Beaverhead
Key facts
- Country
- United States
- Diameter
- 60 km
- Age
- about 600 million years
- Location
- 44.60° N, 113.00° W
No crater shape at all. Shatter cones can be found in outcrops of shattered quartzite and sandstone in the Beaverhead Mountains along the Idaho–Montana border.
Beaverhead is one of the oldest and largest impact structures in North America, estimated to have been about 60 km across when it formed. It lies in the rugged Beaverhead Mountains along the border of Idaho and Montana in the north-western United States, though its remains are no longer quite where the impact happened.
Discovery. In 1990 geologists reported shatter cones, cone-shaped fractures with branching grooves that form only under the shock of an impact, spread over a large area of the mountains. Microscopic study of quartz from the same rocks found planar deformation features, the classic fingerprint of shock. Together these proved that a large impact had taken place, even though no crater shape survives.
What it hit and when. The target was a thick pile of late Precambrian sandstones and quartzites. The age is estimated at around 600 million years, in the late Precambrian, based on the rocks affected and the deposits that followed, but it is not known precisely.
A crater on the move. Much later, between about 100 and 50 million years ago, mountain building in western North America pushed great slabs of rock eastwards along low-angle faults called thrusts. The shocked rocks at Beaverhead are part of one of these slabs, so the remains of the crater have been carried tens of kilometres from where the impact occurred and broken into pieces. Working out the original size and location is like reassembling a jigsaw with many pieces missing.
Visiting. The area is remote, high and rugged, with much of it public land. Check local rules before collecting, and never collect in protected areas.
Why it matters. Beaverhead shows that an impact can be recognised from shocked rocks alone, long after the crater itself has been destroyed. The impactor is unknown.
Did you know?
The remains of the Beaverhead impact were carried tens of kilometres by later mountain building.
It was recognised in 1990 from shatter cones alone, with no crater shape left.
At about 60 km, it was one of the largest impacts in North America's history.