Part 1 of 4-part series

By Jim Rosenbaum

Why study old rocks? Isn’t this the Wisconsin Marine Historical Society? Yes, but let’s consider why the Great Lakes formed where they did. It was not an accident.

Consider also that geological awareness gives you superpowers. Geologists think in four dimensions – our familiar 3D, plus time. Viewing a landscape, a geologist sees not static scenery but millions of years of unfolding change.

Bear in mind that “the Earth is not a geriatric case,” a favorite theme of a former geology professor of mine at Lawrence University. It is popularly believed that the Earth’s processes in its youth were active and hostile, with meteor impacts, volcanic eruptions, earthquakes, and enormous floods all more catastrophic than they are today, not to mention those roaming dangerous, giant animals. A corollary of this belief is that Earth is now conveniently settled down to be inhabited by humans, who require stability to advance civilization.

Don’t count on it.

Geology is destiny, to rephrase an old adage. The formation of the Great Lakes was set in motion about 1.1 billion years ago.

Beginnings of the Great Lakes

In this primer of the geologic beginnings of what we call Lake Michigan, let’s look first at the origin of the basement rock formations, upon which more recent sedimentary layers were deposited. The unruly behavior of the Earth during the formation of the basement rock involved landmasses colliding, splitting, shifting and sliding over each other. Later, oceans formed and disappeared, and uplands and mountains emerged and then eroded away — all leading to the bucolic landscape and comparative tranquility we see around the Great Lakes today.

The oldest rocks found on Earth date from 4.5 billion years ago. This is assumed to be the age of the Earth. At 1.1 billion years ago, geologic events began in our Upper Midwest, setting the stage for the eventual formation of the Great Lakes during the far more recent Pleistocene Epoch (Fig. 1).

Based on the rock record, a massive rift system occurred about that time, forming what resembled a subcontinental-sized horseshoe (Figs. 2, 3, 4). A rift system is a zone in which the motion of adjacent sections of the Earth’s crust diverge much like paired conveyor belts moving slowly in opposite directions. The crust overlying this divergence was split apart and replaced with new crust from cooled magma that rose from the molten mantle, still deep below our feet (Fig. 2).

The immense rift stalled. Movement did not continue long enough to open an ocean basin, but it did result in a system of large blocks of sunken crust between parallel faults. This structure formed an arc of depressions. The northwestern portion of the horseshoe-shaped arc, the best studied, cradles modern Lake Superior like a saucer (Fig. 3). Geophysical surveys indicate that the southeastern portion of the arc underlays lower State of Michigan (Fig.4). Other examples of stalled rift systems include the Great Lakes of East Africa (Tanganyika, Malawi and Albert) and the Lower Rhine Valley in Europe.

What does a rift system look like if it is not stalled? Like the Red Sea, which now separates Africa from the Arabian Peninsula. Eventually, continued rifting will create an entire ocean, as was the mechanism for opening the Atlantic. If the rift had not stalled in what is now the Upper Midwest, there might be an ocean at our back door.

North America was once closer to the equator

Now skip forward about 600 million years from the period of rifting to the first period of the Paleozoic Age, known as the Cambrian. The Cambrian began about 545 million years ago. That’s more than twice as old as the age of dinosaurs, during the Mesozoic Age, which began 250 million years ago. Paleomagnetic evidence from minerals deposited during the Cambrian and later periods suggest that our part of North America was then much closer to the equator, or even south of it. Warm salt water seas covered the much older igneous formations formed by the Mid-Continental Rift. Some paleo latitudes of ancestral North American are shown in (Fig. 5). The continent has slowly rotated counterclockwise, then traveled thousands of miles north.

A misconception is that the Gulf of Mexico extended to Wisconsin. However, that gulf did not exist in the Paleozoic. A large sea of salt water did indeed cover our area, but North and South America (Fig. 5) did not exist in their present forms and were not paired as they are now.

The Michigan Basin

This salt water embayment is known as the Michigan Basin. At times it was completely closed off from the sea. The Michigan Basin extended west-to-east from what is now central Wisconsin to Ohio, and north-south from Ontario to Indiana. The sedimentary rocks of the Michigan Basin (Fig. 6) are likely relics of far more extensive sedimentary layers, most of which have been removed by erosion.

Modern sedimentary basins include the Gulf of Mexico, the Mediterranean and Black Sea, and the Persian Gulf. In these basins, horizontal layers of sand, mud, and coral are still being deposited on the seafloor. Calcium carbonate can also precipitate out of seawater. Under the weight of younger layers, the older ones are gradually compacted and lithified, becoming rock.

