The Maquoketa Formation, often treated in Illinois and Indiana as the Maquoketa Group, is an Upper Ordovician rock unit spread across the central United States and named for Iowa’s Maquoketa River. Deposited in a shallow intercontinental sea, its sediments came largely from the rising Appalachians and partly from the Ozark highlands. Its shale and dolomite strata include the Scales, Fort Atkinson, Brainard, and remnant Neda units, which vary in thickness, composition, and fossil content. Especially valued by geologists, the Maquoketa preserves late Ordovician fossils, records shoreline and sea-level change including an end-Ordovician erosional break, and serves underground as a major aquitard that blocks groundwater flow.