A Deep Subject: Limestone Caverns

by Joel Curatolo

This is the first in a three-part series on caves and caverns. Future columns will cover lava caves, sea caves, and other less comon cave types.

Caves come in various types, each with its own features, problems, and perils. These not only exist in the caves but sometimes extend to the surrounding countryside. In fact, player characters don't even need to enter a cave for it to have an effect on the campaign. In future installments of this series we will be discussing lava tubes, sea caves, and other forms, but this article focuses on the one most familiar: Limestone caverns.

Limestone caverns are what most people think of when they hear the word "cave". These caverns are formed by the action of water on limestone over long periods of time. You may be surprised to find out that limestone and chemically similar stones such as marble are soluble in water, but only slightly. Huge cavern complexes, such as Carlsbad Caverns and Mammoth Caves, have been formed by dissolving enormous volumes of limestone over long periods of time. The volumes of water involved usually requires that these caves form below the water table or have a constant flow of water through them. Changes in the level of the water table, cracks in the original rock, and variations in the composition of limestone layers influence what parts of the original limestone are dissolved. It is also possible to redeposit dissolved limestone as water carrying it evaporates. How does this affect what your player characters see and have to deal with in and around limestone caves? Follow along as a party enters and explores a cave.

As the party approaches the cave the land starts to change. The rocks are generally the same as before, including limestone, shale, and sandstone, but the land forms are different. There are rounded depressions with no outlets known as sinkholes. Sinkholes are formed when part of the cavern roof collapses. The result may be a slight slump in the land above the cave, an actual hole which seems bottomless, or anything in between. Some sinkholes are no more than yards across. Some are almost a quarter mile wide. In the larger-sized depressions the rock at the top of slopes may be of various types, but the lower levels consist of limestone. The bottoms of these depressions can be filled with rubble, dirt, or water. Streams may enter a sinkhole and vanish. The collapse which forms a sinkhole may be gradual or sudden. A sudden collapse usually follows a long dry spell during which water which is partially supporting cavern roof is removed by a drop in the water table. Sinkholes may provide entry to cavern complexes through their sides or bottoms or may be sealed by their own collapse. An actively collapsing sinkhole is quite dangerous with unstable edges and the tendency to increase in size without warning, "swallowing" the surrounding terrain and anything on it. This periodically makes the news when such a sinkhole swallows someone's car (or house) in Florida.

The party may notice streams disappearing into sinkholes, as noted above, or suddenly appearing out of a sinkhole or hillside. Water levels may be the same from pond to pond even though there is no visible connection. This is the result of the local waters being connected through the cave system. It will sometimes be possible to determine that a stream enters the cave system at point "A" and leaves at point "B". This is not to say that the best way from A to B is via the stream. In fact, entering a cave floating down a stream may be the worst possible way in, including being swallowed by a sinkhole.

Entering a cave is usually best done through an opening in a hillside, cliff, or the side of a stable sinkhole. Finding such an opening can be simple ("Look at that big hole in the hill over there!") or it can be a more subtle problem. The best clues to finding an opening to the cave involve finding where something inside the cave is leaving it. Things that may be leaving a cave include water, air, and cave life. A fair sized stream that appears on a hillside may be from an opening large enough to enter the cave from. Air blowing from a small opening may also show a way in that may require digging, explosives, or magic. (No explosives in your world? Tough luck. Try another opening) Another, more subtle, way in which air from inside the cave can lead a party to the entrance is by changing the local microclimate. The air from the cave will always be at a temperature near the year round average for the local climate. For temperate climates this is about 55 degrees Fahrenheit. This means that in the summer the area near an entrance may be cooler than expected, and in the winter, warmer. In deserts, which generally have great daily temperature swings, areas near cave entrances may be warmer at night and cooler during the day. Cave life that can reveal an entrance can be of two types. Creatures that regularly leave and return to caves, such as bats, can be followed back to an entrance. Creatures that live full time in caves that are forced or wander from their normal habitat can act as clues to the existence of cave entrances. Full time cave dwellers are generally blind albino versions of various surface creatures. Most common are insects and crustaceans, and rarely anything more advanced than fish.

