Locations of Silurian reefs in Great Lakes region.
The Great Lakes region of North America contains one of the largest occurrences of Silurian reefs in the world. This map is a reconstruction of the area about 425 million years ago during middle Silurian time. Two large belts of reefs occur along the boundaries of shallow seas and deeper water areas of the Michigan and Illinois basins. Hundreds of individual reefs occur within these belts.