Quartz geodes, interpreted to be silicified anhydrite nodules, are common in the Mississippian Ramp Creek Formation and Harrodsburg Limestone (Sanders Group) of southern Indiana. The abundance of a stenohaline fauna in the Sanders Group and the absence of sedimentologic features indicative of supratidal or hypersaline conditions, such as stromatolites, desiccation crocks, and "birdseye" vugs, suggest a subtidal, normal marine depositional environment. Geodes in the Ramp Creek and Harrodsburg Formations, as well as other geode-bearing strata, are almost invariably associated with dolomite. Seepage reflux of magnesium-rich brines produced during the deposition of the lower St. Louis Limestone is a likely mechanism for dolomitization of the Sanders Group. Replacement of calcite and aragonite by dolomite in hypersaline pore water may have increased the activity of calcium so that supersaturation with respect to anhydrite, or possibly gypsum, was reached and nodules formed in the still unlithified sediments.