Recent discovery of shatter cones and fluidal breccias resembling impactites in Precambrian rock of southwestern Montana signal the possible presence of a large impact structure. The shatter cones are in rocks of the Cabin thrust sheet at the culmination of the southwestern Montana recess in the boundary between the Cordilleran thrust belt and the Rocky Mountain foreland. This study will test the validity of the "impact structure" interpretation by field documentation of shatter cone distribution and orientation and by laboratory analysis of the fluidal breccias. The results of this study will increase our understanding of impact structures and of their possible influence on subsequent orogenic processes.
