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 10 meters 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. The joint virtual project (linked below) 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.