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.

Minnesota Geology Monday - Saganaga Tonalite

Driving north out of Grand Marais (Cook County, extreme Northeast Minnesota) on the Gunflint Trail (County Road 12) leads us to numerous noteworthy locations, but todays post focuses on the Saganaga Tonalite.  In the late 1800s, as mines were being opened in the Gunflint Iron Formation near Gunflint Lake on the Canadian border, the trail was initially developed to lead from Lake Superior to the area.  The Gunflint Trail is now a paved 57 mile road from Grand Marais (on Lake Superior) to Saganaga Lake which is in the Bounary Waters Canoe Area (BWCA).  The Google Earth image below shows the relation or distance between Grand Marais and the Trail's End campground, part of the Superior National Forest.

Next June, my boys and I will be camping at the Trail's End campgound, shown in the lower left of this Google Earth image.  In the upper right of the image, outcrops of the light-colored, granitic Saganaga Tonalite can be seen.

Driving north on the Gunflint Trail, the first exposure of the Saganaga Tonalite that can be seen is at the Laurentian Divide scenic overlook pull out.  Water south of this point flows to Lake Superior and ultimately the Atlantic Ocean, water north of this point flows to the Hudson Bay.  Looking north over Gunflint Lake from the divide are numerous light-colored outcrops of Saganaga Tonalite.  These outcrops are on the Canadian side of the border in Ontario.

The Saganaga Tonalite is considered a quartz diorite due to it's mineral composition of quartz, plagioclase and horneblende.  Because it is lacking potassium feldspar it is not considered a true granite.  The Saganaga Tonalite is considered unique due to the large gray crystals of quartz.  The tonalite has a radiometric age of 2,689 million years.

Because a wind storm (with wind speeds in excess of 90 mph) in the summer of 1999, resulted in a large area of trees being flattened, numerous exposures of the Saganaga Tonalite have become accessible in the Trail's End campground and surrounding BWCA.  Fires, both controlled and wild, have also removed some of the vegetation allowing easy access to many outcrops.