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.
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.
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| Downwellling of Michigan Image Source: Ezra Zeitler, Door Peninsula Powerpoint |
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| Image Source: http://www.aquarius.geomar.de/omc/make_map.html |
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| 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
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| 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.
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| 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
http://jove.geol.niu.edu/faculty/fischer/429_info/429trips/door_county/geolhist.htm
