The scenery of the region between Split Rock and Beaver Bay is the direct result of important events of early geologic time.
Lava flows which had accumulated in this region to a thickness of about 30,000 feet were lifted and arched by a huge mass of molten material rising through the earth’s crust farther north. This mass, the Duluth Gabbro, is now extensively exposed in the Duluth area.
The lighthouse cliff is a fine-grained phase of gabbro named Beaver Bay diabase. The massive, resistant character of which makes the shore of the region particularly bold and rocky.
At the base of the lighthouse on top of the cliff is another kind of rock, light green on color and coarse in texture, called anorthosite. It is this rock, anorthosite, which shows an excellent exposure in the highway cut at Silver Bay and stands up as prominent rounded hills from Split Rock to Carlton Peak near Tofte.
Geologists believe that the anorthosite blocks originated near the base of the Earth’s crust, 20 to 25 miles below the surface, and were broken off and carried up by the diabase magma during the great rifting even 1.1 billion years ago. Another great example of an anorthosite block is Day Hill which rises 250 feet above the lake.
Why is it called “Split Rock” ? (Waypoint #1 Explains)
Today, one common explanation comes from a large, split rock column located 1.5 miles up stream on the Split Rock River.
Another possible explanation comes from early fisherman, who thought that two high cliffs (a mile east of the river mouth) appeared to split apart as they are approached from the Lake.
The earliest use of “Split Rock” appeared on an original lake survey map of British officer, Henry W. Bayfield in 1825. The Split Rock River was also known as Gin-On-Wab-Iko-Zibi, an Ojibwe word meaning Eagle-Iron-River.
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- Display - The Geological Society of Minnesota and Department of Highways, State of Minnesota 1955.
- Geology on Display: Geology and Scenery of Minnesota’s North Shore State Parks by John C. Green. 1996. 70p.