Monday, February 6, 2012

Minnesota Geology Monday - Magnetic Rock

Near the end of the Gunflint Trail, in northeastern Minnesota is a hiking trail that leads to Magnetic Rock.  The trail is approximately one mile in length and mostly uphill (at least on the hike in).  In the picture below, you can make out a small portion of the Gunflint Trail near the trailhead, the picture was taken about 1/4 mile into the hike.

Due to a forest fire in 2007, there are relatively few mature trees or vegetation along the trail.  This fire also did an excellent job of exposing bedrock outcrops throughout the area.  When looking north at several points along the trail, you see numerous exposures of the white, granitic Saganaga Tonalite.

The majority of the trail is on top of the 1,878 million year old Gunflint Iron Formation, which would have been deposited near the shoreline of an ancient sea.  The Gunflint Iron Formation consists of black to gray chert alternating with black iron-rich layers.  A goal this summer is to also acquire better photographs of the Gunflint Iron Formation, in particular the bedding planes, which is largely horizontal in the area.

Along the trail to Magnetic Rock, you are also able to view stromatolites within the Gunflint Iron Formation.  Though the day I traveled to the area a storm was coming in and I needed to complete the hike and set up camp, so I didn't have time to look for the stromatolites.  Maybe that can happen this summer.

Magnetic Rock is a large piece of the iron formation that has been turned vertical, probably by the southwestern trending Rainy lobe during the late Wisconsin glaciation.  Magnetic Rock is almost 25 feet from ground level to the top.  Due to it's height, the rock is visible for some distance along the trail before you actually reach it.

The bedding planes of the Gunflint Iron Formation is largely horizontal in the near vicinity of Magnetic Rock.  The bedding plane of Magnetic Rock is vertical, indicating that it has been moved from it's initial location.

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