The History of Ancient Lake Chicago and Today's Lake Michigan.

The city of Chicago lies in a broad plain that, hundreds of millions of years ago, was a great interior basin covered by warm, shallow seas. These seas covered portions of North America from the Arctic Ocean to the Gulf of Mexico. Evidence of these seas is found in the fossils of coral, such as those unearthed in Illinois quarries at Stony Island Avenue, Thornton and McCook Avenues, or at 18th Street and Damen Avenue, all in Chicago. Evidence may also be found in the fossils in the Niagara limestone bedrock found throughout the Chicago area and extending all the way to Niagara, New York. 

Much later, the polar ice cap crept four times down across the continent, covering the region with ice to a depth of a mile or more. As the climate changed, the ice melted, the last great ice flow, the Wisconsin Glacier of the Pleistocene period, which covered much of the northern half of North America, retreated, and an outlet for the melting water developed through the Sag River and the Des Plaines River Valley around Mt. Forest, in the area known as the Palos.

The Kankakee Torrent poured through those valleys, eventually leaving behind the prehistoric Lake Chicago or Glacial Lake Chicago, the term used by geologists for a lake that preceded Lake Michigan when the Wisconsin Glacier retreated from the Chicago area, beginning about 14,000 years ago.

Lake Chicago's level, at its highest, was almost 60 feet higher than the level of present Lake Michigan, and the lake completely covered the area now occupied by Chicago. Its northern outlet into the St. Lawrence River was still blocked by remnants of the glacier. It drained through the so-called Chicago outlet, a notch in the Valparaiso moraine[1], into the Mississippi system. Its western shores reached to where Oak Park and LaGrange now exist.

LAKE CHICAGO

As the glacier shrank in stages, the major three of which are often referred to as the Glenwood phase (50 feet above the level of Lake Michigan; circa 12,000 years ago), the Calumet phase (35 feet; circa 10,000 years ago), and the Tolleston phase (20 feet; less than 8,000 years ago). After each stage, the next barrier remained solid, stabilizing the lake and creating distinct sandy beaches. If the outlet was formed by a steady erosion of the barrier, it would have been less likely that the well-defined beaches would have been created.

This undated marker is located in the southern portion of Lincoln Park, on the footpath paralleling the east side of Stockton Drive. A second identical marker is located on the same ancient beach ridge 485 feet East-North-East from the first one.

The lake's southern shores were dammed by the hills of the Tinley-Valparaiso terminal moraine systems. As the glacier retreated farther and cleared the northern outlet, the lake level fell further, and Lake Chicago became Lake Michigan. Along the eastern shore of Lake Michigan, the beaches of Lake Chicago were destroyed by erosion, except the highest beach. Much of this beach was also destroyed. The best remaining segments are along the southern tip of Lake Michigan, now known as the Indiana Dunes National Lakeshore.

HOW LAKE MICHIGAN GOT ITS NAME
The first Europeans to see Lake Michigan were French traders and explorers in the 1600s. One of which, Samuel de Champlain (1567-1635), who mapped much of northeastern North America, called Lake Michigan the Grand Lac. It was later named "Lake of the Stinking Water" or "Lake of the Puants," after the people who occupied its shores.

In 1679, the lake became known as Lac des Illinois because it gave access to the country of the Indians, so named. Three years before, Claude-Jean Allouez (1622-1689), a French Jesuit missionary, called it Lac St. Joseph, by which name it was often designated by early writers while others called it Lac Dauphin.

Another story recounts that Jean Nicolet, the first European to set foot in Wisconsin in 1634, landed on the shores of Green Bay and was greeted by Winnebago Indians, whom the French called "Puans." Lake Michigan was labeled as "Lake of Puans" on an early and incomplete 1670 map of the region that showed only the lake's northern shores. However, only Green Bay is labeled as "Baye de Puans" (Bay of the Winnebago Indians) on maps from 1688 and 1708. On the 1688 map, Lake Michigan is called Lac des Illinois.

Jesuit map of Lake Superior (or Lac Tracy) and Lake Michigan (or Lac des Illinois). Map by anonymous cartographer, 1671.

An Indian name for Lake Michigan was "Michi gami" and through further interaction with the Indians, the "Lake of the Stinking Water" received its final name of Michigan, derived from the Ojibwa Indian word mishigami, meaning large lake.

Compiled by Dr. Neil Gale, Ph.D. 

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[1] Moraines are accumulations of dirt and rocks that have fallen onto the glacier surface or have been pushed along by the glacier as it moves.