258 TRANSACTIONS OF THE [may 18, 
The important points of this discussion may then be summed 
up as follows: 
First. The formation in Brown’s Park Valley is lithologically 
different from that to the southeast. 
Second. The southeastern portion resembles the Green River 
Shales. 
Third. The Brown’s Park is softer and more friable than any 
Eocene Tertiary yet observed, and is surprisingly little eroded 
for even a Pliocene formation. 
Fourth. The fragments of bone discovered were not deposited 
earlier than the Loup Fork Miocene. 
From these facts it may be inferred that the beds in Brown’s 
Park Valley represent a Pliocene Lake, which was included be- 
tween two steep quartzitic cliffs and stretched east only as far 
as the Snake River divide. 
Having assigned the beds to the Pliocene, the question natu- 
rally arises: How can we account for the presence of these beds 
—isolated in position and distinct in character from any Ter- 
tiary in the region—and yet lying directly along the course of 
the great drainage factor of the country, Green River? If they 
were deposited in a lake that did not exist until all the other 
Tertiary lakes had disappeared, how do we account for the 
appearance and later the subsidence of this Pliocene lake? The 
explanation lies in the peculiar physiographic position of the 
sediment. It lies, as previously stated, in the U-shaped valley 
formed by the two eastwardly projecting spurs of Weber quartz- 
ite that form the inceptive portion of Uintah fold. The Green 
River cuts into the fold at Horseshoe Canon, some 30 miles 
to the west of the valley, flows east in a deep canon along the 
edge of the quartzite and emerges into the park at the vertex of 
the U. Thence it runs along the low valley for some twenty 
miles only to sweep around to the south and again enter the 
quartzite by the Canon of Lodore, one of the boldest and deep- 
est canons of the Rockies. 
It is a fact no longer disputed that these deep cafions in the 
quartzite by which the river crosses the mountains were first 
established in the softer overlying formations and that these 
formations furnished much of the corrasive material by means 
of which the harder rocks were cut away. As long as the river 
had only these soft rocks to deal with, the forces of erosion and 
elevation balanced each other, and the bed of Lodore Cafion re- 
mained upon the same level as the bottom of the valley. But 
somewhere near the close of the Miocene the forces no longer 
balanced each other. The greater resistance of the hard crystal- 
line quartzite, combined with the gradual elevation, prevailed 
