Paleontology and geology
A shallow, tropical sea covered most of southern Minnesota during the Ordovician and at times, may have flooded the entire state. For much of this period, the skeletons of marine organisms accumulated and contributed to the limy sediments on the sea floor. These sediments eventually became limestone and dolostone. Early in the Ordovician, microbial organisms that formed stromatolites and microbial mats dominated the sea. Later in Ordovician time, “shelly” fossils were most common, chiefly bryozoans, brachiopods, crinoids, and molluscs. At times, winds blew ashes from volcanic eruptions in other parts of North America over Minnesota. These ash layers blanketed the sea floor, sometimes leading to local extinctions of marine life. This map shows the presence of Ordovician rocks in both the northern and southern parts of the state.