I’ve reached the endpoint of Leg 25: The Beaverhead Impact Structure . This is the second largest impact structure yet identified in the United States. (We’ll get to the largest one eventually, be patient.) Per the Earth Impact Database it is 60 km in diameter and was created about 600 million years ago — though the Crater Explorer site says 100 km diameter and an age anywhere from 500 to 1000 million years. In any case it predates or just barely postdates the earliest comb jellies, sponges, and corals. Land based eukaryotic life back then was restricted to plants. So no, no one was watching when it hit.

The diameter is that uncertain because the structure’s essentially invisible. There are some shatter cones and that’s about it. As for where it is, I’m as close as I’m getting to the nominal position given by Earth Impact Database, but that’s about 100 km from the position (back in Idaho, more or less west of here) given by Crater Explorer. Shrug. It’s big and it’s around here somewhere, okay?

It wasn’t always here. It’s allochthonous. Use that word in conversation whenever you can. It means “found in a place other than where they and their constituents were formed”. (“I live in Syracuse these days, but I’m allochthonous.”) The upper crust has moved tens of kilometers to the east (relative to the lower crust) since the impact.

Here’s a map of the Beaverhead Site:

Geologic map of the Beaverhead Impact Structure showing locations of shatter cones, Mississippian and younger rocks of the Cabin Thrust Plate, sandstone, and basement gneiss

Map of the Beaverhead Impact Structure

Fiske, P. S., Hougen, S. B., and Hargraves, R. B., ‘Breccia dikes from the Beaverhead Impact structure, southwest Montana,’ Lunar and Planetary Inst., International Conference on Large Meteorite Impacts and Planetary Evolution, 1992

On closer reading it appears both of the locations mentioned above are right. Where I am is near the middle of that map, which is where the shatter cones were found. The location in Idaho is that of “a large circular gravity, magnetic and topographic anomaly, which could be the root of the impact structure”. The Beaverhead Site is further east, partly because it was originally near the edge of the crater and partly because of the displacement of the upper crust.

Arguably I should have aimed for the (likely) original impact site, not where the impact evidence was found — or else both — but let’s call this good enough.

I’ve gone about 2.5 km further, doubling back for a bit and then heading north, starting Leg 26.

A map showing my progress is here , a spreadsheet with progress detail is here , and a Google Earth KMZ file is in  this Google Drive folder . Present coordinates: 44.623°N, 112.993°W .

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