Local Asteroid Impact Site
Pierce County, WI
Just North of Nugget Lake county park near Plum City there exists evidence for an ancient
asteroid impact crater. Geographically it looks like a circular indentation that is about 4 miles in
diameter. This crater was first discovered by Bill Cordua - a geology professor from UW-River
Falls. [source: Powers, P., 2002, UW-RF student studies asteroid hit, Leader-Telegram, August
17, p. 3B] Christopher Peters, a student at UW-River Falls, spent the summer of 2002 sifting
through core drillings looking for fossils that would provide evidence of the geologic date of the
impact. It was reported that the fossils indicated that the asteroid hit about 450 million years
ago.
A thorough, scientific examination of the impact site is the following reference:
French, B.M.; Cordua, W.S.; Plescia, J.B.; 2004, The Rock Elm meteorite impact
structure, Wisconsin: Geology and shock-metamorphic effects in quartz,
Geological Society of America Bulletin, January/February
Impact Data and Comparisons With Other Craters
Rock Elm Meteorite Impact
400 million years ago
Area was a shallow sea
Meteorite - about 170 meters in diameter, mass of 9x109 kg, impact velocity was about 30
km/sec, energy released in collision was 4x1018 J (or about the energy equivalent in 63,000
Hiroshima size bombs)
Crater - Difficult to spot surface features, has been eroded, deposited upon and is old. Current
size is about 6.5 km is diameter and about 400 meters deep. Original transient crater was
believed to be 4 km in diameter and 1.3 km deep.
Location - About 40 km Southwest of Menomonie, WI.
Dr. Scott's newspaper article about the Rock Elm Meteorite Impact.
(Published in the Dunn County News on July 11, 2004.)
Barringer Meteor Crater
50,000 years ago (much younger than the Rock Elm crater)
Meteorite - about 50 meters in diameter, impact velocity was about 11 km/sec
Crater - Easy to spot surface features, 1.2 km in diameter, about 213 meters deep
Location - Northern Arizona
Chicxulub Impact (caused mass extinction, killed off the dinosaurs) also called the K-T
Asteroid Impact
65 million years ago
Meteorite (or Asteroid) - about 10,000 meters in diameter (10 km), impact velocity of about
28 km/sec, mass of about 1.3x1016 kg, energy released in collision was about 5x1024 J (or about
the energy equivalent in 80 billion Hiroshima size bombs)
Crater - Difficult to spot surface features, 180 km in diameter.
Location - Yucatan Peninsula, Mexico
[The following is on a placard describing the geology at Nugget Lake County Park, Pierce
County, WI. Reprinted and posted with permission from the author in July 2003.]
THE GEOLOGY OF THE ROCK ELM DISTURBANCE
by William S. Cordua, Professor of Geology
University of Wisconsin - River Falls
Nugget Lake Park sits on the boundary of one of the most geologically interesting areas in
western Wisconsin, the Rock Elm disturbance. The disturbance is a circular region
approximately 4 miles across. It contains anomalous rocks and structures thought by some to be
the filled-in scar of an ancient meteor impact. The southern boundary of the disturbance lies
within the park, north of Nugget Lake.
The rocks exposed around Nugget Lake itself show the normal features of rocks in western
Wisconsin. They are flat-lying sedimentary rocks. At lake level is an orangish sandstone called
the Jordan Formation. It is overlain by a gray bluff-forming series of dolostone, limestone and
sandstone called the Prairie du Chien Group. Both the Jordan Formation and Prairie du Chien
Group were deposited in a shallow sea that covered most of the midwest 480-500 million years
ago.
To the north, in the Rock Elm disturbance, this predictable geology abruptly changes. Some
bedrock is suddenly gone or dropped down. Other bedrock formations are strongly folded or
faulted. Older bedrock has been uplifted 100's of feet to the surface. Younger sedimentary rocks
unlike any found elsewhere in the region were deposited in a circular fault-bounded basin.
