Virtual Silurian Reef Site

Silurian Reef Diorama at the Milwaukee Public Museum.

The cities of Chicago, IL and Milwaukee, WI are located between 43° and 42° North latitude on the shores of Lake Michigan. The cities are known for their hot, humid summers and cold, snowy winters. 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, geologists learned that this area was once covered by a tropical sea and Chicago and Milwaukee are built upon rock that was once carbonate mud on this 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 of trilobites, extinct crawling and mud-burrowing arthropods. Meanwhile, dense patches of thousands of pentamerid brachiopods filtered the water for food. In Wisconsin, reefs grew to 10 meters tall in a shallower water environment, while to the south in Illinois, deeper water environments allowed reefs to thrive and grow over 100 meters tall. Up until this time, these were the largest biological structures produced and had the richest biodiversity the world had witnessed.

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 fossil reefs as a vehicle for students to learn the general principles, local details, and environmental significance of the study of the ancient past.

Explore this site to learn about the geology and paleontology of these Silurian reefs.

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, Lime Kiln Park Silurian Reef. [LL]
  • North America, USA, Illinois, Cook, Thornton, Material Service Company Quarry (Thornton Quarry) Silurian reef. [LL]
  • North America, USA, Wisconsin, Milwaukee, Wauwatosa, Schoonmaker Quarry. [LL]