Cause of the Glaciers
Carbon Dioxide in the Ancient Atmosphere
The Fourth "Seacoast"
Lake Erie Statistics
The basins that contain what we call the Great Lakes are primarily the result of glaciation that occurred during the Pleistocene, or glacial epoch (the earlier of two epochs of the Quaternary period) which lasted from 500,000 years before present (BP) to 11,000 BP.
Geologically speaking they are extremely young and are less than 10,000 years old. Though they have abbreviated histories the formation of the basins is extremely complex.
The modern Great Lakes, like all lakes, will be geologically short-lived and are just transitory features of an ever-changing landscape. Shores are constantly being eroded by wave action, especially in some places and throughout the basin during periods of high water such as we experienced in 1987. The lakes will ultimately be destroyed by a combination of sediment filling and downcutting of their outlets by their discharging rivers.
The first spectacular event of this nature will be the demise of Lake Erie, the shallowest of the Great Lakes which has received heavy sediment loading due to human perturbations in the watershed and whose outflow is via the Niagara River and the Niagara Falls.
The Falls have been migrating at a rate of 5 feet/year, which if left unchecked would allow for the draining of the lake in about 25,000 years (Hough 1963).
Though the Lakes have abbreviated histories, the formation of the basins is extremely complex and is based on the interpretation of past events through the analysis of the geologic record. This analysis is based on what the geologists call the "Principle of Uniformitarianism", where the present effects of several geologic agents are used to interpret the happenings of the past (Dorr and Eschman 1970).
[Even for Lake Erie, there may be a role for catastrophism as has been strongly suggested in the demise of the large dinosaurs 65 million years ago -- the asteroid impact theory. However, catastrophism does not generally involve extraterrestrial agents; instead, the agents are usually closer to home, for example heart attacks and earthquakes.
The movement of the southern shore in Lorain County from the Center Ridge to the North Ridge and then to the present location probably occurred catastrophically.]
The Great Lakes are basically the result of glacial scouring of pre-existing stream valleys. The axis of each valley is oriented along a belt of weaker rock and the divides, the areas between the valleys, were developed on more resistant rock.
Basically, the basins are in depressions of "soft" rock (which is erodible) surrounded by more resistant (or less-erodible) "hard" rock.
The basin has two basic kinds of bedrock: very hard crystalline rocks that are part of the Canadian Shield which are from 520 million to 2.7 billion years of age and softer sedimentary rock of the Paleozoic era and are 200 million to 520 million years old.
Lake Superior lies almost entirely within the Precambrian Canadian Shield and its basin is exceptional in being nearly surrounded by highlands. Going outward from the lake in any direction but the southeast, escarpments are encountered that sometimes rise 400 to 800 feet above the lakes level.
A cross section of Lake Superior is shown in the figure below to illustrate Precambrian crystalline rocks at the margins of the basin overlying relatively hard but highly fractured and riddled sedimentary rocks (which have undergone a slight degree of metamorphism) in the depression of the lake.
The lake basin is a low spot in the topography where softer rocks were weathered and eroded away relative to harder crystalline rocks bordering the lake, and harder sedimentary rocks forming Isle Royale and the Keweenaw Peninsula after subsidence due to volcanic activity of the Midcontinent Rift System which formed 1.1 billion years before present (Waters 1987).
During this time, the earth's crust split apart in a gigantic crescent-shaped fracture, lava burst from fissures in the widening crack until hardened layers accumulated to great thickness. Eruptions were repeated many times, located in different areas, so that piles of lava layers resembled tilted, overlapping stacks of pancakes.
Over the area from which these immense masses of molten material had moved upward, the now incredibly heavy crust subsequently subsided, and the primordial Superior Basin was formed.
Lake Michigan lies wholly within the Paleozoic rock province of the region as does all of Lake Huron except for the north-northeastern shore which is on the edge of the Canadian Shield.
As above, the lake basins are in softer erodible rock. Similarly, Lakes Erie and Ontario lie on soft, erodible rock surrounded by less erodible bedrock.
The Green Bay peninsula and the islands rimming Georgian Bay are of relatively hard sedimentary Paleozoic dolomites in the "Niagara series" (named for rock structure forming Niagara Falls).
These rocks form the islands in the Beaver Island "arc" in northeastern Lake Michigan as well. Bedrock forming most of the lower peninsula of Michigan is relatively hard as well.
How did the Great Lakes get where they are presently?
