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.
TO LOG THIS CACHE:
1) Assuming your standing at the same elevation as
the base of the lighthouse, measure your elevation and then
determine what elevation the lake would be?
To log this cache E-mail me
HERE.
Sources:
- 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.