226 JOSEPH B. UMPLEBY 
Furthermore, the third hypothesis is untenable because of the 
topography along certain of the present-day streams. Salmon 
River heads in the Stanley Basin segment of the old valley above 
mentioned but near Stanley leaves it and enters a canyon so 
narrow that there is scarcely room for a wagon road. This gorge 
continues to a point near Challis, a distance of about 50 miles, 
where the stream emerges into a broad, open depression. Fifteen 
miles below Challis the river enters another narrow canyon which it 
follows for about 30 miles to a point near Salmon City, and, after 
flowing for 20 miles through a third broad valley, again enters a 
gorge which it follows nearly to the western border of the state. 
These open stretches along the course of Salmon River cannot be 
explained by local structure or by differences of resistance to 
_denuding processes. ‘Their distribution and individual outline are 
readily interpreted, however, if we consider them to be parts of the 
older valley system. The general depression about Challis has its 
greatest elongation almost at right angles to the course of Salmon 
River, but is itself continuous with an old valley, now filled in many 
places with eruptive rocks, which drained off to the southeast and 
joined the Snake River plains near Martin. The open stretch 
about Salmon City presents numerous exposures of plant-bearing 
Miocene lake beds which extend off to the southeast along the 
present course of Lemhi River and Birch Creek. To an observer 
in the field it is very clear that these open stretches date back to 
the earlier topography and that, though in part filled with Miocene 
lavas and sediments, they have persisted as great depressions 
throughout their entire history.t Salmon River has crossed them 
by headward erosion. 
In the preceding paragraphs I have endeavored to show (1) that 
the old valleys were not developed by down-folding or down-faulting 
and (2) that they were not excavated prior to the planation of the 
region. The remaining hypothesis holds that the valleys are the 
product of erosion and were developed after the elevation of the old 
‘During the past three field seasons many notes have been made on the old 
valleys of southeastern Idaho north of Snake River, and several hundred miles of their 
courses have been sketched. It is expected to assemble this information in a later 
paper. 
