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Geodes in the Fort Payne, Borden, and Salem-Warsaw Formations

Modified from Greb, S.F., Potter, P.E., Meyer, D.L., and Ausich, W.I., 2008, Mud mounds, paleoslumps, crinoids, and more: The geology of the Fort Payne Formation at Lake Cumberland, south-central Kentucky (field trip for the Kentucky chapter of the American Institute of Professional Geologists, May 17-18, 2008:, p. 42-45, with permission of the Kentucky Chapter of the AIPG)

Geodes are rock nodules that are hollow on the inside, although the term “geode” is often used for any quartz nodule or concretion, whether or not it is hollow. On Lake Cumberland, and in much of the outcrop area of the Fort Payne, Borden, and Salem-Warsaw Formations, geodes are common in the bedrock, and as free stones weathered from the bedrock (Fig. 1). Geodes vary in size from less than an inch to as much as two feet in diameter (see for example, Thaden and Lewis, 1962). Most of the geodes in these rock formations have an exterior surface that is bumpy or bubbly, sometimes described as cauliflower-like in appearance. The bumpy outer surface of the geodes consists of a chalcedony rind. Chalcedony is a crypto-crystalline (microscopic-size crystals) variety of quartz. Inside the rind, the quartz nodules, concretions, or geodes are usually filled with solid (massive) or coarsely crystalline quartz.

Figure 1. Geodes and concretions in Fort Payne Formation bedrock.
A). Solid quartz concretion. Approximately 2 inches across.
B). Geode in a vertical exposure of bedrock. Approximately 5 inches across.
C). Concretions and geodes in bedrock (hammer scale).  Breaking Open Geodes


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