Extraterrestrial Impact Craters in Wisconsin

Incoming asteroid!   This nightmare scenario has inspired plenty of bad movies, but the reality of these events is supported by lots of good science. It has happened many times before:  Indeed, Earth is whacked by space rocks from time to time, and the big chunks have significantly impacted the planet while making an abrupt stop here.

The Earth Impact Database lists 190 confirmed impact structures on Earth.  Two of these are located in Wisconsin, and of course the Wisconsin Geology GIS app has them mapped! Read on to find out more about each, and where to look for the geological version of ET.

The "Rock Elm Disturbance"
An asteroid perhaps 600 feet in diameter smacked into what is now eastern Pierce County Wisconsin somewhere between 450 and 500 million years ago, piercing through hundreds of feet of bedrock and leaving a crater nearly 4 miles in diameter in its wake.  The crater has been nearly erased over time by geological deposition and erosion. It is still faintly discernible on the Wisconsin Geology GIS map:  Click here to zoom to the location of the crater,  then study the "hillshade" effect closely.  See it?  Look in the area that is today between County Road CC on the west, County Road HH on the south, County Road S on the east, and 570th Ave on the north. Within a roughly circular area, the drainage pattern is different within the crater than it is in the surrounding area. Drainage and erosion appear to have been influenced by the underlying crater. Some geologists believe this impact was just one of many that rocked the Earth during the Middle Ordovician Period. Several other possible impact craters of perhaps the same age are being studied in Oklahoma, Iowa, and in Lake Superior.

On the GIS map, a red X marks the approximate center of the impact structure (the central uplift) and there is an orange outcrop identified on the south rim where today the Rock Elm and Plum Creeks exit the impacted area.

Cross section of the rock strata in the area of the Rock Elm Disturbance.
Brown, B.A. WGNHS Survey Map 88-7

Topographic relief map of the Rock Elm impact site. Click to enlarge.

The Rock Elm Disturbance was first recognized in the 1940s by geological mapping of the bedrock of western Wisconsin.  Right in the midst of generally horizontal sedimentary strata, something had clearly 'disturbed' the rock layers in this area (hence the name) although at the time it was unclear to stratigraphers and other geologists just what exactly caused this unique geology.

Over the years, the existence of impact structures became more widely recognized in science.  Stratigraphers were not the only geologists involved in identifying impacts like this one.  Minerologists also got into the game.  For example, a mineral called reidite was found in the Rock Elm crater.  Reidite forms under very high pressure and has been found in nature in only 4 other locations on Earth, all within other known impact structures.   Gold, which is also sometimes associated with impact events, has been found in the beds of the streams flowing south out of the Rock Elm crater.  Gold, you say?  If I just distracted you with shiny golden objects, check out a blog article about gold panning in this area---yes, there is public land in and near the impact site.  Here is a very good video by Dr. William Cordua, a geologist who has studied this impact structure in detail. Additional references are linked below, and you can bring up more by doing a Google search on the "Rock Elm Disturbance".  The story is shaping up to be better than any Hollywood movie.
Update:  See also this interactive GIS map:   https://geologyinsight.maps.arcgis.com/apps/webappviewer/index.html?id=09ed828f4c8340419f4c93eb2aa447bd

Glover Bluff
Another impact structure is located in northwestern Marquette County Wisconsin (click here to view the location on the Wisconsin Geology GIS map).  This crater is between 1 and 5 miles in diameter. It is more difficult to recognize on the surface because it is mostly buried under the thick moraines of Wisconsinan glaciation. Quarrying activity has uncovered enough evidence of an impact to make this a 'confirmed' crater.  Fractured bedrock (including shatter cones) and impact breccia surround a central uplift, all telltale signs of a significant extraterrestrial impact. If you visit the area, please keep in mind this area is mostly private land--don't trespass!  Bruce Willis didn't save this area from the asteroid, and he won't save you from the wrath of the locals, either.

Meteoric fall
Smaller chunks--meteors--punch through the atmosphere and land without forming a crater. Once firmly planted on Earth, the meteors become meteorites. Lots of folks run around looking for meteorites--they are valuable and can be sold for more than a few hundred quids.  When Earth-born rocks are mistaken for meteorites, geologists have a solemn duty to inform the stone bearer of the meteorwrong.  There have been some famous meteorite discoveries in Wisconsin, however.  Near West Bend in Washington County, several metallic meteorites have been found--the largest pieces weighed over 400 pounds!  These so-called "Trenton Meteorites" are rather famous among the meteorite hunters.  A meteorite found near Algoma in 1887 is today located in the University of Wisconsin Geology Museum.

Go to the Wisconsin Geology GIS app for more.  Remember you can search the map:  Search terms like "meteor" or "crater" to bring you to known locations of such objects in Wisconsin.

References:

• Earth Impact Database (website), Planetary and Space Science Centre
  University of New Brunswick
• The Rock Elm Disturbance (web page),  Wisconsin Geological & Natural History Survey
• Rare Mineral Found in a Wisconsin Crater, Astrobiology Magazine, 2015
• Trenton Meteorites, Mindat.org