Ancient Peninsula History

This would be the Niagara Escarpment - a very tenacious formation of dolomite limestone that extends all the way from Buffalo, New York through the Fox Valley.  This dolostone composition of the Door Peninsula explains why the peninsula exists in the first place.  Multiple glaciations were unsuccessful in scouring it from the face of the earth. 

About 10,000 years ago when the last ice age visited Wisconsin my part of the world was buried beneath an ice sheet more than a mile thick. The earth's crust is still rebounding from the weight of all of that ice. 

Photo - Wisconsin DNR

Looking at this map you can see how the escarpment transects the peninsula. Visualize the Lake Michigan lobe of the glacier extending south just to the right of the line and the Green Bay lobe of the glacier extending south immediately to the left of the line.  This saucer-shaped geological feature formed from the basin of an ancient inland sea during the Ordovician and Silurian eras some 445 to 420 million years ago.  

Over a period of eons layers of limestone, sandstone and shale were laid down.  The western edge of the escarpment in Wisconsin curves in a semi-circular ridge northeast from Horicon Marsh, toward the eastern edge of Lake Winnebago and the western shore of Door County.  It arcs around Lake Huron, then south through Ontario and ends at Niagara Falls, spanning a total of 650 miles. Some of its edge is underwater or covered by glacial deposits, but much is exposed as cliff outcroppings as high as 200 feet in places.  Geologically-speaking these are old rocks. 

They also happen to be home to some exceedingly-old trees.  Because growing on these cliffs are some of the slowest-growing trees on the planet.  Door County is home to an ancient, old-growth forest.  

Photo - Wisconsin DNR




Last fall, some guys used an incremental borer to extract cores from four of the white cedar trees growing along the edge of the Niagara Escarpment at The Clearing in Ellison Bay. Carefully counting tree rings using a hand lens they discovered that the oldest tree had 397 growth rings.  These slow-growing trees can be more than 300 years of age and have a trunk diameter the size of a 50-cent coin.  The world's oldest red cedar is located on the bluff north of Greenleaf in Brown County.  It is approximately 1,290 years-old and happens to be the only known tree more than 1,000 years of age in Wisconsin.   A 507-year-old white cedar is located on Sven's Bluff at Peninsula State Park and another ancient specimen that is 616 years of age can be found at Fish Creek south of the park. 

It is noteworthy that the peninsula happens to be home to this old stuff.  It sort of makes our short human lifetime here on this earth seem all the more inconsequential and small.