Beaverhead Shatter Cone - Montana 10.1 lb (4,588 g)

The Earth Impact Database lists Beaverhead at approximately 60 km (37 mi) in diameter, making it the second-largest confirmed impact structure in the United States after the 85 km Chesapeake Bay structure in Virginia, and placing it among the ten largest confirmed impact structures on Earth. Larger reconstructions exist: Hargraves, Kellogg, Fiske, and Hougen (1992) argued that the surviving shocked rock represents a piece of the central uplift of a complex crater at least 75 km across, and Fiske and Hargraves (1994) and Carr and Link (1999) both favored a figure near 100 km. Every one of these estimates is inferred from the distribution of shock features rather than measured from a crater rim, because no rim survives.

Beaverhead has no satisfactorily measured impact age, and the two published constraints pull in opposite directions. Kellogg, Snee and Unruh (2003, Journal of Geology) reported ⁴⁰Ar/³⁹Ar dates clustering between 899 and 908 Ma on micas from a breccia zone, interpreted as recrystallized impact glass and therefore treated as a minimum age — implying the impact is at least that old. Carr and Link (1999), working on Neoproterozoic conglomerate and breccia at Leaton Gulch in the northern Lost River Range, Idaho, cautiously argued instead for about 600 Ma or younger on trace-fossil evidence. The 600 Ma figure has since been adopted as the default by most reference compilations, including the Earth Impact Database, despite being the less directly constrained of the two positions. The ~680 Ma shown on our Impacts Map is a central estimate between those competing constraints, not a measurement, and should be read as Neoproterozoic, poorly constrained.

It is a quartz-rich clastic sedimentary rock: a fine-grained, massive feldspathic quartzite, described in the impact literature as feldspathic arenite or arkosic sandstone. 

The lithology is not incidental. Shatter cones develop best in fine-grained, structurally isotropic rock, which is why quartzites and carbonates produce the finest examples worldwide. Hargraves and White (1996) examined Beaverhead cones petrographically and by transmission electron microscopy and found planar fractures, pseudotachylite veinlets, and planar deformation features in quartz and K-feldspar confined to within about 2 mm of the cone surface, which is direct microstructural confirmation that the cone surfaces themselves are shock features.

An impact crater is a landform: the excavated depression itself, with a raised rim, a floor, and an ejecta blanket, as at Meteor Crater in Arizona. An impact structure is the entire volume of rock deformed and shocked by the event, at the surface and at depth, whether or not the landform still exists. Geologists use the second term when erosion, burial, or deformation has removed the crater shape and only the shocked rock remains. Every crater is a structure; most structures are no longer craters.

Beaverhead is the textbook illustration of why the distinction matters. There is no Beaverhead crater. The shocked rock sits within the Cabin thrust plate of the Late Cretaceous Cordilleran thrust belt, indicating it is allochthonous: it was carried tens of kilometers northeastward from the impact site. Hargraves et al. (1992) identified it as the first tectonic fragment of a large impact structure recognized in the geologic record. The buried root of the original structure corresponds to a circular gravity, magnetic, and topographic anomaly near Challis, Idaho (McCafferty et al., 1993), roughly 50 km to the west.