Virtual Silurian Reef Site

Silurian Reef Diorama at the Milwaukee Public Museum.

The city of Chicago, IL is located at 42° North latitude and the city of Milwaukee, WI is located at 43° North latitude on the fresh waters shores of Lake Michigan. The cities are known for hot, humid summers and cold, strong winter blizzards. However, this was not always so. By studying the bedrock of the Michigan Basin, which includes most of Michigan, and parts of Wisconsin, Illinois, Indiana, Ohio, and Ontario, we learned that this area was once a carbonate platform and Chicago and Milwaukee are built upon rock that was once carbonate mud on a tropical seafloor located at 20° South latitude 430 million years ago during the Silurian Period.
In this Silurian sea stromatoporoids and tabulate corals built ancient reefs. Crinoid meadows and bryozoan thickets baffled the strong currents while encrusting stromatoporoids and bryozoans bound and cemented the loose sediment and mud. Orthoconic nautiloid cephalopods jetted about the reefs searching for their prey, crawling, mud-burrowing trilobites, while dense patches of thousands of pentamerid brachiopods filtered the water for food. In Milwaukee shallow water with strong currents produced many reefs 10m tall.In Chicago which had deeper water these reefs thrived and grew over 100 meters tall and were the largest biological structures and had the richest biodiversity in a community the world had produced up until this time.
How do we know this? From over 125 years of research and collecting fossils many of which are in the collections of the Milwaukee Public Museum and the Field Museum. This site uses these fossils reefs as a vehicle for students to learn general principles, local details, and environmental significance of the study of the ancient past.

Associated narratives


Jay describes her 2015 Field Museum Women in Science summer internship project involving digitizing Chicago Silurian reef fossils at the Field Museum.
Alex describes his 2011 summer internship project involving digitizing Chicago Silurian reef fossils at the Field Museum.

Photo Gallery


  • North America, USA, Wisconsin, Ozaukee, Grafton
  • North America, USA, Illinois, Cook, Thornton: Thornton Quarry
  • North America, USA, Wisconsin, Milwaukee, Wauwatosa: Schoonmaker Quarry
  • Europe, England, Shropshire, Much Wenlock