THE OLD EROSION SURFACE IN IDAHO 225 
surface is a product of planation it must antedate the existence of 
such Miocene lakes. 
Still another line of evidence against the down-faulting or down- 
folding hypothesis is the direct observation from numerous 
exposures that folding or faulting is rare along the margins of the 
old valleys. In many places present-day streams have cut verti- 
cally from 1,000 to 3,000 feet or more into the lavas and tuffs 
which occupy the old valleys and thus afford excellent cross- 
sections of the earlier drainage channels. In the many sections of 
this kind which were examined, only one (near Junction, Lemhi 
County, and here the fault is post-Miocene) showed evidence of 
faulting and none presented down-folding. The eruptive rocks or 
lake beds, usually in a nearly horizontal position, abut the older 
formations irrespective of their attitude and in many places the 
lavas contain fragmental material, clearly picked up from the 
surface over which they flowed. Furthermore, the slope of the 
contact in every place observed is toward the lava area. 
The third hypothesis, which supposes that the valleys antedate 
the old erosion surface, is controverted by the shore-line relations 
already referred to. If the valleys were developed before the 
elevation of the old surface of gentle topographic forms, we are led 
directly to the conclusion that within the area planated the upper 
2,800 feet of some of the old valleys remained unfilled throughout 
the period of that planation. Would not those portions below 
base-level have been filled, and that, early in the period of 
reduction ? 
The third hypothesis is also untenable in view of the inequal 
depth of the lavas along certain of the old valleys. Take, for 
instance, the one which extends from near Salmon City, via Prairie 
Basin, Rabbitfoot, Custer, and Stanley Basin, to the Snake River 
plains at Camas Prairie. Prairie Basin, about 1,000 feet below the 
plateau level, Stanley Basin, about 2,500 feet below it, and Camas 
Prairie, yet lower, represent unfilled segments of this old valley, 
while intermediate sections are flooded with lavas which in places, 
as at Twin Peaks in Parker Mountain mining district and at Estes 
Mountain near Custer, rise a few hundred feet above the plateau 
level. 
