R. B. Hargraves, C. E. Cullicott, K. S. Deffeyes, S. Hougen, P. P. Christiansen, P. S. Fiske; Shatter cones and shocked rocks in southwestern Montana: The Beaverhead impact structure. Geology ; 18 (9): 832–834. doi: https://doi.org/10.1130/0091-7613(1990)018<0832:SCASRI>2.3.CO;2
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Shatter cones that point steeply upward are present in abundance over an area >100 km2 in Belt Supergroup sandstone outcrops in Beaverhead County, southwestern Montana. Crushed sandstone with fragments dispersed in a fluidal textured, cryptocrystalline matrix of similar chemical composition has also been found. Absence of cones in adjacent units of Mississippian and younger age suggests a late Precambrian-early Paleozoic age for the formation of this newly discovered impact structure, only a small part of which is preserved in the complexly faulted terrain.