St. Louis Limestone

St. Louis Limestone is a rock formation spanning across the Midwest with layered limestone containing embedded chert and fossil collections throughout. The formation clearly shows the structure of an ancient seafloor with different types of sediments deposited over long periods.

This formation developed during the Mississippian period roughly 330 to 340 million years ago when the area was covered by shallow seas. It was first documented scientifically in 1847 and takes its name from the city of St. Louis in the region.

The formation provides geologists and researchers with valuable data about prehistoric marine environments through its preserved fossil records of corals and bryozoans.

You can explore this formation at many locations across different states where it is visible in quarries, cliff faces, and natural outcrops. The best time to visit is during drier months when access points are easier to reach and safer to navigate.

The limestone breccia and evaporite deposits within this formation are evidence of repeated phases of ancient seas followed by mineral crystallization. These features help scientists reconstruct the exact conditions of the prehistoric environment.

Location: Illinois

Location: Indiana

Location: Missouri

Made from material: limestone

GPS coordinates: 38.61240,-90.20010

Latest update: December 12, 2025 12:08