Great Lakes Atlas

The Glacial Era

There is very little dispute today about whether or not the area surrounding the Great Lakes of North America was indeed glaciated. Current discussions tend to center around the features found in the area, the drainage patterns of the region before the Wisconsin Glaciation, and the stages that combined to form the current lakes system.

The modern drainage pattern for the region includes a watershed that encompasses both peninsulas of Michigan, western Wisconsin and Minnesota and the extreme northern portions of Indiana, Ohio as well as the New York panhandle and Southern Ontario. All rivers and streams in these areas drain into the Great Lakes. From the lakes they flow through the St. Lawrence River and into the Atlantic Ocean.

The water from Lakes Superior and Michigan both flow into Lake Huron through the St. Mary's River (Superior) and the Straight of Mackinaw (Michigan). This water in turn flows into Lake Erie through the St. Claire and Detroit rivers. This also includes the water from the Georgian Bay. Lake Erie drains through the Niagara River into Lake Ontario and then out into the Atlantic Ocean through the St. Lawrence River.

Before the advance of the Wisconsin glacier, the flow of water from the region followed much the same course as it does today. The major difference between the two is in the connection between Lakes Huron and Erie and the St. Clair and Detroit Rivers. Before the ice advance, the water from the current Superior, Michigan and Huron basins flowed into the area now occupied by the Georgian Bay off Lake Huron. All of the water from the current Lake Huron area, south to Lake St. Clair, flowed into this waterway. From there it flowed through the area now occupied by the Trent River to the Bay of Quinte on Lake Ontario ).

The southeast portion of Michigan and Northern Ohio and Indiana were drianed by the preglacial Erigan River that flowed over the land currently occupied by Lake Erie, over the Niagara Escarpment and through the area of Lake Ontario where it merged with the water from the upper portion of the basin and then out to sea through the ancestral St. Lawrence basin

The First Lakes

The advance of the Wisconsin glacier progressed in several major lobes through the region. These lobes were mostly caused by outcrops of highly resistant bedrock of the current Keweenaw, Door and Bayfield peninsulas.

In the west of the region was the Des Moines lobe, moving from north-northwest to south-southeast through Minnesota and Iowa. The Langlade Lobe, Green Bay Lobe and Lake Michigan Lobe all folowed relatively parrallel paths southsouthwestward through Ontario, accross the Superior and Michigan basins and into Wisconsin, Illinois and parts of Iowa. The Ontario Lobe advanced nearly southward over the central St. Lawrence watershed and into western New York and Pennsylvania (Bauder, 2004). The Saginaw, and Huron-Erie Lobes flowed between the Ontario and Michigan Lobes, advancing to the southwest.

The surface rocks of the region were relatively soft Paleozoic shales, salts and sandstones into which river valleys for the drainage system had been cut. These valleys provided the avenues for the ice to follow. The ice divided into lobes and eroded the soft rocks and followed the major river valleys of the time. These valleys were widended and deepend while the tributary valleys were destroyed (Farrand, 1988). The ice moved out of the St. Lawrence watershed and into the Mississippi watershed, but stopped short of the Ohio River to the south.When the glaciers were still in the Mississippi watershed, the melt water could flow freely away from the ice sheet and down into the Gulf of Mexico. When the ice retreated into the preglacial Hudson Bay watershed, however, the melt water was blocked from the sea by the ice itself. This caused the formation of several proglacial lakes along the margin of the ice.

About 14,500 b.p., the ice had retreated and readvanced to build the Valporaiso-Charlotte-Fort Wyane Moraine in northern Indiana and southern Michigan. This moraine system formed a natural dam, stopping the rising water released by the glacial melting from proceeding south into the Mississippi watershed. The weight of the ice had also depressed the lithosphere and caused the bedrock beneath the ice sheet to tilt toward the ice. This further impounded the water against the ice sheet.

The Laurentian Great Lakes were formed nearly 20,000 years ago when the earth's climate warmed and the last glacial continental ice sheet retreated. The glacier, up to 2 miles thick, was so heavy and powerful it gouged out the earth's surface to create the lake basins. Meltwater from the retreating glacier filled the newly created basins. Approximately 3,500-4,000 years ago, the Great Lakes attained their modern levels and area.

The First to Form, Lake Erie

As the lobes shrank back into the valleys that directed their flow, the increased meltwater, combined with the damming of that water by the Valporaiso-Fort Wyane moraines helped to form the first two ancestral Great Lakes, Early Lake Chicago and Early Lake Maumee. Lake Chicago formed in the basin of the Michigan Lobe while Lake Maumee pooled in the Erie basin.

The first stage of Lake Maumee, called Highest Maumee, rose to a level of about 250 meters above sea level. At this point the water found an outlet through the Fort Wayne Moraine and flowed out through the Wabash River valley and then to the Mississippi River. While the water was at this high point, it built beaches along its southern shore. These sand beach ridges still exist today, and were the foundation for many early American trails through the wetlands of the area, and later modern higways. .

