The massive Salmon River Mountains of interior Idaho are bounded on the west by a belt 30 miles wide of post-Miocene, west-tilted normal-fault blocks and west-dipping monoclines. The belt is coincident with the western border zone of the middle Cretaceous Idaho batholith, as it extends from the west edge of the massive interior of the batholith to about the western limit of the border zone of gneisses and schists. West of this belt is the Columbia Plateau province of irregular domal and anticlinal uplifts and northwest-trending normal faults. These structures are superimposed upon east- to northeast-trending, low-grade metamorphic rocks intruded by semi-concordant stocks and small batholiths, products largely of Late Jurassic (?) orogeny. The Idaho batholith has been little deformed, and its border-zone rocks of intermediate competence are broken by concordant structures. Young structures cut directly across the relatively incompetent rocks of the older orogen to the west. The mountains flanking the Snake River and Salmon River canyons are higher than those farther away, suggesting that local isostatic uplift may be compensating for their erosion.
