¶ … region's geologic formation from the Precambrian Era forward, as well as the glaciation processes that were primarily responsible for carving out and meltwater filling of the Great Lakes.
The Great Lakes describes a group of five freshwater lakes located in central North America between the U.S. And Canada, and includes Lakes Huron, Ontario, Michigan, Erie and Superior. The Great Lakes watershed covers about 765,990 km2, with the area being home to approximately one-tenth of the population of the U.S. 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 contains the five Great lakes, which 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).
The Great Lakes basin is a relatively young ecosystem which formed during the last 10,000 years (EPA, 2008). A number of tectonic events shaped the Great Lakes region, 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 rip apart the continent a billion years ago (Davis, 1998).
The foundation for the present Great Lakes basin was set 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 stresses which 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 which were inhabited by numerous life forms, including corals, crinoids, brachiopods and mollusks. The seas deposited lime silts, clays, sand and salts, which consolidated into limestone, shales, halite, sandstone and gypsum (EPA, 2008).
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 inched forward, the glaciers, which were up to 2,000 meters thick, scoured the earth's surface, leveling hills and forever altering the previous ecosystem. Valleys created by the river systems of the previous era were deepened to form the basins for the Great Lakes. After thousands of years the climate began to warm, which in turn melted and slowly shrank the glacier. This era was followed by an interglacial period during which vegetation and wildlife returned. The entire cycle then repeated several times (EPA, 2008).
Before 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...
The basins also owed 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 213m below sea level, or more than 397m below the basin's outlet at Saulte Ste. Marie. Compare this to the Erie basin, the shallowest, with a floor approximately 110m above sea level or 64m below its outlet near Niagara Falls. Also, the floors of the Superior, Huron and northern part of the Michigan basins are mostly irregular and uneven because of resistant bedrock layers. By contrast, the floors of the Ontario, Erie and southern part of the Michigan basins are relatively smooth due to the "softness" of the underlying bedrock (Larson & Schaetzl, 2001).
A partial record of the most recent glaciation has been well preserved in the Great Lakes watershed, but for preceding glaciations, however, the stratigraphic record is incomplete. This gap occurred primarily because, for glaciations that preceded the last, most of the sedimentary record has either been completely eroded away by subsequent glaciations, or is buried too deeply to be easily studied. However, 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).
During the Wisconsin glaciation the Laurentide ice sheet, which was centered in northern and eastern Canada, expanded southward, eventually covering the entire Great Lakes watershed. After having reached its maximum extent and fluctuating near that position for several thousand years, the southern margin of the Laurentide ice sheet began its retreat northward. The retreat was interrupted by several major readvances, with the final one occurring around 10 ka (Larson & Schaetzl, 2001).
The last glacier began its retreat approximately 14,000 years ago. As the glaciers melted, the resulting water, known as meltwater, filled the massive holes left by the glaciers. As the ice retreated, the St. Lawrence River Valley evolved as the outlet to the Atlantic Ocean, and lake levels eventually dropped to current levels (Great Lakes Information Network, 2004).
The record of glacial and postglacial lakes in the Great lakes watershed includes bars, lake floor sediments, abandoned spillways and channels, along with wave-cut cliffs, beach ridges, and deltas indicating abandoned shorelines. The spillways in particular are significant because they controlled the level of lakes and were periodically subject to ice blockage, isostatic uplift and downcutting. As the southern margin of the Laurentide ice sheet receded, large proglacial lakes formed in the lake basins between the ice margin to the north and the high topography to the south. Deglaciation of the north rim of the Superior basin by approximately 9ka defines the end of the glacial history of the Great Lakes watershed. Nonetheless, isostatic uplift continued and played a major role in the postglacial evolution of the Great Lakes (Larson & Schaetzl, 2001).
The uplift and shifting ice fronts caused significant changes in the depth, size and drainage patterns of the glacial lakes. Drainage from the lakes occurred variously through the Illinois River Valley towards the Mississippi River; through the Hudson river Valley; through the Kawartha Lakes (Trent River); and through the Ottawa River Valley before entering their present outlet through the St. Lawrence River Valley (EPA, 2008).
The geologic history of the Great Lakes basin did not end with the retreat of the most recent glaciers; the ongoing evolutionary process of landscape development continues today. These processes have resulted in a number of features, including…
