Inside the Tollway
One of the shortest segments of the Illinois Tollway traverses one of the biggest ravines in the ground in the Midwest.
A portion of the 5-mile segment of the Tri-State Tollway (I-294/I-80) between Hazel Crest to near the Indiana border runs across the Thornton Quarry. This 1.5-mile long, 450-foot-deep pit ranks as one of the largest commercial quarries in the world, according to the Thornton Historical Society.
Motorists on this stretch may see the quarry as one big canyon, but the Tollway was actually built on a dryland dike that splits the north quarry from the main pit area to the south.
One of the nation’s oldest limestone mining operations, Thornton Quarry began mining operations in 1836 – a year before Chicago was incorporated as a city and the population of the fledgling city stood at around 4,200 people.
Owned and operated by Hanson Material Service Corp. since 1938, Thornton Quarry produces more than 7 million tons of rock products a year. Contractors use these products for asphalt and concrete pavement mixes, asphalt roof shingles, fertilizers and various building materials. Materials from this quarry have been used in asphalt and concrete mixes on many Tollway roadway and bridge projects, as well as on pavements throughout Illinois, Indiana, Michigan and beyond.
The history of Thornton Quarry is much older. Fossilized remains of ancient sea creatures dating back more than 600 million years have been discovered in the quarry. Ancient crabs, squids and coral once thrived when much of North America was underwater as part of a vast tropic reef.
During the Silurian Age, about 450 million years ago, the coral took in sea-water and processed out the lime, according to the Thornton Historical Society. Lime deposits from the coral hardened and produced limestone. When the coral died, it remained on top of the limestone and became part of the formation. Young coral then laid eggs on the limestone and started the process all over again. The result was coral and other dead sea creatures became encased in the layers of limestone.
Today, the north quarry serves as an emergency reservoir for the Chicago area to prevent floods during heavy rains. Just below the toll road, a concrete dam separates the two sections of the quarry to prevent reservoir water from flooding into the main pit. The dam is 120 feet high and was built on rock 200 feet above the quarry floor. It is one of the biggest dams in Illinois, according to the Metropolitan Water Reclamation District of Greater Chicago.
The north quarry is capable of draining nearly 8 billion gallons of excess water through a 109-mile network of connections to the Deep Tunnel system operated by the MWRD. Opened in 2015, the reservoir prevents stormwater and sewage from backing up into streets and flowing into area rivers and creeks. After a storm, water is pumped back through the tunnels to the MWRD’s 130th Street treatment plant, where it is processed and returned to waterways flowing toward the Mississippi River.