Jan 8, 2014

Thornton Quarry - Underneath Chicago is a Seabed Reef Turned Into Limestone

These pictures are of a diorama at the Milwaukee Public Museum of plants and animals which lived on and formed Silurian-age reefs. Fossil crinoids, brachiopods, trilobites, ammonoids, gastropods and corals are found in Thornton Quarry rock. 

Modern reef environments are found in warm, shallow oceans. The water is usually clear of silt or even wind-blown sediments. Many of the reef inhabitants are filter feeders and sediment prevents feeding.

The area which we call the Chicago Region is itself part of a larger geologic region, regionally known as the Niagaran Escarpment, marked by outcrops of Silurian dolomites and limestones. Niagara Falls cuts through this rock which is exposed in a giant ring around the middle three Great Lakes and centered on the state of Michigan.

The silver-grey to white dolomite pebbles found in builder's piles and as landscaping stones may come from local quarries. Thornton Quarry rock is uniquely recognizable by black flecks of asphalt mixed in with highly fossil ferous reef rock. Thornton Quarry rocks kept half (or less) submerged in household vinegar produce pink dolomite crystals.

Click the link below to read more about Chicago and the Thornton Quarry.

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