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2003. Geological Survey (U.S.) Minerals Information Team. This point shapefile represents construction minerals operations in the United States. The data represent commodities covered by the Minerals Infor... National Atlas of the United States.
1984. Tobin, B. D. and Weary, David J. This polygon shapefile is a digital version of U.S. Geological Survey Open File Report 2004-1352, Engineering Aspects of Karst. The open-file repor... National Atlas of the United States.