The present day Great Lakes are the result of over deepening of a pre-existing drainage system that was eroded by glaciation. Extensive glacial deposits cover most of the bedrock formations of the Great Lakes region except for the Canadian Shield.
There is no evidence that the Great Lakes occurred before these glaciations. There is evidence that stream drainage systems did. Boring through glacial till shows these systems: the Mississippi drainage extensions into the Great Lakes region from the south and the St. Lawrence system to the east.
Cause of the Glaciers
There were at least 4 glaciations in recent times each marked by advances and retreats:
Nebraskan which began about 1 million BP,
Kansan about 700 thousand BP,
Illinoian about 225 thousand BP,
and the Wisconsin about 22 thousand BP.
["There were at least 4 glaciations in recent times each marked by advances and retreats" does not mean that the snow fell in Northern Canada and that then a sheet of ice advanced southward.
Snow from the polar vortex?
A more likely scenario is that the Arctic ice cap melted, but the Arctic remained cold. The result was "lake effect" snow of almost unimaginable magnitude which may have blanketed Avon with fifty feet of snow in July, reflecting sunlight into space.
In a relatively few years, there was a mile-high sheet of ice formed in place, to stay for "100,000" years. The "lake effect" snow stopped when the Arctic Ocean froze over again. The conveyor-belt action of ice melting under pressure at the bottom of the sheet was responsible for the glacial grooves and the deposit of materials from many thousands of miles away.
This is a somewhat different picture of what happened in contrast to the advance and retreat of ice-lobe fronts presented below. The "lake effect" theory does not explain the end of the last ice age. The ocean current theory is a more comprehensive explanation for rapid climate change.
Professor Maureen Raymo has data showing that the Himalayan rock face is supplying salts which react with dissolved CO2 to form carbonates which are sequestered in sea shells. As the Indian plate continues to ram into the Asian plate, more rock face is exposed.
At the end of the Cretaceous (65 million years ago) the atmospheric CO2 concentration was about 1000 parts per million (ppm). When the concentration dropped to about 500 ppm, the grasses gained a definite advantage. Despite all our efforts to release CO2 into the air, the CO2 concentration has not been able to return to 500 ppm.
For those who pay attention to the Gaia hypothesis, the best efforts of the fire-making animals may fail to prevent the increasing severity of ice ages and the eventual steady-state ice-ball earth.]
During the glacial maxima the whole of the Great Lakes area was ice covered. The history of the modern Great Lakes began approximately 15,000 BP with the retreat of the Wisconsin ice sheet.
Glaciation was the result of long term climatic changes in which summers were not warm enough to melt the previous winter's snow. With each successive year more snow accumulated and after many centuries snow layers hundreds of feet thick were formed.
The tremendous weight pressed the lower accumulations of snow to ice, then the ice to a plastic state; the ice began to move away from the centers of accumulation, southward across the North American continent. The movement of glacial ice was not a regular, linear progression across the continent, but rather in distinct tongues or lobes.
Lobes advanced and melted back, sometimes from different centers and thus in different directions. While one lobe melted back to expose an area of land, another progressed from a different direction toward the same area, carrying different materials and leaving different kinds of rock debris.
The movement of ice in lobes occurred because each followed a preexisting lowland, through which the ice was guided by higher ground on both sides. When the terminus of an ice lobe melted at about the same rate as the ice moved ahead, the forward edge remained at about the same location.
However, this meant that much rock debris carried by the ice was deposited in one place, as northern ice continued to arrive and melt. Thus, the glacier lobe acted like a giant conveyor belt carrying boulders, gravel, sand, and silt through the lowland to dump it all in great heaps at the glacier's margin. The result was a range of great hills ---- terminal moraines.
A number of factors and major events led to the formation of the present Great Lakes during this retreat. During the early phases of retreat, depressions south of the glacial ice dams filled with glacial meltwater and normal runoff from the surrounding areas formed pre-glacial lakes (ones out in front of a glacier) such as Lake Chicago and Maumee.
These lakes drained southward into the Mississippi River valley. As the ice sheet retreated and readvanced over the next 8,000 years, discharge channels from previous scourings were uncovered and altered. At one point pre-glacial lakes that were located where present day Lakes Huron and Erie lie, coalesced and collectively drained across the lower peninsula of Michigan to Lake Chicago.
