Rock Elm Disturbance
Rock Elm Disturbance is an impact crater in Pierce County, Wisconsin, about 40 kilometers (25 miles) southwest of Menomonie. It’s named for the nearby town of Rock Elm. The crater is about 6 kilometers (3.7 miles) wide and dates to the Middle Ordovician period, roughly 430–455 million years ago, based on fossils in the crater fill.
The meteor that created it is estimated to have been about 170 meters across, with a mass of about 9 billion kilograms and an impact speed of around 30 kilometers per second.
Rock Elm may be part of a group of Middle Ordovician meteor events that hit roughly at the same time, around 469 million years ago, including Decorah crater in Iowa, the Slate Islands crater in Lake Superior, and Ames crater in Oklahoma.
At the center is a raised area about 0.8 kilometers wide and 2.4 kilometers long, showing the impact pushed up rocks such as breccia and the Mount Simon Sandstone beneath the surface.
To the crater’s south edge lies Blue Rock, an exposed faulted portion of Prairie du Chien sandstone that can be seen at Nugget Lake County Park.
Researchers from the University of Puerto Rico found a rare high‑pressure mineral called reidite at the crater’s center. Reidite forms under the intense heat and pressure of a meteor impact and is the oldest known example of this mineral. Other sites with reidite include Xiuyan crater in China, the Chesapeake Bay impact crater in Virginia, and Nordlinger Ries in Germany.
This page was last edited on 3 February 2026, at 14:37 (CET).