Later stages of Lake Maumee (Lowest and Middle) had a lower water level because the northward retreating ice had exposed two lower outlet channels. The Lowest Maumee drained through the Grand River and into Lake Chicago. The Middle Maumee was unable to drain through the Grand River directly, as readvancing ice had blocked that passage. However, there was another channel through what is called the Imlay Outlet. This outlet also flowed westward through Michigan, but did not connect direclty to Lake Chicago, but merged with the Grand River west of Lake Maumee.

Many advances and retreats of the glacier formed several lake stages, the highest of which was Lake Whittlesey (225 m) which drained indirectly through central Michigan and the Grand River. The most recent stage has been identified as Lake Lundy. Some evidence suggests that Lake Lundy may have drained eastward through the Mohawk Valley in New York. With the further retreat of the ice sheet, a new channel was opened over the Niagara Excarpment. This large release of water formed Lake Iroquois in the Ontario Basin. From there the water flowed out to sea through the Mohawk Valley.

The quick outflow of this water caused the level of the lake to drop 46 meters, which would have practically drained the lake except for the deeper parts of the western Erie Basin. The isostatic rebound of the bedrock caused by the removal of the ice sheet slowly alowed water to accumulate in the entire Erie Basin by raising the elevation of the Niagara Escarpment.

The Upper Great Lakes

The Upper Great Lakes (Michigan, Huron and Superior) went through similar stages of high a low water levels, coinciding with the advance and retreat of the major ice lobes in each basin. Lake Chicago was nearly forced from its banks with the Lake Border advance about 500 years after the lake was formed.

The Port Huron advance ended about 13,000 b.p. with three major lakes in existence, the Chicago, the Saginaw in the Huron basin, and the Wittlesey in the Erie basin. Each of these three lake drained through the Chicago-Illinois river valley. The Wittlesey drained through the Ubly channel into the Saginaw and the Saginaw drained through the Grand River into Lake Chicago. Lake Wittlesey and Lake Saginaw are actually believed to be parts of one lake, Lake Arkona, but seperated by an advance of the Huron Lobe.

1988). Lake Saginaw was a shallow, but widespred lake that was about 80 kilometers inland and 35 meters above present Lake Huron.

After the Port Huron advance, the ice retreated above the Straits of Mackinac and exposed a lower outlet at Kirkfield, Ontario. This outlet lead into the Trent Valley from the Georgian Bay and into Lake Ontario. With this new, lower outlet, the waters of both Lake Michigan and Lake Huron were diverted from the Chicago outlet and into the Atlantic Ocean (Farrand, 1988). During this same interval the earliest lake formed in the Superior basin, Lake Keweenaw. This lake occupied about two-thirds of the basin.

One last major advance in the Upper Great Lakes sector again divided Lake Huron and Lake Michigan. Afterward the ice began its final retreat that eventually would see it leave the continent. This retreat allowed for the formation of Lake Algonquin, which was a merging of the waters of the Michigan and Huron basins with those in the Superior Basin. This lake was drained by both the Chicago outlet and through the St. Clair River into the newly established lakes Erie and Ontario.

As the ice continued to retreat, progressively lower outlets were revealed across Ontario through the North Bay and the Ottawa River. These lower and lower outlets caused the water level in Lake Stanley (in the Huron Basin) to fall. This in turn caused water in Lake Chippewa (in the Michigan Basin) to flow through the Mackinac River.

With the weight of the overburdening ice sheet removed, the process of isostatic rebound began. Rising by about 30 cm per century (Leelanau, 2004), the crust underlying the North Bay began to rise. This caused the water levels in the upper basin to rise as well, forming the Lake Nipissing Great Lakes, which occupied the Michigan, Huron and Superior basins. This rebound is still taking place today, with a rebound of 53 cm per centrury occuring in the Northern Superior Basin.

The rate of crustal reboud differs in different parts of the region. The areas rising fastest are those that had a heavier ice load, and also the areas that were covered in ice most recently. Because the lower basin crust were rebounding more slowly, the northern outlets eventually rose above the level of those to the south. This caused the closure of the North Bay outlet and a reopening of abandoned outlets through the Illinois River and through the St. Clair river into Lake Erie.

When the North Bay outlet was abandoned, the entire water discharge from the upper basins bagan to flow through the Illinois River and the St. Clair River. The Illinois River in the area of Chicago is underlain by limestone, which risisted downcutting. The channel of the St. Clair River is situated in unconsolidated till. As the water form Lake Algonquin flowed through, the elevation of the St. Clair River steadily declined. Eventually, the level fell below that of the Illinois River. The western outlet was abandoned and the entire discharge of Lakes Superior, Michigan and Huron began flowing through the St. Clair exclusively. .

With the redirecting of the Upper Basin flow through the St. Clair River at Port Huron, the configuration fo the Great Lakes reached a stage that they essentially hold today. There have been some shifting in shorlines and lake levels, but the hydrologic flow has been consinstent since the abandonment of the North Bay and the Illinois outlets.