhaps 

 in a very thin sheet, to distances of 20 and 30 miles from the shore-line. 



