This paper examines the geological processes and history that created the Great Lakes, North America's largest freshwater system. It traces the region's geology from the Precambrian Era—when volcanic activity and mountain formation began—through the deposition of Paleozoic sedimentary rocks, to the dramatic reshaping by Pleistocene glaciers approximately 10,000 to 12,000 years ago. The paper discusses bedrock composition, glacial drift deposits, crustal warping, and competing theories about lake-basin formation, concluding that while glacial processes clearly played a major role, significant uncertainty remains about the precise mechanisms involved.
During the Ice Age, a great deal of the northern part of North America was engulfed in glaciers. As the climate warmed, these great ice caps advanced and receded many times, creating a complex and diverse geography in what is now the Great Lakes region.
The Great Lakes is a chain of inland lakes, including Lake Ontario, Lake Erie, Lake Huron, Lake Michigan, and Lake Superior (Eichenlaub 1979, p. 5). These lakes stretch from New York to Minnesota. Due to their size as a major waterway, they have played a significant role in our understanding of geology.
Spanning a surface area of 95,000 square miles, the five lakes represent the greatest stretch of fresh water on the planet. Even Lake Ontario, the smallest of the lakes, is the 14th largest in the world. Nearly one-seventh of the U.S. population lives in the region of the Great Lakes.
The Great Lakes are actually one of the youngest natural features on the North American continent, formed approximately 10,000 to 12,000 years ago from glaciers that made deep troughs in the earth. Ice sheets remaining in these troughs melted and formed the Great Lakes basin (Eichenlaub 1979, p. 5). However, the area surrounding the Great Lakes is the result of geological processes that began billions of years ago.
Approximately three billion years ago, during the Precambrian Era, there was a great deal of volcanic activity and resulting stresses, which formed massive mountain structures. Early sedimentary and volcanic rocks were folded and heated into complex structures, which eventually eroded. Today, these rocks comprise the rolling hills and small mountain remnants of the Canadian Shield, which makes up the northern and northwestern portions of the Great Lakes basin.
This paper will discuss the various geological processes that formed the features and natural resources of the Great Lakes, as well as provide a summary of the geologic history of the area.
The Great Lakes are sites of dynamic biological, chemical, and geological processing of materials from both the terrestrial and offshore zones (Forsyth 1996). The waters and natural resources are created by a complex set of geological processes that are often unique to coastal environments.
The Great Lakes are dominated by their coastal nature. The lakes are oceanographic in scale, meaning that they are large enough to be influenced by the Earth's rotation, yet they are also closed basins in which the influence of coastal processes is much greater than that of most coastal marine systems. Therefore, the Great Lakes clearly demonstrate how complex geological processes interact in a coastal system. As a site for studying geological processes, the Great Lakes have many advantages, including size and a closed basin morphology.
According to Cox (1999), tilted shorelines of ancient high-level beaches indicate that less violent crustal movements continued after the Great Lakes had formed. The crustal warping provides the clue to the cause of the currents that excavated Lake Ontario. Most geologists have assumed the uplift of the Great Lakes area was a slow process, caused by "rebound" of the Earth's crust when the ice melted. I.C. Russell wrote (Russell, I.C., p. 100):
"The amount of change in level shown by the warping of the beaches about Lake Ontario is considerable, and illustrates the character of the slow upheavings and subsidences known to be in progress over wide areas of the Earth's surface. It is stated by Gilbert that 'the old gravel spit near Toronto, belonging to what is known as the Davenport Ridge, is 40 feet higher than the contemporaneous gravel spit on which Lewiston is built; at Belleville, Ontario, the old shore is 200 feet higher than at Rochester; at Watertown, N.Y., 300 feet higher than at Syracuse; and the lowest point in Hamilton, Ontario, at the head of the lake, is 325 feet higher than the highest point near Watertown. From these and other measurements..., we learn that the Ontario basin with its new attitude inclines more to the south and west than with the old attitudes.' The general tilting has thrown the waters of Lake Ontario westward and flooded small tributary valleys so as to drown them and make miniature fjords. Movements in the Earth's crust were also in progress during the long period in which the ancient lakes of the Laurentian basin were making their various records, as shown by the fact that the abandoned beaches do not lie in planes parallel with each other."
According to Cox (1999), Russell's words—"The general tilting has thrown the waters of Lake Ontario westward"—suggest a violent displacement of the overlying waters at a time when the region was submerged. The environment in which the basins of Lake Ontario and other Great Lakes were excavated, and the drumlins were formed, was one of catastrophic flooding.
Observation of existing ice sheets and glaciers indicate that they invariably flow downhill from elevated regions (Cox 1999). Still, the postulated, hypothetical flow of the ice of the glacial theory was uphill out of basins and depressions like the basin of Lake Ontario. The ice is assumed to have flowed uphill out of Lake Ontario, over the steep cliffs of the Niagara Escarpment, and flowed uphill towards the area of northwestern New York.
There are many geologic characteristics of the Great Lakes, including bedrock. The geologic setting of the Great Lakes basins began billions of years ago, with the formation of bedrock. The foundation for the present Great Lakes basin began about three billion years ago, during the Precambrian Era. Most bedrock is made up of sedimentary rock of Paleozoic age (which can be up to 600 million years old). However, the rock around Lake Superior, the deepest of the Great Lakes, is much older igneous and metamorphic rock, Pre-Cambrian in age (up to two billion years old).
