During Silurian time, the Indiana part of the Wabash Platform was a shallow-water area between the proto-lllinois and pro-to-Michigan Basins and a site of growth of hundreds, or perhaps thousands, of reefs. Today, most reefs of northern Indiana are dolomite, but some are dolomitic limestone, and a few are limestone of high purity in deposits that can be mined by open-pit methods.
Four of the five generations of reefs of Silurian age in the Great Lakes area have been recognized in northern Indiana. All known oldest-generation reefs in Indiana (second-generation in the Great Lakes area) are dolomitic. A reef in Carroll County containing commercial amounts of high-calcium limestone is the next oldest (third generation), with roots in basal Louisville limestone, and a limestone reef of similar size in Grant County is of a younger (fourth) generation, with its roots in upper Louisville limestone and dolomite. Numerous slightly younger reefs with mostly mixed dolomitic-calcitic compositions are also of this generation, but all known examples are too small to be commercially important. One small limestone reef has been found in the youngest (fifth) generation, with roots in limestone of the Kokomo Limestone Member of the Salina Formation.
All known limestone reefs are restricted to an area of six counties in north-central Indiana, although no apparent depositional environment as revealed from study of surrounding inter-reef rocks has been found to account for any restriction. Dolomitization is more likely related to the textures and lithologies of the individual reefs.