Water creates the caves in Virginia by dissolving limestone. Rainwater seeps down through the soil, becoming slightly acidic as it passes through decaying organic matter such as leaves. The acidic water slowly transforms the calcium carbonate, the predominant mineral in limestone and dolomite. (Dolomite includes more magnesium than ordinary limestone, and is formed when magnesium ions replace calcium ions in the original limestone formation.)
The chemical weathering creates calcium bicarbonate, which easily dissolves in water. Biological activity by sulfur-based microbial communities may also contribute, as at Cesspool Cave along Sweet Springs Creek in Allegheny County.1 As the calcium carbonate weathers away, the rock will erode and create a "karst" landscape with springs, sinkholes, caves - and in Virginia, eroded remnants of cave systems at Natural Bridge, Natural Tunnel, and Natural Chimneys. Acid rain may also be speeding up the creation of caves, by increasing the acidity of rainwater.
The chemical weathering process is invisible to us when it occurs underground, but we can see the same process in cemeteries aboveground. Look at old marble gravestones, and notice how the once-clear letters have eroded away. The calcium in the marble (which is metamorphosed limestone) has dissolved in just a few decades, until the carved letters on many gravestones are no longer legible.
Some minerals, such as silicon dioxide (quartz), are very hard to dissolve. Go to a Virginia beach and you'll see predominantly quartz sand grains, rather than calcium carbonate grains. Quartz is not very reactive. Silicon dioxide is about the last mineral to dissolve, as rocks are washed down from the Appalachians. The quartz resists, to the bitter end, the inevitable fate of dissolving into the ocean. Granite headstones, with a high percentage of quartz, retain their lettering longer.
The granite rocks of the Blue Ridge, and the sandstone ridges of Massanutten Mountain, are not riddled with caves like the limestone valleys in Virginia. The metamorphic bedrock of the Piedmont also lacks caves. There are only a few places in the Piedmont where limestone outcrops on the surface and caves might form naturally.
Where calcium carbonate (limestone) is the bedrock, caves will be more common. A map of cave locations in Virginia shows that nearly all the caves are west of the Blue Ridge, in the limestone Shenandoah Valley and the equivalent valleys south of Augusta County. There is one cave in York County, where Cornwallis supposedly took shelter during the bombardment before surrendering on October 9, 1781 - but most if not all of that hole in the hillside was excavated by people.
When the groundwater emerges back at the earth's surface at the top of the water table, it's known as a spring. Most are gentle seeps at the headwaters or edges of the many creeks in Virginia, but in some larger springs you can see water literally bubbling up from underground.
In limestone areas, springs are caves-in-process. Once the ground water drops, those underground passageways carrying water to the spring will be filled with air... in other words, a cave. Today there may be a small pipe in a hillside near a road where tourists can fill a jug with "mineral water." Come back in a few thousand years, however, and you might find a sign at the same location advertising cave tours, where some calcium-enriched groundwater will have created "speleothems" (stalactites hanging from the cave roof, stalagmites on the cave floor, flowstone on the cave walls, etc.).
Very few springs develop into caves in Virginia. Springs are common in the Blue Ridge, the Piedmont, the Coastal Plain, and in the Appalatchian Plateau where caves are uncommon. In those physiographic provinces of Virginia, groundwater will dissolve minerals too... but the resulting tiny voids underground will be filled by other grains of quartz, felspar, or other minerals. In limestone country, the surrounding rock will support the weight of the overlying sediments and allow the voids to grow into the large rooms visible today on commercial cave tours.
The location and shape of cave passages and rooms reflect the geology of a particular site. The water underground flows gradually to the spring, and it follows the lines of least resistance. In all the upheavals forming the Virginia landscape, the limestone has been cracked in places - and the water will follow those weak spots until it reaches the surface. The calcium carbonate dissolves fastest where the water is flowing and where the crumbled rock surface is exposed to chemical action.
