Most caves are formed by the dissolving of bedrock by underground water (groundwater). Nearly all caves open to the public are of this type. These caves are called SOLUTION CAVES formed in rocks such as limestone or dolostone. They are part of a unique kind of landscape containing sinkholes, sinking streams, and springs. This landform type is called KARST, named for a plateau in western Slovenia. About 10-15% of the world’s surface is karst. Water that seeps through the soil enlarges the widest cracks in the underlying bedrock. As the soil subsides into the largest of the growing openings, surface depressions gradually form, which are called SINKHOLES. As sinkholes enlarge, greater amounts of water are funneled into the widening cracks below them. With time, the fastest-growing cracks become caves. Most underground water emerges as springs in nearby valleys. If a cave grows large enough, its ceiling may collapse, forming additional sinkholes. The most extensive karst landscapes in the U.S. are located in Kentucky, Missouri, Tennessee, southern Indiana, northern Alabama, and southeastern West Virginia.
The area around Spring Valley is also a karst landscape. Many farm fields are dotted with sinkholes, especially in the springtime when snowmelt is seeping into the ground. The village of Spring Valley was named for the several springs which emerge in and around the community.
There are several other types of caves, most of which are small. Shelter caves are formed by weathering of weak rock layers overlain by stronger layers. Many, like at Mesa Verde, have provided shelter for humans. Lava caves are formed during volcanic eruptions, when the surface of a lava flow solidifies and the interior molten material flows out from beneath creating a void. Many lava tubes are found in Hawaii. Sea caves are formed by the erosional action of waves along steep seacoasts as in California, Oregon and Washington.
A generally accepted definition of a cave is as follows;
- Must be large enough for a person to enter.
- Must be naturally formed
- Must be able to go in far enough to enter total darkness.
The question is often asked “What is the difference between a cave and a cavern?”. Technically, there is no difference. Oftentimes, people use cavern to indicate a very large cave but they are still the same. A comparison would be “cave is to cavern” as “man is to male”.
Cave Passages and Patterns
Many caves have, or have had, water flowing through the passages either on a permanent basis or periodically such as during rains. These caves are often flooded and can fill with water or mud very rapidly. This rapid movement of water carrying mud, silt and sand, causes enlargement of the cave by erosion. Other caves, such as Crystal Cave, do not have active water flows. From about 27,000 to 10,000 years ago, as the glaciers moved through the area, water did enter Crystal Cave, depositing the mud which filled the passages.
Crystal Cave is a series of interconnecting passages, rooms, and domes. Each forms from the rock being dissolved. The passages are called FISSURE PASSAGES or simply fissures. They are straight, narrow and have formed along the joints in the rocks. They are usually much deeper then they are wide. In Crystal Cave, the fissure passages have been filled with silt and mud. As you walk through, in most cases, you are not walking on the floor of the cave. You are walking on silt or mud which fills the lower portion of the passage. This mud is known to be several feet deep in many places.
ROOMS are the large areas where joints intersect, or simply put, where the passages have joined together. Some rooms are simply areas of large passages that look like rooms because they are larger than the surrounding passages.
All but the simplest caves consist of arrays of intersecting passageways that form distinctive patterns. Different parts of the same cave may exhibit different patterns, and more than one pattern can be superimposed in a single location. Crystal Cave is a NETWORK cave. It is a “maze” of intersecting passages and rooms formed by the widening of nearly all major joints over a large area. The straight, high and narrow passages form a pattern like city streets. Look at a map of Crystal Cave and notice how the cave forms a “boxy” pattern.
There are many other types of solution cave patterns including branching (60% of all solution caves), anastomotic (5-10% of caves), spongework (5%), ramifying (5-10%), and single-passage (>2%). Network caves include about 15-20% of all caves formed by solution.
After a cave forms, it often has a variety of minerals deposited throughout. Mineral deposits in caves are called SPELEOTHEMS (popularly known as “cave formations). Speleothem shapes and mineral types are classified according to the environment in which they formed. Most of these deposits are made of calcium carbonate that precipitates from water (carbonic acid). This is the same calcium carbonate that makes up the limestone bedrock in which the cave is located.
When the carbonic acid seeps downward from the surface, it dissolves the limestone bedrock, becoming saturated with calcium. When the carbonic acid enters the cave on the ceiling or along the walls, it enters an air-filled void which has far much less carbon dioxide than the acid. The droplet of carbonic acid will begin to lose its carbon dioxide into the air just like soda pop loses carbon dioxide once it is opened. This reduces the acidity of the liquid and also the amount of dissolved calcium the droplet can hold in solution. The result is the calcium precipitates as crystals of calcite.Return to Cave Geology