Five hundred million years ago, the land that is now called Illinois was covered by a shallow tropical ocean, filled with corals, cephalopods, and trilobites. The soil, plant, and animal matter that settled into the earth created today's layers of sandstone and limestone. Over the next few hundred millions of years, the ocean transformed into a muddy swamp, with giant trees that died and compacted into coal. The mud. seashells and sand from this era became shale, as the land became drier.
Starting about two million years ago, several waves of glaciers came down from the north, covering the land with thick layers of ice. As the ice flowed and then retreated, it piled up ridges of rocks and clay at the edges of ice sheets. These moraines are some of Chicago area’s highest points today. Glaciers also carved out and melted into a large lake covering all of what would become Chicago's metropolitan area with a flat bed of terrain.
About 10,000 years ago as the last glacier melted, water in the lake drained out to make it much smaller than it is now, and the level of the lake fluctuated over the next several thousand years until the shoreline settled around 2,500 years ago.
The shoreline then became a smooth curve of sandy beaches fronted with low dunes up to 15 feet high. Stretching under the water was sand for up to a mile off the shore, covering a layer of clay.
Humans have always shaped this environment, but settlers have transformed the Chicago lakefront in ways that were previously unimaginable, harming the natural environment and violating agreements with the Native people of this land in order to create profit.
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By Frank B. Taylor, Frank Leverett - The Pleistocene of Indiana and Michigan, History of the Great Lakes; Chapter XVI, Glacial Lake Arkona; Frank B. Taylor; Monographs of the United States Geological Survey, Vol LIII; Frank Leverett and Frank B. Taylor; Washington, D.C,; Government Printing Office; 1915, Public Domain.