432 ELIOT BLACKWELDER 
top of Pole Mountain, at an elevation of 9,000 feet, may correspond 
in both origin and age to the Medicine Bow plateau. It rises high 
above the remarkably even surface later to be described as the Sher- 
man peneplain. In the absence of more satisfactory evidence, I can 
only suggest as a plausible hypothesis, that this Medicine Bow plateau 
surface and Pole Mountain are remnants of the one developed by the 
Eocene cycle of denudation. When neighboring regions have been 
examined with these problems in mind the hypothesis may be either 
verified or cast aside. 
Mid-Tertiary sedimentation.—Reference has already been made 
to the Tertiary sediments which underlie the region east of the Sher- 
man uplift. ‘These consist largely of clays, soft sandstone, and con- 
glomerate. As a rule the finer sediments are evenly stratified, and 
the colors are prevailingly light, being either white, gray, or yellow. 
The conglomerates are cross-bedded and locally indurated. It has 
been customary in the past to speak of these and similar deposits 
throughout the western mountain region as “lake beds,” on the theory 
that fine stratified sediments could be deposited only in seas or in lake 
basins. More recently this origin has been called into question by 
Davis, Chamberlain, and others. It is now fairly well established that 
most of these Tertiary formations resemble those now being made by 
agerading rivers, while some others bear a striking resemblance to 
eolian deposits.. In the region under discussion it is highly probable 
that the Tertiary formations were deposited by streams in association 
with other terrestrial agencies, and the predominance of fine sediments 
indicates merely that the currents, whatever their nature, were of 
relatively weak transportive power. 
The Oligocene beds resting on the eroded Eocene surface seem 
to imply that the erosive activities of the Eocene epoch were later sub- 
verted and gave way to aggradation. It is well, however, to bear 
in mind the alternative hypothesis that, without any marked change, 
a sheet of Piedmont alluvium which had been accumulating farther 
eastward during the Eocene epoch gradually spread westward as it 
was built higher and the highlands were reduced. In either case, the 
fact remains that the Eocene valleys were filled with clays and sands, 
and these sediments finally buried the lowland east of the Sherman 
uplift to a depth of several hundred feet. Embayments of this sheet 
