Keweenaw Fault
The fault is more than 100 miles (160 kilometers) long and extends from the tip of the Keweenaw Peninsula in the northeast to a termination near the Michigan-Wisconsin border in the southeast. The fault is most likely younger than the Jacobsville Formation and the Devonian Period.
A seismic event in 1906 claimed to be an earthquake has been attributed to a rock burst, as the area has been significantly mined.
The Natural Wall
![](http://upload.wikimedia.org/wikipedia/commons/thumb/0/01/Pumpellyite-%28Mg%29-Quartz-21371.jpg/220px-Pumpellyite-%28Mg%29-Quartz-21371.jpg)
One significant feature along the Keweenaw Fault is known as the Natural Wall. The Wall is a near-vertical slope of the Jacobsville Sandstone which dips as steeply 85°. The steep dip of the Jacobsville Sandstone is the result of it being folded due to motion on the fault.
Interstate State Park at St. Croix Falls, Wisconsin is an example of the Keweenaw Fault. On its trails visitors can find Tholeiitic basalts.
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Tholeiitic basalts - Clam Falls Volcanics, Mesoproterozoic
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Tholeiitic basalts - Clam Falls Volcanics, Mesoproterozoic