High-calcium (>95 wt % CaCO3) limestone units commonly occur in pre-Pennsylvanian rocks that crop out along the Mississippi and Ohio Rivers in the western and southern parts of Illinois. The Mississippian Glen Dean, Haney, Ste. Genevieve, St. Louis, Salem, Ullin, and Burlington Limestones; the Devonian Wapsipinicon, Grand Tower, and Backbone Limestones; the Silurian St. Clair Limestone; and the Ordovician Dunleith Limestone all contain high-calcium limestone intervals more than 10 ft (3.3 m) thick.

Aggregate is now produced from most of these units, but significant new exploration targets for quarriable stone exist in Calhoun and possibly Jersey Counties for the Dunleith Limestone, in Union County for the Backbone Limestone, and in Alexander County for the Dunleith. Possibilities for underground mining exist in Adams County for the Burlington Limestone and in Pulaski County for the Ullin. Limited high-volume markets for products from these undeveloped sources and, in some cases, transportation problems may constrain rapid commercial development of these deposits.

Reflectance measurements by the Illinois State Geological Survey on high-calcium limestone samples from most of these deposits range from 57.6% for the Wapsipinicon Limestone from Rock Island County to 88.9% for the Burlington Limestone in Adams County. Values for some of the deposits exceed 80% and therefore compare favorably with reflectance specifications for filler and coating clays.

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