Bedrock Geology and Mineral Resources of Putnum County, Indiana

Abstract

Bedrock units from the Mansfield Formation (Pennsylvanian) to the Edwardsville Formation (Mississippian) crop out throughout Putnam County. Their present distribution is largely due to erosion and nondeposition. These rocks are on the northeast edge of the Illinois Basin and dip westward about 25 feet per mile. Local structural anomalies, such as small-scale monoclinal and synclinal features, punctuate the otherwise flat-lying strata in some places. The Mississippian-Pennsylvanian unconformity is evident in areas where the Mansfield Formation (Pennsylvanian) overlies rocks as old as the Edwardsville Formation (lower Middle Mississippian) and as young as the Elwren Formation (lower Upper Mississippian). The minerals industry of Putnam County is based almost entirely on limestone. Five quarries produce limestone raw materials for Class A through Class C aggregate, pulverized limestone, agricultural limestone, and cement. Sand and gravel from valley-train deposits of Wisconsinan age are the only other geologic commodities currently produced (1982). The once-healthy clay-shale industry is now dormant, though moderately extensive deposits of both raw materials remain throughout the county. Minor shows of fair-quality to poor-quality coal are exposed along the Pennsylvanian outcrop, but these deposits are not considered to be commercially valuable.