In the rock-making process, sand becomes sandstone; mud becomes shale; coral, shells, and calcium carbonate form limestone. During the lithification of limestone, magnesium can partially substitute for calcium, producing dolomite, which is tougher than limestone. This chemical substitution is relevant to the future formation of Lake Michigan. Here are the differing properties of these sediments:

Sandstone, shale, and limestone differ in their resistance to erosion. Shale is the softest, and readily breaks up or even dissolves when exposed to rain and frost. It is uncommon to observe shale at the surface, as it readily breaks down into the soil. Dolomite is far more resistant to erosion.

Evolution of the Michigan Basin

The sediments of the Michigan Basin were originally deposited as horizontal layers, as are all sediments. However, these sedimentary rock layers now dip gently toward the center of the basin. For example, consider the crucial Niagara dolomite, which is one layer within the Silurian formations. The Niagara dolomite is found locally at the surface in active quarries in Lannon, Wis., west of Menomonee Falls. But in central Michigan, the same Niagara dolomite is found 7,000 to 8,000 feet below the surface (Figs. 7, 8).

Sedimentary layers continued to be deposited in the Michigan Basin from the Cambrian to at least the Permian Period, 250 million years later. Why did such a thick sequence accumulate? Relic faults that were part of the Mid-Continental Rift may have resulted in a weak zone in the basement crust. Geologists speculate that these faults were later reactivated by the massive weight of the overlying sediments of the Michigan Basin. This may have caused the gradual sinking of the basement floor, accounting for the increased thickness of the sedimentary formation toward the center of the basin. Another explanation offered is that downward buckling of the crust under the Michigan Basin resulted from stress at the periphery of the Appalachian mountains to the east, which also formed during the Paleozoic Age.

A “platter” underlies Lake Michigan, State of Michigan & Lake Huron

Another common misconception is that the Niagara Formation was deposited as a ridge, extending from southeast Wisconsin up through the Door Peninsula. Not true: The ridge, which in many places forms a cuesta (a one-sided butte), is only the outer rim or lip of a vast, shallow platter or disk-shaped rock formation (Figs. 7, 8). From Wisconsin, this formation descends east under Lake Michigan, then under the state of Michigan, and under Lakes Huron and Erie, eventually returning to the surface at Niagara Falls, for which the formation is named. But remember the Great Lakes did not exist during the Paleozoic Age. Their formation comes later.

Geomorphology, the study of landscapes

OK wise guy, you might ask, if the Niagara formation was not deposited as a necklace of hills, why does it form a ridge? Answer: Most hills and even mountain peaks are the result of differential rates of weathering. More resistant rocks, such as the Niagara dolomite, erode slowly, resulting in highlands. Areas underlain by softer rocks erode relatively easily, forming valleys.

Well known peaks such as Everest, the Matterhorn, or those in the North American Rockies were not the result of individual upheavals or thrusting of rock units. Initially, regional uplift was likely caused by the collision of plates of the Earth’s crust. However, following uplift, these broad plateaus were dissected by the erosive power of rivers and glaciers, leaving behind individual peaks.

Part 1: Is Earth a geriatric case?
Part 2: The rock of ages
Part 3: Wisconsin, a virtual glacial ghost town
Part 4: Post-glacial events
Food for thought, suggested field trips, and references

Other charts:

Photo at top of page:  Jim Rosenbaum, on a Lake Michigan beach at Klode Park, Whitefish Bay, Wis., holding large plastic disks to illustrate the rock strata beneath the State of Michigan, Lake Michigan, Lake Huron, the Bruce Peninsula and Georgian Bay, in Canada, and Door County and Green Bay.  Photo by Dan Patrinos

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Jim Rosenbaum holds a B.A. from Lawrence University and an M.S. from Stanford University, both degrees in geology. Born in Milwaukee, now living in Whitefish Bay, Jim’s long walks along Lake Michigan and formative years in Arizona and Colorado led to his interest in natural history. An avid sailor, he’s negotiated the coastal waters of Lake Michigan along Wisconsin’s shoreline for many years. An article he wrote on beach erosion was included in Focus on Environmental Geology (1976) and in Environmental Geology (1983), both edited by Ron Tank of Lawrence University and published by Oxford. In the 1980s, he migrated into the book business, from which he recently retired. He also holds a B.B.A. from the University of Wisconsin – Milwaukee. A member of the Wisconsin Marine Historical Society, he also belongs to the American Geophysical Union.