The Beaverhead impact structure is the 2nd largest impact structure within the U.S. It lies within the states of Idaho and Montana. Estimated at in diameter, it is the 9th largest impact crater on Earth. With an estimated age of 600 million years (Neoproterozoic), the impact's original shatter cones along the impact structure's perimeter provide some of the structure's only remaining visible evidence. It is named for the Beaverhead region of southwestern Montana in which it was first discovered. See also Chesapeake Bay impact crater List of impact craters in North America References Further reading Carr, J and Link, PK, 1999, Neoproterozoic conglomerate and breccia in the formation of Leaton Gulch, Grouse Peak, northern Lost River Range, Idaho: Relation to Beaverhead Impact Structure, in Hughes, S.S., and Thackray, G.D., eds., Guidebook to the Geology of Eastern Idaho: Pocatello, Idaho Museum of natural History, p. 21-29. Aerial Exploration of the Beaverhead crater Proterozoic impact craters Impact craters of the United States Landforms of Idaho Landforms of Beaverhead County, Montana