Cave formation is the process by which a cave is created. Cave "formations" are the unusually-shaped rocks that you see inside a cave, such as stalactites.
As the water level drops and a cave forms underground, the water-filled passages gradually become air-filled. The fancy terms: a "phreatic" channel becomes a "vadose" channel.
Most of the surface water reaching the top of the air-filled passage will continue to drip down into the stream of water that emerges as a spring. Once the humidity in those passages drops below 100%, however, some of the surface water will evaporate into the atmosphere of the cave.
When a drop of water evaporates underground in a cave, it may leave behind a tiny deposit of the calcium carbonate that the drop had dissolved during its journey down from the earth's surface. Thin films of calcite can grow into half-inch wide "soda straws" with a drop of water on the bottom. Such soda straws can grow over a foot long. Once a piece of grit or a grain of calcium carbonate blocks the water's flow through the middle of a soda straw, the film of water will drip along the outside of the formation. This process creates - over hundreds or thousands of years - the stalactites that hang from a cave ceiling.
If the drips of water fall to the cave floor before evaporating completely, they carry a portion of the dissolved calcium carbonate with them. When the water droplets then evaporate on the cave floor, they form stalagmites that grow upwards from the floor of the cave to the ceiling. Stalagmites are often located just below a stalactite, and when the two finally grow together a "column" is formed.
stalagmites form at construction sites, when cement leaches from structures
Other cave formations include flowstone, cave coral, helictites, and a variety of other shapes that reflect the rate of evaporation, the chemical composition of the local rock, and even tiny wind currents in the cave. The colors in the formations reflect the local minerals. Water flowing through iron ores (hematite and limonite) will create reddish/yellow bands alternating with the white calcite, creating formations that resemble slabs of bacon with layers of fat and lean meat. Manganese will create black streaks on the cave walls and a black coating on the rocks in a flowing stream.
The rate at with formations grow varies, but can be rapid. Limestone and marble buildings less that 100 years old (such as the Lincoln Memorial) may have soda straws several inches long, showing that the chemical process of dissolving calcium carbonate is not restricted to caves and gravestones.
cave formations (speleothems) at Grand Caverns (Rockbridge County)
Cave formations are rarely created on the earth's surface, because wind and rain erode the soft calcium carbonate faster than it can be created through evaporation. There are a few locations where the calcium in the water may be deposited outside a cave or spring.
The most-visited such site is Mammoth Hot Springs at Yellowstone National Park. At Falling Springs near Covington, at Falls Ridge Preserve in Montgomery County, and at Natural Bridge in Rockbridge County, water emerges from a spring still loaded with dissolved limestone. It deposits travertine (a form of calcium carbonate) along the edge of the pools and on the streambed just below the spring, and can coat sticks and leaves with a film of rock within just a few months.
The formations are fragile, however. One careless step by a visitor can destroy decades of rock formation. In addition, cave visitors are warned "don't touch" because the oils on our fingers will block the continued growth of a cave formation. The water droplet with its thin film of calcite will slide off the formation, before it can evaporate and deposit another addition to the "living rock."
Some caves are exposed when erosion from the surface intersects the cave, removing the roof and exposing the cave formations to wind and rain. It takes only a short time (a few decades) to erode away the obvious signs that there was once a cave at that location. There's a small stream entering the Shenandoah River about a mile downstream of the Route 50 bridge, where the old formations are just barely visible today along a streambank that was once a cave passage.
Natural Bridge is the last remnant of a cave roof