A cave shield forms as calcite-rich seep water under hydrostatic pressure is forced from tiny cracks in a cave wall, ceiling, or, occasionally, floor. As this seep water loses carbon dioxide to the cave air, calcite is deposited as parallel extensions to the cracked walls. The result: two thin, sandwiched disks separated only by a capillary, sheet-like void that feeds their growth.

Often, seep water feeding a shield does not lose all of its carbon dioxide at the shield rim. Additional CO2 loss by water slowly dribbling from the shield rim gives rise to draperies, as seen in photos 1 and 2. If a shield becomes clogged, perhaps due to sealing of its rim during dry spells, backed-up seep water may find escape through perforations in the shield disks. This may form stalactites or masses of tangled helictites as in photos 3, 4, and 5.

Photo #6 shows a series of miniature shields forming where water has been squeezed through a crack in a flowstone wall. These sort of incipient shields are also known as welts.

AUTHOR: Djuna Bewley