Geology of Door County


Geology of Door County
As you think of Wisconsin and its geologic history you may start to get bored, fall asleep, and try to argue that there really is nothing significant in Wisconsin’s timeline. However, Wisconsin, geologically has been through more than a war torn hero has.
Roughly 425 million years ago Wisconsin was much different than it was today. Instead of a flat and what some might call plain landscape, was a shallow sea that was thriving with life which geologists call the Silurian period. As glaciers advanced and retreated the sea levels moved accordingly. This caused a rise and fall in deposits of sediment throughout much of modern day Wisconsin and Michigan. Over time and layer after layer, sediment eventually compressed creating limestone. As compression increases so does temperature which over time eventually caused chemical change to the limestone creating dolomite. These dolomite formations that were once ancient sea bottoms are now what we see as rock outcrops as we drive up to Door County.

Downwellling of Michigan
Image Source: Ezra Zeitler, Door Peninsula Powerpoint
Image Source: http://www.aquarius.geomar.de/omc/make_map.html

 
Due to a process called down-warping that may have occurred from the presence of a subduction zone after the Silurian period, the Michigan Basin was forced into the shape of a bowl (below). As softer underlying sandstone layers were eroded away the stronger dolomite eventually gave way creating large cliffs that we see throughout the Door Peninsula. This layer of dolomite that was pushed into a bowl shape beneath Michigan spans an incredible distance. With its western edge in the Door Peninsula the bowl, which is officially referred to as the Niagara Escarpment, reaches all the way under Michigan, across Ontario, and New York where you can see being eroded at Niagara Falls today.
Image Source:
http://wisconsingeologicalsurvey.org/ice_age.htm
 
This is only the beginning of Door County’s geologic history. During the Pleistocene, geologically known for the Laurentide Ice Sheet, this massive glacier’s advancing and retreating has had dramatic effects on the Door Peninsula. Looking more specifically at the glacier, the Lake Michigan Lobe (left) has deposited erratic’s and glacial till that can be over a hundred feet thick in places. The resistant dolomite escarpment had its influence on the glacier as well making a large divide between Green Bay lobe and the Michigan Lobe of the Laurentide Ice Sheet. If you look at the map to the left, you can see the eastern Michigan Lobe has created the Lake Michigan basin we still see today.
 
 
 

 
Results of the Geologic History
Photo Source: Zac Hilgendorf
Glaciers have scraped almost all of the topsoil that in most regions would filter rain runoff. As the water percolates down to the dolomitic limestone it has no problem making its way through to the water table. This is because dolomite tends to naturally fracture both horizontally and vertically. In Wisconsin stress fractures in the bedrock are exacerbated by the expansion of ice located in cracks in the winter. On top of the naturally fracturing dolomite, limestone in solution has a tendency to dissolve, creating larger cracks and in some cases caves. “The county has some of the longest and deepest caves in Wisconsin. Horseshoe Bay Cave is more than 1,800 feet long, with passages four to thirty feet wide, and Paradise Pit is more than 1,600 feet long.” (Fraser, 2008)

With the layers above the water table extremely permissive to runoff Door County has had issues with the quality of their water. In some cases the pesticides being sprayed on the many apple and cherry orchards have leaked directly into the water table that many homes and businesses have their wells tapped in to.
Photo Source: Zac Wolmendorf
As we drove around the Door Peninsula we saw on almost every property the presence of septic mounds. These mounds vary in size but have a horizontal exaggeration and appear only about 5-feet high. Layered with sand, gravel, and other filterable materials a pump moves the properties waste to the top of the mound allowing for further filtration. Although these methods are rather efficient they are also outdated and newer even more efficient methods have taken their place. Trying to manage sewage doesn’t come without a cost and is also one of the largest burdens of living in the Door Peninsula.






Sources Cited:
Hart, John Fraser. 2008. My Kind of County: Door County, Wisconsin. Chicago: Columbia College’s Center for American Places Press

http://jove.geol.niu.edu/faculty/fischer/429_info/429trips/door_county/geolhist.htm