Rock Elm
Key facts
- Country
- United States
- Diameter
- 6 km
- Age
- about 430 to 455 million years
- Location
- 44.72° N, 92.23° W
A subtle round basin about 6 km across in the farmland of Pierce County, Wisconsin. Disturbed sandstone is exposed at Blue Rock, in Nugget Lake County Park.
Rock Elm is an eroded impact structure about 6 km across in Pierce County, western Wisconsin, in the United States. It is best known for a remarkable mineral found at its centre.
What it hit. The impactor struck a sequence of Cambrian sandstones and Ordovician dolomites and sandstones on the floor of a shallow sea. The rebound of the crater floor lifted Cambrian sandstone, normally buried deep below, into a central uplift about 2 km across. The surrounding rocks were broken and faulted, and the crater was later partly filled with younger Ordovician sediments.
Proof of impact. Rock Elm contains shocked quartz, grains crossed by microscopic planes produced by shock, and impact breccia. Geologists have also found grains of reidite, a rare, extremely dense form of the mineral zircon that forms only at very high pressures. The reidite at Rock Elm is among the oldest known on Earth.
Age. Fossils in the sediments that filled the crater show that it formed in the Middle Ordovician, roughly 430 to 455 million years ago.
Why it matters. Rock Elm is one of several Ordovician craters in the American Midwest that formed during a time when the Earth was struck unusually often. Its reidite shows how shock minerals can survive for hundreds of millions of years. The impactor is unknown.
Visiting. Nugget Lake County Park has outcrops of faulted sandstone at its southern edge. Do not hammer or collect in the park.
Did you know?
Rock Elm contains reidite, a rare high-pressure form of zircon.
Fossils in the crater fill date it to the Ordovician.
A central uplift brought deeply buried sandstone close to the surface.