140 JOSEPH BO MPERBY. 
here and there forming low hills. Near Leesburg, in Lemhi County 
the plateau surface is also well preserved. Here it is about 8,500 
feet in elevation, and breaks off abruptly along the canyon of 
Salmon River, which flows in a gorge 5,500 feet deep. 
Equally significant with the high flat-topped areas are the 
innumerable level-crested divides which extend out from them, 
and with other more isolated summits form a veritable labyrinth 
of highland tracts: Also worthy of note, as indicating that streams 
once flowed near the summit levels, are occasional rock-cut ter- 
races which parallel the plateau surface but at slightly lower 
Fic. 1.—A portion of Poverty Flat, which has an elevation of 9,600 feet, and 
is bordered by canyons as much as 5,000 feet deep. The spur on the right shows 
the highly inclined beds across which the flat is developed. 
levels. One of these occurs along the west side of Spring Moun- 
tain, and another along the south side of Poverty Flat. 
These several features clearly indicate a plateau surface, now 
deeply dissected, and when it is remembered that it is cut across 
highly inclined rocks of diverse composition, 1t seems equally 
clear that it could only have been developed by profound erosion. 
The region was reduced to gentle relief, and later raised to its 
present elevation. 
Correlation and extent.—Mr. Lindgren describes the same surface 
in west-central Idaho, and concludes thatt ““The whole mountain 
region should probably be regarded as a vast plateau. ... . dive 
uplift of this plateau and its intricate and deeply cut drainage 
system evidently antedate the Miocene period.” These high- 
Waldemar Lindgren, Twentieth Annual Rept., U.S. Geol. Surv., Pt. III (1900), 77. 
