204 



ELIOT BLACKW ELDER 



If the Eocene sediments were deposited in basins excavated 

 below the peneplain, the sediments should have been derived in part 

 from the slopes and divides of the basins themselves. On both sides 

 of the Gros Ventre Range, however, the thick coarse conglomerate 

 in the Lower Eocene consists not of local rocks but almost entirely 

 of varicolored quartzite with occasional porphyries. Since there are 

 no rocks of this character entering into the make-up of the Gros 

 Ventre Range, these pebbles must have been imported; and the 

 nearest appropriate outcrops are about loo miles to the northwest. 



Eocene i^a//ey 



^oe/9/a/n 



Fig. 19. — Diagrams illustrating various hypotheses as to the relation and age 

 of the peneplain: (o) as an Eocene plain with valleys excavated in it and then filled; 

 (6) as an Eocene plain covered with sediments and then warped; (c) as a Pliocene ( ?) 

 plain cut across Eocene and older rocks. 



Considerations of time are also somewhat adverse, since upon 

 this hypothesis the planation, involving the most massive and 

 resistant rocks, must have been completed in that fraction of the 

 Eocene epoch between the folding of the Paleocene (Fort Union) 

 strata and the deposition of the Lower Eocene (Wasatch) beds. 

 In the same interval of time, spacious depressions must also have 

 been excavated beneath the new peneplain for the reception of the 



