During the Silurian Period in earth history, 443-417 million years ago, much of North America was covered by a shallow tropical sea. A reef system known as the Racine Formation, flourished in what is now Wisconsin and Illinois.
The rich limestone and dolomite bedrock attributed to these formations are what the cement and quarry industries found so appealing about Milwaukee County. This area was once the Story Quarry. The reef, which is the oldest and best known example of Silurian formations in North America, is listed on the National Register of Historic Places and is a National Natural Landmark. Remnants of a smaller reef outcrop can be found at the edge of a parking lot on 68th & State Streets, which was once the site of Schoonmaker Quarry.
Silurian Period reefs are noted for their abundant marine life, and examples of the many wondrous organisms can be seen in dioramas at the Milwaukee Public Museum. Both of these protrusions are found to be very poor in fossil evidence, but on closer examination you may notice color changes in the rock, and in some places even a layering effect from the growth of the reef and ocean sediments.
What do you see happening to the reef now that it is exposed to the elements? Please email me your answer when logging the cache.
- Dott, Jr., R. H., & Attig, J. W. Roadside Geology of Wisconsin. Missoula, Montana: Mountain Press Publishing, 2004.
- Dept. of Geology.Virtual Silurian Reef. Milwaukee Public Museum, 1998.
- Dept. of Geology. Geology Section. Milwaukee Public Museum, 2004.