Monday, February 27, 2012

Minnesota Geology Monday - High Falls of the Pigeon River

The Pigeon River flows along the United States and Canadian border and is home to the highest waterfall in the state of Minnesota.  The High Falls of the Pigeon River is a waterfall of approximately 120 feet in height.  The Pigeon Rivers source is very near the Laurentian Divide, the area of high ground that separates water from flowing to Hudson Bay to water flowing to Lake Superior.  

Downstream from the waterfall, the Pigeon River has cut a fairly deep gorge through the Rove Formation.  The Rove Formation is an 1,850 million year old sedimentary formation consisting of layers of shale, sandstone and siltstone deposited in an ancient sea.



The sediments in the Rove Formation were deposited horizontally in the Animikie Basin.  The source of the Rove sediments are Archean rocks found to the north.  





Above the Rove Formation are the 1,110 to 1,105 million year old Logan intrusions, a large mafic intrusion that consists of numerous vertical dikes and horizontal sills in the area.  At the High Falls, the Rove Formation is capped by a diabase sill that is resistant to erosion.  Since the diabase is resistant to erosion, Pigeon River flows over the diabase and erodes the much softer sedimentary rocks of the Rove Formation.







Downstream of the waterfall are many diabase boulders that have been eroded out of the sill and transported downstream.



The High Falls are easily accessible via trails from the visitor center and rest area at the Grand Portage State Park (http://www.dnr.state.mn.us/state_parks/grand_portage/index.html).  The trail to the falls is approximately 1/2 mile in length and is paved.  The last length of trail is a raised wooden walkway.  There are several overlooks at the end of the trail giving visitors different vantage points to the waterfall.