Winnetka’s Geological History

Gazette Article by: Charles W. Shabica, Ph.D.
Appeared in the Gazette: Spring 1999

*Tracing Our Changing Lakeshore*

During the past 14,500 years as the Laurentide ice sheet receded from the Great Lakes Basin, the surface of Lake Michigan fluctuated between 45 feet higher and 200 feet lower than today. These extremes were caused by glacial ice that alternately blocked and opened outlets for lake drainage. Evidence for high lake stages can still be seen in Winnetka’s rolling topography. Twelve thousand five hundred years ago all of Hubbard Woods and downtown Winnetka occupied a peninsula of land in glacial Lake Chicago, the ancestor of Lake Michigan.

Green Bay Road now runs south along the axis of the peninsula that split into two at Ridge Avenue and Willow Road. Most of the area to the east including Kenilworth was under water. Today, as you drive by Village Hall south on Ridge Avenue past the fire station, you are following the west fork of land that disappeared under the lake about where Saints Faith, Hope and Charity School is located.

To the northeast, storm waves broke against clay bluffs located east of Glencoe’s present shore. Sand eroding from the bluffs was carried along shore as a thin beach that swung southwest by Lloyd Park past the hill east of the Village Green. As the lake receded, beach sand was transported south along the east fork of land by Green Bay Road, to Church Road and past the present entrance to Indian Hill Club. Here the upland ends, and a six-mile-long sand spit now called the Wilmette Spit begins. At that time, the west side of Winnetka including Northfield was a huge marshland and shallow lagoon system.

The evidence for low levels of Lake  Michigan is now under water and therefore, a lot harder to find and study. In 1989 a grove of oak, hickory and ash tree stumps was discovered submerged in 85 feet of water about 15 miles off Chicago. Radiocarbon dates indicate that they were living 8,100 years ago along the banks of a river on the shore of a much smaller Lake Michigan, called the “Lake Chippewa low water stage.” Then the walk from the Village Green to the beach was about eight miles. The trees, up to three feet in diameter, were later drowned by rising lake water and buried in mud. They now are preserved in excellent, although waterlogged condition.

For the last 2,000 years, lake level fluctuations have held within a six-foot range, depending primarily on amounts of precipitation falling directly on the lake. As a result, bluff erosion rates have decreased, greatly reducing the sand supply to the system.

With nineteenth century development, intensive construction of piers, groins, and seawalls stabilized the eroding bluffs, with few exceptions, where we see them today. This is confirmed by the presence of mature trees, many nearly one hundred years old, on the bluff surfaces. No natural beaches now exist in Winnetka; all are held by human-built structures.

The geological future of Winnetka’s lakeshore is the subject of intense study by coastal scientists and engineers. They all agree that how to cope with an eroding near-shore lake bed is today’s primary challenge. For the next one hundred years, despite wave downcutting of the lake bed, the shore is predicted to occupy its present location. However, with deeper water close to shore, the coastal protection structures of choice will be shore-parallel barrier islands, not the shore perpendicular piers and groins of the past century. But that’s another story.

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