POST-CRETACEOUS HISTORY OF WESTERN WYOMING 107 



areas and coincident filling of others. It appears, therefore, to be 

 the more promising factor in this case. That the new state of things 

 continued almost, or quite, without interruption for a long lapse of 

 time is shown by the fact that the eroded surface is now buried in 

 some places by a thickness of apparently conformable sediments 

 which cannot well be less than 4,000 feet thick and may easily be 

 much more. Remnants of these beds are now to be found in all 

 the principal lowlands of the district, such as Green River basin, 

 Jackson Hole, and Wind River valley, and also in the Mount Leidy 

 highlands at the northwest end of the Wind River uplift. The 

 strata vary considerably from place to place, as they might be 

 expected to in view of their terrestrial origin and the irregular 

 surface upon which they were deposited. 



The origin of these Tertiary deposits is not wholly obvious, 

 and their designation as "lake-beds" by the Hayden Survey is 

 open to grave doubt. It is safe to say that they are not marine, 

 for instead of marine fossils they contain the remains of land 

 plants and land mammals, as well as of freshwater fishes and 

 mollusks. As working hypotheses we may suppose that they 

 could have been deposited (a) by lakes and marshes, {b) by graded 

 streams upon their floodplains, (c) by wet-weather streams making 

 alluvial fans, or (d) by the wind upon dry land surfaces. It is 

 not inherently improbable that each of these agencies may 

 have played an important part. The problem compels more 

 analysis. 



The Pinyon conglomerate (Fig. 6) of the Mount Leidy high- 

 lands has already been mentioned as highly suggestive of the work 

 of powerful aggrading streams with constantly shifting channels. 

 It appears to be identical with deposits being made today by those 

 means, and no other agency seems competent to produce it. The 

 pebbles are nearly all quartzose in composition and are in large 

 part foreign to the district. This suggests that they have been 

 transported far, and that they are the residue of long-continued 

 attrition. If this be granted, it may be taken to indicate that the 

 Pinyon conglomerate was deposited largely by aggrading rivers of 

 considerable length and power, rather than chiefly by local creeks 

 from the surrounding mountains. 



