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In page St. Louis:

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Limestone and dolomite of the Mississippian epoch underlie the area, and parts of the city are karst in nature. This is particularly true of the area south of downtown, which has numerous sinkholes and caves. Most of the caves in the city have been sealed, but many springs are visible along the riverfront. Coal, brick clay, and millerite ore were once mined in the city. The predominant surface rock, known as St. Louis limestone, is used as dimension stone and rubble for construction.[citation needed]