Excerpt from the Ashfall Fossil Beds (Orchard, NE) website:

About 12 million years ago, a volcano in southwest Idaho spread a blanket of ash over a very large area. One or two feet of this powdered glass covered the flat savannah-like grasslands of northeastern Nebraska.

Most of the animals that lived here survived the actual ashfall, but as they continued to graze on the ash covered grasses, their lungs began to fill up with the abrasive powder. Soon their lungs became severely damaged and they began to die.

The smaller animals died first (smaller lung capacities) and finally, after perhaps three to five weeks, the last of the rhinos perished.Their bodies were quickly covered by the blowing and drifting ash.

Undisturbed except by an occasional scavenging meat-eater, the skeletons of these animals are preserved in their death positions, complete with evidence of their last meals in their mouths and stomachs and their last steps preserved in the sandstone below.

http://ashfall.unl.edu/ashfallstory.html

The website explains that the ash came from a volcano in Idaho. The caldera is called the Bruneau-Jarbidge Eruptive Center. To see its proximity to Yellowstone, here's a page with a map.

http://www.usu.edu/geo/shervais/Research%2...

I estimate the as-the-crow-flies distance between this extinct volcano in Idaho and Ashfall Fossil Beds in Nebraska to be about 700 miles. I wonder how much farther significant amounts of the ash traveled, and in what directions. During the 1980 Mount St. Helens erpution, noticeable ashfall didnt extend much farther than Spokane, which is about 250 miles from Mount St. Helens. Ashfall in Spokane measured around two-tenths of an inch. *Ashfall covering the fossils at the Nebraska site measured up to ten feet.

It sounds as if the Yellowstone eruption is going to be the mother of all volcanic eruptions...well, a doozy anyway. Yikes. I think I'll move to New Mexico.

*Edit: At the Ashfall Fossil Beds website, I found this photo and accompanying info:



An aerial view of a portion of the Ashfall Fossil Beds site. This photo was taken during the Park's development and shows a series of 6-meter sampling grids layed out on the newly-exposed surface of the ash bed. The overburden (sand and sandstone) and the upper 3 to 5 feet of volcanic ash have been removed, leaving the fossils safely buried under another 5 feet of ash. The Rhino Barn (not yet built when this photo was taken) now covers the area at the bottom of the picture. Exploration has indicated that this is a very small portion of the actual bone-bed. Much work remains to be done here, and many more exciting discoveries will be made in the future, perhaps even while you are watching!