Subsurface data in Indiana and adjacent states indicate subaerial erosion of the Trenton Limestone, karst topography and large solution cavities in the upper part, and a regional unconformity between it and overlying shale of Cincinnatian age. The Trenton Limestone thins southeastward across Indiana and terminates along a major northeast-trending structural alinement. This alinement, marked by the eastern border of the Ozarks in Missouri, the Findlay arch in Ohio, and the St. Lawrence River valley, was a zone of structural inflection during Trenton time. East of this hinge line, contemporaneous deposition (of the Lexington and Cynthiana Formations) continued into Cincinnatian time without interruption.

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