1. Maps showing thickness and limestone-dolostone ratios of selected Paleozoic carbonate units in the northern midcontinent, U.S.A. Yale Public content 1996. 2002-06-01T04:00:00Z. 2016-05-18T18:44:14Z. Geological Survey (U.S.). The Survey ; For sale by U.S. Geological Survey Information Services. English. Public. Middle West. Paleozoic.