Shields are the formation that Lehman Caves is best known for. They are not as rare as was once thought, and have been found in at least 80 caves in the United States. Caves that have shields often have them in large numbers. Lehman Caves has an unusually large concentration of shields, more than 300. Shields consist of two round or oval parallel plates with a thin medial crack between them. The medial crack is thought to be an important clue to their formation. Shields tend to form in caves with highly fractured limestone (like Lehman). Shields grow at all sorts of angles from the ceiling, wall, and floor of the cave. The most accepted theory for how shields form relates to fractures in the bedrock. Water under hydrostatic pressure moves through thin fractures in the limestone. As it enters the cave passage by means of capillary action, the water deposits calcite on either side of the crack, building plates of calcite with a thin, water-filled crack between them. Shields may be decorated with popcorn or helictites on the top and along the medial crack, and draperies and stalactites on the bottom. Sometimes the speleothems on the bottom plate get so heavy they pull the shield apart.
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