The Montana Mountains contain part of a resurgent dome complex within the southwestern section of the McDermitt caldera. Exposed intracaldera units consist of welded tuffs, lavas, domes, and epiclastic deposits. The majority of volcanic units have a comenditic composition, but a few units are nonperalkaline and biotite bearing. A deformed series of comenditic welded tuffs occur in a strongly folded rheomorphic zone more than 500 m thick. The geometry of vitrophyres, flow bands, foliations, and lineations causes the rheomorphic zone to superficially resemble a series of intrusive domes. However, the contacts between units are conformable, the vitrophyres are continuous from one fold to the next, and the units display pumice, shards and other features common in welded tuffs. The deformed tuff sequence lies on top of biotite tuffs and epiclastic deposits that are tilted and faults (but not folded0 and beneath relatively undeformed tuffs and lake deposits. This relationship suggests that their large-scale folds in the deformed sequency developed shortly after caldera collapse.