As one moves north through the disturbance one first crosses a fault and encounters a belt of
folded and faulted Prairie du Chien Group rocks. These are well exposed at Blue Rock within
the park. Other outcrops are found further north along both Rock Elm Creek and Plum Creek
within the park and along County Highway HH northwest of the park. These rocks in places
have been tilted nearly vertical. This whole belt has been dropped down relative to its usual
elevations in the region.
Along Hwy HH one crosses a major fault. This fault drops the Prairie du Chien Group rocks still
more. At the surface one now sees shales and sandstones that are not found elsewhere in western
Wisconsin. These rocks were deposited in a circular basin formed after the major folding and
faulting stopped. The shales, gray thin-bedded clay-rich rocks, are well-exposed in outcrops and
roadcuts stretching from the entrance of the park north passed the little town of Rock Elm. A
white to buff colored sandstone was deposited on top of the shales. This sandstone caps many of
the hills in the Rock Elm region.
In the center of the disturbance, about 1 1/2 miles north of the park, a low range of wooded hills
marks an area of uplifted bedrock that punches through the basin fill of shale and sandstone.
This central uplift exposes tilted beds of Mt. Simon sandstone, a formation usually in flat layers
buried 700 feet below the surface in this area.
The north edge of the area, north of Rock Elm, is a fault zone. This fault is circular and
completely encloses the disturbance. It is traceable back around to Nugget Lake Park, passing
through the park north of the boat launching area. But don't worry! These faults aren't active
and haven't moved in millions of years.
One idea for the formation of the disturbance is that it is an ancient meteor crater. The meteor
impact caused the folding and faulting and formed a crater that later filled with sediment.
Central uplifts can form in large meteor craters when the middle of craters, after being pushed
down so far, slowly rebounds, in the same way that the dents in your pillow disappear after you
get up in the morning.
A different idea for the disturbance is that it originates from an internal burst of pressurized gas
or water, channeled along very old faults. The gas burst would cause the folding, faulting and
uplift. It would also form an explosion crater which would later fill with sediment.
The Rock Elm region has been the site of scattered finds of gold and diamonds in the stream
sediments. Minding companies have explored the disturbance as recently as 1988. The gold is
present as very tiny particles called "flour gold". During gold placer mining in the 1880's at least
10 tiny diamonds were discovered. Most were very small, the largest being 2 kts. Although not
gem quality, they were pale blue to yellow in color. So far no local bedrock source for these
minerals has been found. Their connection, if any, to the odd bedrock geology remains, like so
much else about the disturbance, a mystery.
THE GEOLOGY OF BLUE ROCK
The rocks in Blue Rock are limestone, dolostone and sandstone that were deposited in a shallow
sea about 480 million years ago. Ripple marks and burrows from prehistoric soft-bodied animals
are evidence for this environment. Most bedrock in western Wisconsin is flat, but Blue Rock is
different! The rocks here are bent with dips up to 30 degrees into a down warp known as a
syncline. Bedrock further north along both Plum Creek and Rock Elm Creek are also bent,
sometimes standing nearly vertical.
Blue Rock is part of the Rock Elm disturbance, a circular area of anomalous bedrock and
structures about 4 miles in diameter. The older bedrock, such as Blue Rock, is extensively
folded and faulted within the disturbance. Some geologists think the Rock Elm disturbance is an
old meteor crater. Others think faulting or a burst of pressurized gas or water from the earth's
interior could have formed the disturbance.
Maps to the site: large scale, small scale (campground)
Nugget Lake County campground web site
(Small Scale, Nugget Lake campground trails with location of rock strata showing signs of an
ancient asteroid impact. Copied and posted with permission.)
Aerial Photos of the Area
Summer 2003, Trip to Nugget Lake County Park
Expedition to explore a rock outcropping that show signs of an ancient asteroid
impact.
Group of hikers beside the tilted outcropping of rock along Plum Creek.
I (Dr. Scott) am standing in the stream next to the tilted rock strata.
Another view of the rock strata at the underlook.
More of the tilted rock outcropping.