500 million years ago during the Ordovician period, a shallow tropical sea covered much of North America, and its sandy seashore extended diagonally across the southern part of what is now Minnesota. At that time, Minnesota was near the equator. An abundance of aquatic life lived in this sea, including corals, trilobites, clams, and snails. Over millions of years, the carbonate shells from these marine organisms deposited on the ocean floor and accumulated to form the limestone that is so familiar in southeastern Minnesota.
This limestone is an important part of the Twin Cities story. A layer of Platteville eventually formed the shelf over which St. Anthony Falls dropped, creating the waterpower for the manufacturing center of Minneapolis and the head of navigation at St. Paul. Moreover, limestone was quarried and used as a building material. The buff and gray color of Minnesota limestone can be seen all over the Twin Cities, from hundreds of basement foundations, to its earliest territorial buildings, to 19th-century landmarks like the Washburn A Mill, the Pillsbury A Mill, and the Stone Arch Bridge. It continues to be a favored building material, especially the Kasota Stone visible on the Wells Fargo Tower, LaSalle Plaza, the Federal Reserve Bank, and the new Target Plaza. One doesn’t have to go far in Minneapolis and St. Paul to find the remains of the sea creatures that inhabited our state hundreds of millions of years ago.
~David Stevens, Minneapolis, MN