Geologic Formation and Glaciation of the Great Lakes

Overview of the Great Lakes Region

The Great Lakes describes a group of five freshwater lakes located in central North America between the United States and Canada, comprising Lakes Huron, Ontario, Michigan, Erie, and Superior. The Great Lakes watershed covers about 765,990 km², and the area is home to approximately one-tenth of the population of the United States and one-quarter of the population of Canada. The Great Lakes watershed includes some or all of eight U.S. states as well as a Canadian province, and the five lakes taken together represent the largest unfrozen freshwater body on Earth (Larson & Schaetzl, 2001).

The area is rich in natural resources. Oil and natural gas have been produced from subsurface formations in Michigan, Indiana, Ohio, and southwest Ontario. Since the late 1800s, nearly 2 billion barrels of oil and 10 trillion cubic feet of natural gas have been produced. Underground mines near Detroit have produced large amounts of rock salt created by Silurian-age evaporite deposits. Large quantities of bromine, potash, chloride, and sodium have also been mined from these layers, with limestone, gypsum, and dolomite mined from surface quarries in the outcrop areas. Clay for ceramics and bricks, along with sand and gravel for construction, are mined from surface-level glacial deposits (Gillespie, Harrison, & Grammer, 2010).

Precambrian and Paleozoic Foundations

The Great Lakes basin is a relatively young ecosystem that formed during the last 10,000 years (EPA, 2008). However, a number of tectonic events shaped the Great Lakes region long before that, beginning with the assembly of the first pieces of continental crust in North America from roughly 3.5 to 2.6 billion years ago. This assembly acted as the nucleus for further continental development. The area continued to undergo crustal collisions and rifting until the Mid-Continental Rift left the final scar on the Great Lakes region when another rifting sequence began to tear apart the continent approximately one billion years ago (Davis, 1998).

The foundation for the present Great Lakes basin was established during the Precambrian Era about 3 billion years ago. During this period, which occupies about five-sixths of all geological time, there was a high level of volcanic activity and immense crustal stresses that formed massive mountain systems. Early sedimentary and volcanic rocks were shaped and heated into complex structures, which were later eroded. Today these formations appear as gently rolling hills and small mountain remnants of the Canadian Shield, which forms the northern and northwestern areas of the Great Lakes basin. Granitic rocks of the Shield continue southward underneath the Paleozoic sedimentary rocks, forming the lower structure of the southern and eastern sections of the basin (EPA, 2008).

The Paleozoic Era brought repeated flooding to central North America by marine seas inhabited by numerous life forms, including corals, crinoids, brachiopods, and mollusks. These seas deposited lime silts, clays, sand, and salts, which consolidated into limestone, shales, halite, sandstone, and gypsum (EPA, 2008).

Pleistocene Glaciation and Basin Formation

Throughout the Pleistocene Epoch, continental glaciers repeatedly advanced southward over the Great Lakes region. The first glacier began its advance more than a million years ago. As they moved forward, the glaciers — which were up to 2,000 meters thick — scoured the earth's surface, leveling hills and permanently altering the previous ecosystem. Valleys created by the river systems of the preceding era were deepened to form the basins for the Great Lakes. After thousands of years, the climate began to warm, which melted and slowly shrank the glaciers. This era was followed by an interglacial period during which vegetation and wildlife returned. The entire cycle then repeated several times (EPA, 2008).

Before the glaciations of the Quaternary Period, the Great Lakes watershed experienced long-term sub-aerial erosion. Little evidence remains from this period, but it includes fragments of former bedrock valley systems that formed on the preglacial bedrock landscape, the best known of which is the Laurentian drainage system. Quaternary glaciers are believed to have removed or buried saprolite and karst (Larson & Schaetzl, 2001).

The Huron, Ontario, Michigan, Erie, and Superior basins originated primarily due to channeling of ice flow along major bedrock valley systems that existed before glaciation. The basins also owe their origins to increased glacial scouring and erosion in areas of relatively weak bedrock (Larson & Schaetzl, 2001).

It is believed that the ancestral Great Lakes were shallower than the current Great Lakes and that they were significantly deepened by glacial scour, which varied considerably between basins. The Superior basin, for example, is the deepest of the five basins, with a floor approximately 213 meters below sea level — or more than 397 meters below the basin's outlet at Sault Ste. Marie. By contrast, the Erie basin, the shallowest, has a floor approximately 110 meters above sea level, or 64 meters below its outlet near Niagara Falls. The floors of the Superior, Huron, and northern part of the Michigan basins are mostly irregular and uneven because of resistant bedrock layers, whereas the floors of the Ontario, Erie, and southern part of the Michigan basins are relatively smooth due to the softer underlying bedrock (Larson & Schaetzl, 2001).

A partial record of the most recent glaciation has been well preserved in the Great Lakes watershed. For preceding glaciations, however, the stratigraphic record is incomplete. This gap occurred primarily because most of the sedimentary record from earlier glaciations has either been completely eroded away by subsequent glaciations or is buried too deeply to be easily studied. South and west of the Great Lakes watershed, the record of earlier glaciations is better preserved because glacial erosion was less severe (Larson & Schaetzl, 2001).