A variety of rocks are exposed in the Duluth area. They range
in age from the 1.8 to 1.9 billion old Thomson
Formation to the 800 m.y. old sedimentary rocks of the Fond du Lac Formation. Volcanic
and sedimentary rocks of the North Shore Volcanic Group were formed
from the Mid-Continental Rift system about 1.1 billion years ago.
The main groups of rocks in Northern Minnesota include:
1. The Thomson Formation, which is exposed at Thomson Dam in Carleton
County, Minnesota
2) North Shore Volcanic Group overlays the Thomson Formation and
is exposed from Duluth at Ely's Peak, north to Grand Portage.
3) Duluth Complex
intrudes the North Shore Volcanic Group and is exposed from Duluth
at Bardon Peak north to Grand Marais and as far west as Babbitt
forming a crescent shaped body.
4) Sedimentary basin infill of the Fond du Lac formation exposed
at Fond du Lac in the vicinity of Mission Creek.
The Keweenawan (upper Precambrian) lavas in the Superior region were deposited at different times in overlapping accumulation. There are 100's of lava flows. The North Shore Volcanic Group is 6000-9000 meters thick (Picture 3). The lowest (oldest) basalts are exposed at two ends. One is near Duluth and the other is near Grand Portage (see picture 2). The basalts that are exposed progressively get higher (younger) from these ends toward the midpoint which is near Tofte. Sometimes the general trend of this is interrupted locally by instrusions. Lavas are intruded by a great variety of intrusive rocks. An example would be the Beaver Bay Complex, North of Silver Bay. Also you could have interflow sedimentary rocks (mostly sandstone) that occur between lava flows. These were deposited during times of non-volcanism. Interflow sedimentary rocks are lenticular in shape and they fill crevices between and within the lava.
Basaltic flows are most common in this area. All of the basalt flows are dipping toward Lake Superior.