The Old Rock Schoolhouse in Prairie du Chien, Wisconsin, is built of Ordovician dolostone. This dolostone is the result of dolomitization of a limestone deposited on the seafloor in the Ordovician period, about 450 million years ago. It thus presents one with at least two intriguing facts of historical geology: (1) that sea level was high enough in the Ordovician to flood continental interiors like Wisconsin, and (2) that subsurface fluids later transported enough magnesium to convert vast masses of limestone (made of calcium carbonate) to dolostone (made of calcium-magnesium carbonate).

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