The Hagerman Formation: Multi-Use Geology

The Hagerman Formation, a layer of sand and silt compressed to fragile sedimentary rock, is part of the larger Idaho Group. It’s world famous as the source of the Hagerman fossils. A large piece of the Formation is preserved as the Hagerman Fossil Beds National Monument, which has produced thousands of fossils across more than 220 species. The most famous fossil is probably the Hagerman Horse, a Pliocene era, zebra-sized ancestor of the modern horse. But the fossil beds also contain everything from mastodon fossils to sabertooths (saberteeth?) to shrews.

The Hagerman Formation is a small element of the deep layers mud, sand and gravel deposited by the series of ancient lakes that are collectively known as Lake Idaho. At some of Lake Idaho’s various peak levels, it extended from 60 miles west of Boise east to the Twin Falls area. Boise itself was Lake Idaho lakebed, hundreds of feet…