The flow of the water across Michigan was greater than the present day flows of the St. Lawrence River. Today's remnant of that waterway is the Grand River. Around 8,000 BP flows from the Great Lakes drained northeasterly as well as to the east through present day Rome, New York and from there via the Mohawk River to the Atlantic Ocean.
The major flow patterns and general configurations of the modern Great Lakes were fixed about 5,000 BP, except for a lowering of lake levels and diversion of all discharge through the Erie-Ontario basins to the St. Lawrence drainage.
Another factor that affected the Great Lakes is isostatic rebound. Simply stated, the earth's crust in the Great Lakes region has risen vertically since glaciation, with the greatest amount of uplift occurring in the more northern part of the area. At times of extensive glaciation areas north of the Missouri and Ohio river systems were covered with up to 10,000 ft of ice.
The surface of the earth, in these places of isostatic rebound, was depressed under this tremendous weight. Calculations indicate that there was approximately one foot of depression for every three feet of ice cover so in these regions the earth's crust was depressed up to 3,333 ft or almost 2/3 of a mile.
As the weight of the ice was lessened with retreat, the earth's surface began to rebound, analogous to a rubber balloon returning to its original shape once squeezing has stopped.
Because of the earth's considerable mass and semi-solid configuration there was a time lag between when the weight was removed and the earth's crust rebounding.
There is evidence that some of the rebounding is still going on today in the Hudson Bay region. This movement had a dramatic effect on the Great Lakes region in post-glacial times. When the ice first disappeared from the region, and before much crustal rebound had occurred, the northern parts of the basin were at a much lower elevation than they are now.
Early after deglaciation, many of the basins drained northward though outlets now abandoned because they have since been raised above other outlets farther south.
Post-glacial uplift in the southern part of the region was much less, if it even occurred at all, because the thin ice cover typical of the marginal edge of the glacier did not depress the crust.
August 2000; Nature 406, 695 - 699 (2000); By PAUL N. PEARSON AND MARTIN R. PALMER
"Atmospheric carbon dioxide concentrations over the past 60 million years
Knowledge of the evolution of atmospheric carbon dioxide concentrations throughout the Earth's history is important for a reconstruction of the links between climate and ... the Earth's surface temperatures.
Although atmospheric carbon dioxide concentrations in the early Cenozoic era (about 60 Myr ago) are widely believed to have been higher than at present, there is disagreement regarding the exact carbon dioxide levels, the timing of the decline and the mechanisms that are most important for the control of CO2 concentrations over geological timescales.
Here we use the boron-isotope ratios of ancient planktonic foraminifer shells to estimate the pH of surface-layer sea water throughout the past 60 million years, which can be used to reconstruct atmospheric CO2 concentrations.
We estimate CO2 concentrations of more than 2,000 p.p.m. for the late Palaeocene and earliest Eocene periods (from about 60 to 52 Myr ago), and find an erratic decline between 55 and 40 Myr ago that may have been caused by reduced CO2 outgassing from ocean ridges, volcanoes and metamorphic belts and increased carbon burial.
Since the early Miocene (about 24 Myr ago), atmospheric CO2 concentrations appear to have remained below 500 p.p.m. and were more stable than before, although transient intervals of CO2 reduction may have occurred during periods of rapid cooling approximately 15 and 3 Myr ago."
The Fourth "Seacoast"
Thousands of years ago, the melting mile-thick glaciers of the Wisconsin Ice Age left the North American continent a magnificent gift: five huge freshwater seas collectively known today as the Great Lakes -- Lake Superior, Lake Michigan, Lake Huron, Lake Erie and Lake Ontario. From the westernmost tip of Lake Superior at Duluth, Minnesota, to the easternmost tip of Lake Ontario at Watertown, New York, the five lakes stretch a thousand miles across the heartland of both the United States and Canada, creating nearly 9,500 miles of ocean-like shores.
Dubbed "the nation's fourth seacoast," the U.S. Great Lakes shoreline alone totals more than 4,500 miles -- longer than the U.S. East and Gulf coasts combined.
The North American Great Lakes are unique among the world's large lakes in that their basins are linked together and form one continuous drainage basin.
Starting in Lake Superior, the water flows out the lake's southeastern tip down the St. Marys River into Lakes Michigan and Huron, which actually are two halves of one lake. From there, the water flows southward down the St. Clair River at the southern tip of Lake Huron through "little" Lake St. Clair and out the Detroit River to Lake Erie. Leaving Lake Erie, it flows north via the Niagara River and over Niagara Falls into Lake Ontario. It then flows northeast down the St. Lawrence River -- the last link in a 2,000-mile-long waterway that ultimately connects Minnesota to the Atlantic Ocean.
