The muddy brown hills and rolling farmland here look like others in Wisconsin. Tall grasses, cornfields and a bubbling brook yield to rocky outcroppings and rows of trees.
But scientists years ago saw something different about those rocks and concluded an ancient catastrophic event occurred here, although what type of calamity remained a mystery.
They believe they have finally solved the puzzle: A 650- to 700-foot (200- to 210-meter) meteorite crashed into the earth at a speed of up to 67,500 mph (108,000 kilometers per hour).
The impact 450 million years ago dislodged rocks and created a massive hole in a 4-mile-diameter (6.4-kilometer-diameter) area called Rock Elm about 70 miles (110 kilometers) east of Minneapolis, three scientists said in an article published in the Geological Society of America Bulletin.
How the hole faded away
Over time, shale, dirt and sediment filled the hole to make the impact site virtually indistinguishable from the surrounding land. A shallow sea covering Wisconsin at the time of the impact likely blunted the meteorite’s effect.
The report said the impact at Rock Elm released more than 1,000 megatons of explosive energy, lifted the earth at the center more than 1,650 feet (500 meters) and sent shock waves through the rocks, crushing them.
“They were at ground zero, so they got the brunt of it,” said William S. Cordua, of the University of Wisconsin-River Falls, and one of the paper’s authors.
The confirmation of what happened here millions of years ago is significant to geologists seeking to trace geological patterns, said Don Yeomans, an astronomer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.
Although they’re not spectacular-looking, to Cordua and other scientists the rocks here have always appeared different than those just a few miles away. They’re tipped at an angle in many places, reflecting the damage inflicted millions of years ago.
Worldwide, there are only about 200 such impact formations, and only a couple of dozen in the United States. They are believed to have occurred only every few hundred thousand years.
The first modern indication of anything wrong here came in 1942, when a UW-Madison graduate student spotted the differences in soil and quartz and mapped out the area for more study.
“Mostly after its discovery it was pretty well-ignored,” said Bevan M. French, a former NASA geologist who is a research collaborator at the Smithsonian Institution. Even so, the area has been known among amateur geologists and farmers as an anomaly.
Since the 1980s, Cordua has trudged through grassy fields and muddy bogs looking for answers about Rock Elm. He started writing about the formation in 1985, and although he suspected it was formed by a meteorite, he couldn’t prove it.
“What I’ve been trying to do is hope that people who study more of these things would get interested in it. And that finally happened,” Cordua said.