The Giant's Range Granite and Laurentian Divide

The Laurentian Divide separates the watershed of the streams that flow north to the Arctic Ocean from the watershed of the streams that flow southeast throught the Great Lakes to the Atlantic Ocean. The divide is formed by a prominent array of hills known as the Giants Range. The name "Laurentian" is used because the granites forming the ridge are similar to, and were once thought to be related to, granites of the Laurentide Mountains in Quebec. Although this connection is no longer made the name has remained.

The Giants Range is made up mostly of several types of granite that formed several kilometers deep in the Earth's crust about 2.7 billion years ago. Uplift and erosion slowly brought the granites to the surface.  They have formed a highland throughtout time because they are resistant to erosion. In the road cuts near the parking lot, crisscrossing bodies of darker and lighter granite record several successive intrusions of molten rock. Because of the complexity of the rocks, this site is known as "Confusion Hill" to local geologists. 1

Wayside Rest Sign
The sign at the left is an easy iindication one has reached the Laurentian Divide. The divide is in a road cut about 4 miles north of Virginia on Highway 53. In the roadcut it is easy to see the granite part of the granite-greenstone terrane of northern Minnesota. The greenstone (Ely Greenstone) is a particular name for a chlorite shist metamorphic rock.

The granite is part of a batholith. A batholith is a large discordant rock formation formed underground as molten rock cut across layers of pre-existing rock and slowly cooled. Because it cooled slowly, insulated by the ground above it, larger crystals of granite rocks were formed instead of the fine grained crystals of rhyolite. The Giants Range goes from just north of Hibbing (The "Hill of Three Waters" is in the Hull-Rust Mine) to Babbit and rises from 200 to 400 feet above the surrounding area.
The intrusions ("wavy" parts) and inclusions (more angled and smaller parts) of the rocks combined with the metamorphic changes make the geologic time table difficult to determine. Radiometric determinations of the rocks place the time of formation about 2.6 billion years ago. About 1.8 billion years ago the Biwabik Iron Formation was placed on top of these older rocks and then about 11,000 years ago glaciers left the landforms we see today.

The lighter colored rock is called tonalite and the darker is diorite. Other dark more angular patches are metamorphosed volcanic or sedimentary rock that the grantitic magma intruded. Tonalite2 is a rock intermediate between granite and diorite (a different website than just above).  For most people the differentation between tonalite and granite could only be made by chemical analysis of the minerals.
Giants Granite1
Giants Granite2
Giants Granite3

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1. Geology Plaque placed by the Minnesota Geological Society, 1998

2.  The 1911 Edition Encyclopedia