Thornton Quarry is one of the largest commercial stone quarries in the world. It consists of three large quarries connected by tunnels and is operated by the Material Service Corporation, which produces aggregates, stone sand, metallurgical stone, mineral filler and several other products. Over nine million tons of stone have been mined from Thornton in a single year. Thornton Quarry exposes a Silurian reef that is 90 m thick and about 2 km in breadth. Areas of massive reef core are present, but most of the reef consists of inclined flank beds.
Thornton Quarry is located on the southern edge of the Chicago metropolitan area. The quarry can be seen from Interstate 80-294, which crosses its northeastern portion.