LARAMIE REGION, WYOMING 433 
of alluvium extend westward up into the valleys in the Sherman high- 
lands, thus burying the outcrops of the upturned Carboniferous rocks, 
as already described. Even the divides between these valleys were 
more or less covered by the same deposits, for at Granite Canyon 
Tertiary beds are found lying on the top of one of these divides. 
Fic. 2.—(Stereogram.) The Laramie uplift in the Oligocene epoch. The east 
flank partially buried by terrestrial deposits. 
No trace of the middle Tertiary sediments has been found within 
the southern part of the Laramie basin. Just what the meaning of 
this fact may be is not now clear. While the Eocene surface (of which 
the Medicine Bow plateau is possibly a remnant) was being buried 
and thus preserved, east of the mountains, by the continued up-build- 
ing of the Tertiary strata, corresponding denudation of the Sherman 
and Medicine Bow uplifts was probably in progress; but the record 
of it, if preserved, is too obscure to be read with any degree of 
confidence. 
The exact age of the Tertiary sediments is a matter of considerable 
importance in working out the history of this region. According 
to Darton the lowest beds are the Chadron sands and the Brule clays, 
both of Oligocene age. These are followed by the conglomeratic 
Arikaree formation, which he assigns to early Miocene time. The 
close correlation of the Tertiary formations of the Great Plain is a 
matter of considerable difficulty, and perhaps the determinations 
thus far made should not be accepted too exactly. If, however, they 
are approximately right in this case, we may assign the epoch of 
