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    Cretaceous - Niobrara formation; Carlile shale; Greenhorn limestone; Graneros shale; Dakota sandstone (1)
    Geology -- Iowa -- Plymouth County -- Maps (1)
    Geology -- Iowa -- Sioux County -- Maps (1)
    Geology -- Nebraska -- Dakota County -- Maps (1)
    Geology -- Nebraska -- Dixon County -- Maps (1)
    Geology -- South Dakota -- Clay County -- Maps (1)
    Geology -- South Dakota -- Union County -- Maps (1)
    Geology -- United States --Maps (1)
    Groundwater -- Iowa -- Plymouth County -- Maps (1)
    Groundwater -- Iowa -- Sioux County -- Maps (1)