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Thornton Quarry Reef Outcrop

Reef core with flank beds dipping off of it to the right and left at Hanson Material Service Quarry (Thornton Quarry). (Note pick-up truck for scale.) The tunnel just visible over the top of the reef core leads to another quarry pit.
Reef core with flank beds dipping off of it to the right and left at Hanson Material Service Quarry (Thornton Quarry). (Note pick-up truck for scale.) The tunnel just visible over the top of the reef core leads to another quarry pit.

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.

Photo Gallery

Pentamerid brachiopod fossils at Thornton Quarry, Racine Formation, Silurian reef. Pentamerids are a type of brachiopod that grew to sizes of over 10 cm, and they represent one of the largest types of dwellers within Silurian reefs. A thickened beak area served as a weight to stabilize the shell in the sediment, and there was no fixed attachment. Pentamerid brachiopods often lived as clumps of individuals, as shown in this reconstruction. Pentamerid brachiopods occur abundantly in discrete beds on the reef flanks at Thornton. The shell material has been dissolved, and the brachiopods are preserved as internal and external molds. The abundant pentamerid brachiopod at Thornton is Kirkidium knightii, a species described originally from the Silurian of the Welsh Borderland.
© The Field Museum - CC BY-NC
Boulders were emplaced by sliding down the flank of the reef. They contain abundant corals and stromatoporoids and were derived from the reef crest. This is a Silurian tabulate coral fossil at Thornton Quarry, Racine Formation, Silurian reef.
© The Field Museum - CC BY-NC
Reef rubble - a small section of the reef broke off from the reef core and rolled down the flank beds and is now covered by more flank beds. Closely-spaced, light-colored vertical lines are drillholes. Left to right inclination of flank beds is also visible. Inclined flank beds of the reef overly nearly horizontal pre-reef deposits.
© The Field Museum - CC BY-NC
Pentamerid brachiopod packstone at Hanson Material Service Quarry (Thornton Quarry). Silurian reef Racine Formation.
© The Field Museum - CC BY-NC
Flank beds at Hanson Material Service Quarry (Thornton Quarry). Silurian reef Racine Formation.
© The Field Museum - CC BY-NC
Reef core with flank beds dipping off of it to the right and left at Hanson Material Service Quarry (Thornton Quarry). (Note pick-up truck for scale.) The tunnel just visible over the top of the reef core leads to another quarry pit.
© The Field Museum - CC BY-NC

Sites

  • North America, USA, Illinois, Cook, Thorton, Thornton Quarry, Illinois. [LL]