Dr. Michael Chrzastowski, senior coastal geologist with the Illinois State Geological Survey, explains the geologic evolution of the area:
“About 14,000 years ago, the Lake Michigan lobe of the Laurentide Ice Sheet began to recede, leaving rings of glacial till called moraines (piles of clay, gravel, and sand) atop the bedrock. The Highland Park moraine, destined to become incised by the ravines, extended past our current lake’s boundaries and stood over 100 feet high.
“About 10,000 years ago, the weight of the still-retreating ice pushed down on the land so much so that it opened a lower-than-lake-level exit to the north, through Canada’s Ottawa River. The result: Lake Michigan’s water level dropped an astounding 250 feet in a very short time, exposing the older lakebed for 20 miles, to the shores of the now much smaller lake, which geologists call Lake Chippewa. Rain and melting snow quickly developed channels on the exposed lakebed floor. (Also interesting, a forest formed on the lakebed; tree stumps found at the bottom of today’s lake date to 8,300 years ago.)
“About 8,000 years ago, the lakebed streams’ headward erosion reached the Highland Park moraine. As streams touched the moraine, they washed out sand and gravel, cutting a V-shaped channel into the hill. Moraine erosion continued in two directions: run-off of rain and snow in streams from the top, plus “chewing” in from the bottom from the lakebed streams. And thus the ravines began to form. (Myth #2 says that glaciers formed the ravines; actually, glaciers formed the moraines, while streams formed the ravines.) The process was quick, lasting just 5,000 years, between 8,000 B.C. and 3,000 B.C. (This dispels Myth #3, which assumes that ravines are old— they’re actually quite young, geologically speaking.)
“As the glaciers at the northern end of the lake continued to melt, their weight decreased, allowing the land to gradually rise again, which closed off the water exits to the north. (This rebound continues today. Land around Hudson Bay continues to rise one meter per century.) Lake levels rose again, and by about 3,000 B.C., Lake Michigan had refilled close to today’s levels. Waves then chewed away at the moraine (which once extended farther east), forming the bluffs that we see today."
