Geology of Minnesota
Split Rock region
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 father north. This mass, the Duluth Gabbro, is now extensively exposed in the Duluth area.
The lighthouse cliff, standing 100 feet high, is a fine-grained phase of gabbro named Beaver Bay Diabase, the massive, resistant character of which makes the shore of this region particularly bold and rocky.
At the base of the lighthouse on top of the cliff is another kind of rock, light green in 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.
Erected by the Geological Society of Minnesota and the Department of Highways, State of Minnesota, aided by a grant from the Louis W. and Maud Hill Family Foundation. 1955
(Apparently was supposed to be replaced by #233 in Minnesota History Along the Highways in 2003, but never was.)