However, this stair-step arrangement of basins is relatively new and resulted from the slow rise of the land as it rebounded from the depressing weight of the mile-thick ice sheets.
Lake Erie and southern Lake Michigan (Lake Chicago) were first unveiled by the glacier about 10,000 years ago. Both originally drained to the southwest, out the Maumee-Wabash-Ohio and Des Plaines-Illinois rivers, respectively, to the Mississippi River. About 9,000 years ago, the early stage of Lake Superior, called Lake Duluth, drained southwest out the St. Croix and Mississippi rivers, along what is today the Minnesota-Wisconsin border.
About 7,000 years ago, as the last ice left Lake Michigan, the land to south of the lakes had risen high enough that the lakes no longer drained in that direction. Lake Ontario came into being, and the Niagara River became Lake Erie's outlet.
As the glacier retreated into Canada, it temporarily made Lakes Superior, Michigan and Huron into one huge body of water called Lake Nipissing, which had the unusual quality of having three outlets -- via the Ottawa-St. Lawrence rivers, Detroit-St. Clair rivers and Illinois-Mississippi rivers.
Lake Huron continued to drain eastward out the Ottawa-St. Lawrence rivers until about 5,000-6,000 years ago. Lake Michigan continued to drain out the Illinois River where Chicago now stands until about only 3,000 years ago, when the Great Lakes finally assumed their present shapes.
Lake Erie Statistics
The Great Lakes today hold an estimated six quadrillion gallons of water -- a fifth, or 20 percent, of all the drinkable water on the surface of Earth.
If all the water in the Great Lakes were spread evenly over the continental U.S., the 48 states would be flooded under more than nine feet of water!
The surfaces of the lakes total more than 94,000 square miles -- covering an area about the size of the entire state of Oregon.
The awesome sizes of the Great Lakes amaze just about everyone seeing them for the first time. These lakes not only look like oceans, they often behave like oceans. They have coastal currents and large, short-term changes in water levels called seiches (pronounced "say-shez") that resemble tides. Like the oceans, the lakes also moderate the temperature of the air and increase the amount of rain or snow that falls on the land around them.
Lake Erie is the fourth-largest Great Lake and the world's twelfth largest freshwater lake. Erie is about 210 miles long, 57 miles wide and about 570 feet above sea level.
Bordered by Michigan, Ohio, Pennsylvania, New York and Ontario, it has 856 miles of shoreline, giving it a surface area of just over 9,900 square miles -- slightly larger than the state of Vermont.
Though the lake bottoms out at 210 feet, it averages only 62 feet deep. Because of its saucer-like shallowness, Lake Erie has a reputation among sailors of being quick to "kick up her heels," raising waves of frightening size in even a modest gale.
Erie may well be the most used, most enjoyed and perhaps even the most loved lake of the five. Erie forms part of the top of the U.S. "industrial crescent" -- the majority of U.S. and Canadian cars are made in this region, and it is a principal steel-producing area.
It also supports the second-largest sport fishery on the Great Lakes today, and its walleye fishery is generally considered to be one of the best in the world.
Erie's water quality problems were legend during the environmental movement of the late 1960s, when the "dead" Great Lake became a national symbol of the effects of pollution and neglect. Fortunately, Lake Erie's flushing time is less than three years -- the shortest of all the Great Lakes -- and the lake has been the quickest to respond to U.S. and Canadian efforts to improve waste treatment and reduce pollution of the Great Lakes.
At Erie's eastern tip near Buffalo, N.Y., its water flows north into the Niagara River, racing downstream at 750,000 gallons per second. In a 35-mile stretch between Lake Erie and Lake Ontario, the river elevation drops 326 feet, nearly 200 feet of it in one drop -- Niagara Falls, one of North America's most famous geographic features and one of the natural wonders of the world.
A few miles west of Niagara Falls lies the Welland Canal, perhaps one of the most impressive man-made structures in the Great Lakes region. Operated by Canada, the 26-mile-long canal contains eight locks that lower and lift cargo ships around the falls.
After the falls, the river again swings east and empties into Lake Ontario, the last of the five Great Lakes.
For Further Information:
GIFTS OF THE GLACIERS - Wisconsin Sea Grant
HISTORICAL SKETCH OF AVON, OHIO, TO 1974