The spire of brown volcanic rock at White Mountain, long regarded as a filled volcanic conduit associated with eruption of the Eocene Wapiti Formation, is actually an erosional remnant of volcaniclastic sedimentary rocks of the Eocene Cathedral Cliffs Formation that has been transported 10 to 15 mi southeastward as part of a Heart Mountain detachment fault block. Prevolcanic Paleozoic carbonate rocks, which comprise the western part of this block, are metamorphosed and strongly folded, in contrast to the rocks of other Heart Mountain fault blocks. This metamorphism ends abruptly downward at the Heart Mountain fault plane. Rounded fragments of metamorphosed carbonate rock in Heart Mountain fault breccia and in related clastic dikes show that the metamorphism occurred before the faulting, presumably as a result of the Cathedral Cliffs igneous activity.
Igneous rock fragments in the Heart Mountain fault breccia at White Mountain seem to have been derived from allochthonous sills and dikes that predate the Heart Mountain fault. Therefore these fragments are not evidence for volcanic activity during Heart Mountain faulting, nor for introduction of volcanic gases along the fault plane as suggested by Hughes (1970a).
A fault plate would have to be large and coherent to be buoyed up by volcanic gas or other fluids. The present widespread distribution of the remnants of the upper plate indicates that the upper plate broke up to form a number of fault blocks very soon after the Heart Mountain faulting began.