The initial, tiny water channels underground grow and grow over time. When the erosion on the surface cuts down to intersect the underground conduit, the result is a large spring. Erosion will continue to cut the earth's surface even lower - remember, the Appalachians may have been as high as the Himalaya Mountains at one time, and the Coastal Plain shows how much has eroded in 225 million years. When the surface level drops, the groundwater will drop and emerge to the surface at a lower elevation, leaving the old spring high and dry. The spring where the water used to emerge will become a cave entrance, and the old passages in the water-filled conduit will become cave rooms.
In many cases, however, the roof of the cave will collapse as the water level drops and the ground dries out. Some of the old ceiling of a cave may exist for a brief period of time as a "natural bridge," before the entire roof collapses. Sometimes a wall of the cave will remain, as at Natural Chimneys, before it too erodes away.
A rubble-filled path in the ground may be all that marks the old route of the water leading to the spring. That path may erode faster than adjacent limestone, creating a small valley. That wrinkle in the surface topography may be all that humans ever see of the conduits leading to a former spring - even in limestone areas, few springs will grow into caves large enough for people to visit.
In Florida, there is little topographic relief and the caves are still filled with water. Divers at the large Florida springs are now exploring the water-filled caves, using scuba gear to seeing the rooms and passages. Most Florida bedrock has not been uplifted high enough above the level of groundwater to have air-filled passages, and Florida Caverns State Park offers the only commercial cave tours in the state. (As the springs discharge dissolved bedrock, Florida loses about 3 feet of elevation every 38,000 years to "underground erosion."2)
Sections of some Virginia caves are still filled with water too, because the water table has not dropped completely below that section of rock which dissolved underground over thousands of years. Divers who take the risk of pushing past the water-filled sumps in Virginia caves may rise up into other passages and rooms filled with air. In addition to overcoming the technical challenges of diving in a narrow cave passage, and facing the thrill of danger (cave diving is far from a safe activity...), cave diving offers a unique thrill of discovery. The portion of the cave on the other side of the water-filled passage is likely to be a pristine wilderness area, never seen before by any other human. Ever. There are no other places in Virginia where you can make a credible claim that you are the very first to see it.
The limestone bedrock west of the Blue Ridge was formed 500 million or so years ago, long before the Appalachians were uplifted, but the caves in the limestone may have been created only in the last few thousand or million years. In geologic time, caves are relatively new. However, even "new" caves with deep layers of sterile silt and mud, washed in over the years, may harbor evidence of previous visitors, including both animals and early Americans over the last 10,000 or so years.
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.
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 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.
As the groundwater dissolves the limestone, it creates voids and removes the rock that supports the surface. At the ground surface, sinkholes form where the ground has subsided underneath Some valleys have no "exit" where a stream leads to a river. Instead, the water drains into the sinkhole, enters the cave, and then exits the cave at a spring before reaching the river. Some surface streams also lose water to a subsurface conduit, and can even appear to dry up before reaching a river. A classic "losing stream" is Sinking Creek in Giles County. Most of the water in the stream sinks underground about a mile before the stream reaches the New River, leaving a rocky streambed at the mouth of the creek that is filled with water only during storms.
The term karst topography describes the landforms in a region with a large number of caves, sinkholes, and losing streams. USGS quad maps use hatched countour lines to indicate sinkholes, and cavers carefully explore karst areas in hopes of discovering an entrance to a previously-undiscovered cave.
Nearly all caves show signs of rockfalls from ceilings, and a cave room may grow so large that it collapses. Mother Nature does not excavate rooms according to the same engineering designs used by coal miners, leaving pillars to support the roof.
Natural Tunnel and Natural Bridge show what happens when just a portion of the roof collapses. Natural Chimneys was formed by erosion of the limestone ceiling and one side of a former cave.
The general rule is that limestone caves are located near the surface, in the top 1,000 feet. The acidic water is concentrated there. By the time the surface water reaches a deeper depth, it has been neutralized and is no longer able to dissolve calcium carbonate. In New Mexico, however, some very deep caves have been formed by acid fumes rising up from underground gas deposits, so there is an exception to every rule.
Cave tours sensitize visitors to the unique values of the cave environment, and the majority of visitors to caves are urban residents whose only experience underground is going into a basement. Cave guides prepare for four inevitable questions:
Proposed Limestone Overlay District in Loudoun County - zoning controls based on geology
Source: Loudoun County Mapping System