Tropical Seas over the Great Lakes of North America, 440 mya - 420 mya

After the ice age at the close of the Ordovician and the beginning of the Silurian Period, the oceans warmed up and the part of the North American continental plate where the Great Lakes now lie become flooded with the warm and rising ocean. At this point, North America was right on the equator, as the continent continued drifting north. While most of North America, including the areas that are now Wisconsin, Michigan, Illinois, Indiana, and Ontario were mostly submerged, some parts of North America were above the sea level, and even those parts that were under the ocean were not very deep. So, for a period of about 20 million years (most of the Silurian Period), many marine creatures and their products as well as bits of eroded rock and soil flowed into the shallow seas and sank to the bottom, where they were deposited as limestones and shales that are now sometimes exposed at the surface near the shores of the Great Lakes.

Over this period of flooding and deposit, it is fairly clear that the shallow and warm seas did not have strong ocean currents. Rather, they were encircled by land and reefs, and in some cases, slowly filled up with sediments, becoming tidal flats, and at some point later in the Silurian Period, some of the seas became massive freshwater lakes, and in other cases, the seas became shallow areas where water evaporated and left the sort of minerals and rocks that form when seawater evaporates. By the time of the early Devonian, after the Silurian, the land had mostly risen up above the level of the seas, and swampy flat lowlands and massive shallow lakes replaced the shallow tropical seas and reefs that had dominated during the Silurian. Much of the continent that is now covered by the Great Lakes had been submerged before, in the Cambrian and Ordovician, and much of it would be covered by the sea again later in the Devonian and the Carboniferous periods, but the reefs of the Silurian period's shallow tropical seas left some of the more interesting rocks and fossils to be found in places such as the western shores of Lake Michigan.

Links about the Silurian Period:

1. The Michigan State University description of the pinnacle reefs formed in the Silurian Period in what is now the Great Lakes region.

2. A description of the sedimentary rock sequence in the Michigan Basin provided by Michigan State University.

3. Michael Tuffelmire shares his account of the geologic history of the Great Lakes region and Michigan.

4. The Illinois State Museum displays some fossils from the Silurian period reefs that exited near Chicago.

This work by Eric & Chun-Chih Hadley-Ives is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.