Once the party has entered the cave their first impressions will probably be of mud and debris. The floor of the cave will be covered with rock fallen from the ceiling, material settled from the water that formed the cave, or debris swept in from the surface. Commonly, the floor and walls will be wet with water seeping from the surface. If the water from the surface has traveled through cracks in limestone and carries dissolved limestone into the cave it is possible to have stone formations deposited. Stalactites form like icicles from the ceiling of the cave. Small amounts of limestone are deposited as water drops hanging from the ceiling evaporate. Water that drops to the floor of the cave then evaporates leaving behind mounds of formerly dissolved limestone forming stalagmites. When a stalactite and a stalagmite meet and grow together they form a column. A number of columns in a line that meet and grow together is called a curtain. Deposits that form on the walls of the cave and cover other surfaces in a sheet-like manner are called flowstone. These formations can only form if water from above containing dissolved limestone enters the cave. If the climate above is dry, water flow is blocked by impermeable rock, or no limestone is contained in the water, the formations will not grow.

The overall layout of limestone caves is of passages and rooms of varying heights widths and lengths. These rooms were almost all connected at one time but rooms may have been cut off by rock falls or sediment built up over the years. Your party may find passages with any slope, any width, or any height. Passages can twist and turn or end at a pit or blank wall of rock. All is not chaos however. Rooms and passages are most commonly found in roughly horizontal layers. This is due to the changes in the level of the water table and the horizontal bedding of many limestones. The lowest levels of the caves are generally below water level. The horizontal layers of rooms, when found, are often connected by roof/floor collapse or vertical passages dissolved from flaws in the original rock or following secondary compositional differences. The result is a three dimensional maze which can have a few yards or many miles of passages.

There are several major hazards that a party faces traveling through these caves. The greatest hazard is disorientation resulting in getting permanently lost. Falls and other mishaps due to slippery conditions and dangerous climbs are another danger. Falling into a subterranean stream can result in a swift death if the character is swept into the depths of the cave. It is possible to have members of the party become stuck when trying to force their way through small passages. Cave explorers have become so firmly wedged in areas of poor access that they could not be extracted despite the best efforts of rescue crews with modern equipment. Lack of air can be a problem if the party enters a small closed space with restricted air flow from the rest of the cave. The generally cool nature of these caves also produces the possibility of death by hypothermia if the party can not keep warm. Hypothermia is particularly a risk if the explorers are soaked, whether from falling into a pool or merely belly-crawling through mud in a low passage.

There are other wonders to be found in limestone caves. One of the less wonderful wonders is the result of having a million bats living in a cave for thousands of years. At night the bats leave to forage for food. During the day they remain in the cave, usually hanging from the ceiling. While they hang they ... how should I put this? ... they excrete solids. Lots of solids. For thousands of years. This produces very thick layers of guano. This material is a moderate biohazard, which is to say that if you muck about in it you're likely to catch something nasty, and makes excellent fertilizer.

Sound can travel great distances through caves but may follow odd paths which make it difficult to trace to its source. It is not uncommon to have sounds seem to come from the opposite direction from their actual source. Sound is not absorbed by the rock walls, it just bounces. This also results in the seeming amplification of sound in the cave. Other minerals, aside from limestone, can be deposited. It all depends upon what dissolved compounds are in the water. For example, there is a cave in Pennsylvania which has a room in which has tiny glowing spots on the ceiling. The glow is caused by radioactive elements in the minerals crystallized on the ceiling. Some deposits lead to impossibly delicate and beautiful formations. Traces of minerals such as iron can tint the cave deposits various colors. If fossils are common in the limestone through which a cave is cut, they can be seen in the walls, making for some interesting decor.

This concludes our brief tour of limestone caverns. If you would like to know more, your local public library will be able to supply you with books on the subject. There is no real substitute, however, for a visit to an actual cave. Many areas of the United States, and elsewhere in the world, have caves which are open to the public through guided tours, often with stairs, paved walkways, and even elevators. While the scientific accuracy of some of the commercial tours is often shaky, the impact of travelling down into a cave and entering an environment so unlike our everyday world cannot be surpassed.