Mountain-making geological processes, including volcanoes, intrusions, and metamorphism, created these igneous and metamorphic rocks. These processes occurred during the Pre-Cambrian period. Igneous and metamorphic rocks are the most resistant rocks found in the Great Lakes region, meaning that the land around the Lake Superior basin is very high and hilly. These rocks sit below the younger, Paleozoic sedimentary rocks in the south. In the north, these rocks rise closer to the surface.
At the start of the Paleozoic Era, marine seas, which were inhabited by various life forms, flooded the majority of central North America repeatedly (Forsyth 1996). These life forms included corals, crinoids, brachiopods, and mollusks. Over time, the waters deposited lime silts, clays, sand, and salts, which slowly consolidated into limestone, shales, sandstone, halite, and gypsum.
During the Pleistocene Epoch, continental glaciers spread over the Great Lakes region from the north. The first glacier started growing more than one million years ago. As these glaciers expanded, they covered the surface of the Earth, hills, and basically transformed the previous ecosystem. Valleys that were created by river systems during the previous era were deepened, forming the basins for the Great Lakes. Several thousand years later, the climate warmed up, melting and shrinking the glacier. Next, an interglacial period occurred, and vegetation and wildlife returned to the Great Lakes region. The whole cycle has since been repeated many times.
Sand, silt, clay, and boulders were created by these glaciers and can be seen in various mixtures and forms (Russell 1900). These deposits are known as "glacial drift" and include features like moraines, which are linear mounds of poorly sorted material, flat till plains, till drumlins, and eskers made of sands and gravels deposited from meltwater. Areas that have a significant amount of deposits of sands and gravels are usually good for groundwater storage. In addition, they are great sources of sand and gravel for commercial extraction.
As the glacier moved away, meltwater flooded the front of the ice. At the time, the land was depressed from the weight of the glacier, so large glacial lakes were created. These lakes were bigger than the present Great Lakes. This resulted in beach ridges, eroded bluffs, and flat plains above lake levels.
As the glacier receded, the land rose. This rapid uplift and the shifting ice fronts transformed the depth, size, and drainage patterns of the Great Lakes. While the uplift has slowed down, it still takes place in the northern part of the basin. This, along with evolving long-term weather patterns, demonstrates that the lakes will probably continue to evolve.
Throughout history, thick glaciers and tropical forests have covered the Great Lakes basin. However, these changes occurred before humans occupied the basin. Today, environmental concerns are based on the belief that human activity may be changing the climate at a much quicker rate than ever seen before.
The "greenhouse effect" is a natural phenomenon that occurs when water vapor and carbon dioxide in the atmosphere absorb heat from the Earth and radiate it back to the surface. As a result, the Earth stays warm and habitable. However, humans have increased the carbon dioxide present in the atmosphere so much that many researchers believe the concentration will double its pre-industrial levels by the next century.
Climatologists have determined how the increase of carbon dioxide emissions will affect the climate in the Great Lakes basin. Warmer climates will likely result in increased evaporation from the lake surfaces and evapotranspiration from the land surface. This will supplement the percentage of precipitation that is returned to the atmosphere. Studies reveal that the amount of water contributed by each lake to the overall hydrologic system will drop by 23 to 50 percent, decreasing the average lake levels.
Nearly 4,600 million years ago, the material making up the early Earth's crust is believed to have gone through an extensive period of heating, melting, solidifying, remelting, mixing, and solidifying again. As a result, primitive rock crust formed, cracked, and partially melted, then formed again.
Over the next billion years, the landscape consisted of dark-colored volcanic masses, with no vegetation or life. Volcanoes covered the area with hot lava. The resulting volcanic cones and lava flows were thick and strong. During this time, the iron ore deposits of the Great Lakes were formed.
Millions of years later, the layers of volcanic rock created a huge load on the underlying crust, causing the crust to decline. The great forces that occurred during this event made the volcanic rock layers slowly bend until large folds were created in the layers of rock. When bent, breakage occurred in some rocks.
The downwarp of the crust under its big load of folded volcanic rock caused parts of the crust to melt billions of years ago. This newly melted rock, granite, started to rise up through the crust and creep into the folded volcanic regions. The effect of the folding, breaking, and intruding was a massive uplift of the area. Eventually, mountains rose.
Fast-flowing rivers, glacial ice, and the pounding of waves against the shore occurred over the next 300 million years, wearing down most of the mountains. There was no vegetation protection to slow down these geological processes. As a result, the mountains disintegrated.
The eroded rock debris built up as thick layers of sand, gravel, silt, and clay, eventually transforming into sedimentary rock. Thus, volcanic activity was the major geological process in the Great Lakes area. Gradually, the sand grains cemented together, forming sandstone, which can be found in all the lakes.
Major dune systems formed at the mouth of large rivers as glacial material was transported from inland areas. The dunes of the Great Lakes were formed by a blend of wind, water, and vegetation. Great Lakes dunes have some of the biggest assortment of dune types and dune zones in the world.
Great Lakes dunes are made up of four basic dune types (parabolic, perched, linear, and transverse) and five major zones (beach, foredune, trough/interdunal pond, blowouts, and backdune).
While there are many theories regarding the geologic origins of the Great Lakes, there are still many unanswered questions and gray areas regarding this topic. The problem of explaining the origin of the Great Lakes was explained by Sir J.W. Dawson in his book Acadian Geology:
You’re 82% through this paper. Sign up to read the full paper.
Sign Up Now — Instant Access Already a member? Log inAlways verify citation format against your institution’s current style guide requirements.