Exploring Chicago’s Geoheritage

Roy Plotnick
5 min readJan 25, 2021
Palmisano Park, Chicago. Original quarry high wall in background.

I recently participated in a workshop on America’s Geoheritage. What is geoheritage? One definition is that this an area with geologic features with “significant scientific, educational, cultural, and/or aesthetic value.[i]” One form of geoheritage sites is: “places where geologic features or landscapes played a role in cultural or historical events”. Although there is an unofficial list of geoheritage sites maintained by the National Park Service[ii], the naming of such sites is still in its infancy (thus the workshop). Many locations in Chicago and the surrounding region are prime candidates for eventual designation as culturally and historically significant geoheritage sites. I will discuss some that can be easily visited, even during a pandemic.

The geology and topography of Chicago were shaped by two major episodes in geologic history, separated by hundreds of millions of years. Some 420 million years ago, during the Silurian, the region and indeed most of North America was covered by a shallow tropical sea. Like shallow tropical waters today, in places reefs grew to the surface. Huge deposits of lime mud were deposited, which were transformed over time into the carbonate rock dolomite. These rocks have great economic value; they can be used for building stone, as gravel, or processed to form cement and plaster. Since these deposits were located close to the surface, numerous quarries sprang up across the region as the city grew. The oldest and longest surviving of these was the Bridgeport (Stearns) Quarry, located near 27th and Halsted St., which opened in 1833 and remained operational until 1970, when it was purchased by the city. Reaching a depth of 350’ and conveniently close, the former quarry was an ideal landfill site. After reaching capacity, it was transformed into an inviting park (Palmisano Park) that opened in 1998. One of the remarkable features of the park is a portion of the original highwall of the quarry, that you can go up to and touch in one area.

Far south of the Bridgeport Quarry, at around 175th St. and Halsted, is the vast Thornton Quarry complex. Opening sometime in the 1850’s, this still operational quarry is world-renowned because it exposes a huge Silurian age reef. One of the quarry pits has recently become a reservoir for the Tunnel and Reservoir Plan (Deep Tunnel Project), a massive flood control project. The Thornton Reef has been visited by generations of geologist and paleontologists. Although the quarry is currently closed to tours, you can still see the flank beds of the ancient reef while driving along Interstate 80/294.

Thornton Quarry

The second geologic episode that shaped the area was the late stages and aftermath of the last glacial period. Some 21,000 years ago, almost a kilometer of ice stood where Chicago is today. Some 14,000 years ago, it began to retreat, in fits and starts. When it stopped retreating for a time, it left the hills that surround the Chicago area, the moraines. As it continued to withdraw, a lake form along the southwest boundary of the ice, with the other shore being the morainal hills. This lake, indeed, all of the forming Great Lakes, drained through a huge river that cut through the moraines near what is now Palos Park, forming a huge valley. What is now Chicago sat underneath the lake until Lake Michigan retreated to its current shoreline. The former lake bottom was covered by numerous lakes and marshes (thus the long-term problems with flooding).

You can see the valley of the ancient river, the Chicago Outlet, by visiting the Swallow Cliffs Woods in Palos Park and climbing the 125 steps of the Swallow Cliff Stairs to the top. You are now standing on top of the south bank of the river. Below your feet are the till deposits that form the moraine. Looking to the north, you can see the other bank of the river about a mile away.

View across the Chicago Outlet Valley from the Swallow Cliff Stairs, Palos Park

Before 1900, all the water on the east side of the moraines drained into the Great Lakes through the Chicago River, while that on the west side eventually drained towards the Mississippi, forming a continental divide. The path of the former river, however, produced a low spot that allowed easy portage between the two drainages. This was well known to the indigenous inhabitants of the area and was part of their trade routes. It also became known to European explorers, who recognized the potential importance of the portage and the routes to it, leading to the founding of Chicago at the Great Lakes access. The western end of the former portage can be visited at the Chicago Portage National Heritage at Harlem and 47th St., in Lyons, Illinois.

In 1838, the Illinois and Michigan Canal (I & M Canal) was built through the portage, making it possible to travel from New York to New Orleans strictly along inland water routes. This kickstarted the early growth of the city. It also made it possible to convey building stone quarried in Silurian dolomite of the Joliet region to the city. This yellowish stone, known as Joliet Limestone, was used in 1869 to build Chicago’s iconic Water Tower.

Chicago’s location on the former lake bottom made (and makes it) prone to flooding. This was exacerbated by Chicago River, which was used for sewerage, flowing into Lake Michigan, which was the source of drinking water. You can see the problem. In 1900, therefore, the canal was greatly expanded to reverse the flow of the Chicago River. The ancient drainage now carried Chicago’s waste towards St. Louis along the Sanitary and Ship Canal. Not far from Palmisano Park is the awkwardly named Park Number 571, which sits along on the junction of the South Branch of Chicago River, the South Fork of the South Branch (aka Bubbly Creek), and the entrance to the Sanitary and Ship Canal.

All of these sites: the Water Tower, Chicago Portage National Monument, the Swallow Cliff Stairs, Thornton Quarry, Palmisano Park, and Park 571 reflect the intimate relationship between the geologic history of Chicago and the growth and development of the area. They are excellent candidates for designations as geoheritage sites. In a future essay, I will discuss other sites near and along the I & M Canal, within the I & M Canal National Heritage Area.

[i] https://www.geosociety.org/gsa/positions/position20.aspx

[ii] https://www.nps.gov/subjects/geology/unofficial-register.htm



Roy Plotnick

Paleontologist, geologist, ecologist, educator. Professor at the University of Illinois at Chicago. Author of Explorers of Deep Time.