Speleothems are secondary mineral deposits formed in caves, usually in limestone. All speleothems are the product of flowing water: seeping water, water drops, or pooled water that carry molecules of minerals (often calcite, aragonite, or gypsum) in solution and deposits them when the chemical equilibrium changes, making them grow. Speleothem types are based on their morphology, or how they look, rather than their mineral compositions. Stalactites and stalagmites, which (respectively) grow straight down from the ceiling or straight up from the floor, are the most common form of speleothems. Stalagmites, stalactites, columns,and coralloids are usually the result of water drips. Flowstone, draperies, and canopies typically form from flowing water. Shelfstone, cave rafts, rimstone dams, "bottle-brush" stalactites, and pool spar typically form from pooled water. Seeping water can form a variety of speleothem types such as helictites, moonmilk, boxwork, crusts, frostwork, anthodites, and shields. No one has been able to explain the origin of all speleothem types, however, and causes as diverse as wind flow, crystal growth anomalies, and electrical activity have been suggested.

Helictites deserve special mention due to their exotic shapes. Unlike stalactites and stalagmites, a helictite is a speleothem that changes the orientation of its axis as it grows, sometimes more than once. The result is a wild array of different forms, curving or angular, threadlike to massive antler forms, that make helictites appear to be gravity-defying. Helictites are the result of capillary action, where water is drawn through a tiny tube at the center of the formation to its emergence at the end. The shapes they end up in often receive fanciful names: butterflies, angel wings, blades, antlers, and others. No matter the geology, helictites are the cavers' "pretties," the easter eggs in caves, the extra reward for following air and water into the earth. Not all of the following pictures are from wild caves (caves that have not been commercialized), but we're sure you'll forgive us the labeling!

For additional information on speleothems and their origins, see the following reference: Hill, Carol A., and Paolo Forti. 1997. Cave minerals of the world, second edition. National Speleological Society, Huntsville, Alabama, 463 pp. Available from the NSS Bookstore.

For more pictures of cave formations, see Dave Bunnel's various pages:

• Helictites
• Stalagmites
• Stalactites
• Shields