Cave Geology in Depth

In Depth Geology of Lehman Caves

Caves have drawn people since prehistoric times. For some, the draw was a belief in the supernatural. Caves figured into the religions of many ancient cultures. The Greeks went to caves for oracles. Many Olmec sculptures depict a priest coming out of an underground void. The Lakota believe that a cave in the Black Hills may be the place of creation. Today, cavers are drawn to caves by the mystery. The unknown in darkness ahead compels cavers to go ever deeper into caves. Scientists look to caves for their geologic secrets, unique biota, and reserves of information about natural history. For over a hundred years, people have been going deep into Lehman Caves and wondering at its secret mysteries.

Just what is a cave? A cave is a naturally occurring underground cavity. There are many different types of caves, including solution caves, tectonic caves, boulder caves, sea caves, and lava tubes. Tectonic caves tend to be relatively small and can form in almost any type of rock that has been highly fractured. Lava tubes are the fastest forming type of cave. They form as a flow of lava is running down an inclined surface. The surface of the lava flow cools from contact with the air and hardens to rock, while the inside keeps flowing. When the inside drains out, the result is a black tube of rock. Some caves may have ice in them seasonally, or year-round. Depending on the shape of a cave and its entrance, caves can trap cold air and contain permanent ice. Lava tubes can often be cold air traps. Solution caves, such as Lehman Caves, can have a great variety of formations and passage patterns.

Solution caves form in a rock that dissolves in acidic water. Limestone, dolomite, gypsum, salt, and marble are examples of rocks that form solution caves readily. The bedrock of Lehman Caves is low grade marble (it was only lightly metamorphosed), but it is usually referred to as limestone. The Pole Canyon Limestone was deposited under a shallow sea during the Cambrian Period (over 500 million years ago). Limey ooze and hard, calcium-rich, parts of sea life settled on the sea floor in a layer that was over 1,000 feet thick in some places. With time and pressure, these sediments solidified into limestone. Limestone is made of the mineral calcite, which is calcium carbonate in chemistry (CaCO3).

Caves in limestone form by the chemical dissolution of the rock. Water is always the agent for cave development. Very rarely does physical abrasion by gravel in moving water play a role in cave formation. Lehman Caves formed mostly by chemical means. The calcite in the limestone can dissolve in water if the water is at least weakly acidic. The acid at work in Lehman Caves was carbonic acid. Carbonic acid is familiar to all of us. It is in soda pop. Where did the acid come from?

The carbonic acid forms when water combines with carbon dioxide. The carbon dioxide might come from the air or from biological activity and decay of organic material (roots, leaves, etc.) in the soil. The carbon dioxide levels in air in soil can exceed 10%, which is 300 times higher than the carbon dioxide concentrations of the air we breathe. The acid-forming reaction is:

H2O + CO2 --> H2CO3 (carbonic acid)

The water and carbonic acid solution then seeps down into the limestone. The acid reacts with the calcite to dissolve it in the liquid.

CaCO3 + H2CO3 --> Ca+2 + 2(HCO3-) (calcium bicarbonate solution)

Many factors influence the amount of limestone that a body of water can dissolve. Temperature and carbon dioxide content are a few of the factors. Most dissolution of limestone happens in the aerated zone (vadose zone), where the acidic water first contacts the limestone. This is just below the soil zone. It does not form caves, but instead dissolves the rock from the top down. This can contribute to the karst topography that often characterizes limestone regions. Karst topography refers to land that has many sinkholes, caves, sinking streams, and limestone pillars. It is most often found in areas of soluble rock with a high rainfall. Some sinkholes are collapsed caves. The water that dissolves the top layer of rock may quickly be saturated with calcite. It may seem surprising that the same water can travel downward and dissolve more limestone to form caverns underground when it mixes with other water.

The next location of high dissolution of limestone is at, or just below, the top of the water table. This is the place where most caves form. There are three main reasons for this:

• The water in the saturated zone (phreatic zone) moves slower than water percolating down, so it is in contact with the rock for a longer period of time to dissolve it.
• The top of the saturated zone (water table) receives acidic water from above (it is closer to the source), so more dissolution happens near the top of the saturated zone than deeper.
• Because of a quirk of solution chemistry, when water of two different chemistries or temperatures mix, the resulting water can dissolve even more calcite than either of the first two waters. Even if both waters were saturated with calcite, when they mix they can dissolve even more calcite. Because the incoming surface water and groundwater often have slightly different chemistries (such as carbon dioxide levels), a lot of dissolution of limestone happens in the zone where they mix (water table). This is completely a chemical effect, not the result of physical mixing and churning.

Because of these factors, cave passages will develop at about the level of the water table, or just below it, if the water table remains at a relatively constant height for a long time. This is why Lehman Caves is relatively level, even though the bedrock is inclined at a great angle.

Lehman Caves formed by this process of a weak carbonic acid dissolving away the rock. The groundwater was probably only slowly moving (no raging underground whirlpools), so the chemical processes completely formed the cave passages. Physical erosion, or scouring, did not play a role in the formation of Lehman Caves. No one knows how long it took the cave to form. Caves often tend to be in the range of hundreds of thousands to a few million years old. Lehman Caves is probably not more than a few million years old, at oldest. It is also not known when the water drained out of the cave. This could have happened because of uplift of the mountain range, climate change, and/or down-cutting of surface streams. Dating the cave, especially in relation to key regional geologic events, could shed some more light on the specifics of how the cave formed.

Yet even today the water seeping through the soil above the cave forms carbonic acid. The acidic water dissolves a little of the bedrock above the cave passages. Most of the limestone is dissolved at the boundary of the rock and soil. This probably does not weaken the cave substantially, until the surface wears down to intersect a cave passage, forming an entrance. The calcite stays in solution until the water reaches the air of the cave passage. At this point the water usually redeposits some or all of the calcite it contains in solution. The reasons for this vary slightly.

Degassing is one main cause of calcite deposition. The carbon dioxide content of the groundwater entering the cave passage is about 250 times higher than that of the air. So when the water contacts the air, it degasses just like a soda pop when you open it. Without the carbon dioxide, the calcite cannot stay in solution. The chemical reaction reverses.

Ca+2 + 2(HCO3-) (in solution)--> CO2(gas) + H20(water) + CaCO3 (calcite)

The water continues on, but it does not carry as much calcite in solution. Another major method for deposition of calcite is evaporation of the water it is dissolved in.

Either way, calcite is deposited. Depending on what shape the calcite takes, it may be called by different names. Travertine is one name for the calcite in the cave. Dripstone is a generic caver term that encompasses any cave decoration caused by dripping, splashing, or seeping water. Speleothems are cave decorations formed after the cave passage has formed, such as dripstone. (Speleogens are features like scallops that form in the bedrock while the cave is forming.)

People often want to know how old the formations are. Broken columns have growth rings that look like tree rings, but there might be thousands of years between each period of deposition. It is not possible to count the rings to date a stalactite. Formations in Lehman Caves have not been dated, so we can only guess by observing the current growth rates. Yet these current rates may not be at all similar to growth rates in the past. Soda straw stalactites that are growing on broken formations in Lehman Caves are mostly less than an inch long. Most of these formations were broken between 1885 and 1922, or roughly a century ago. Keep in mind that because soda straws are hollow, they grow longer much faster than wider speleothems do. The same amount of calcite may be deposited per year on other formations, but the change would not be as noticeable if it is spread over a greater surface area (i.e. flowstone or large columns). Growth rates vary depending on the amount of calcite in solution and the drip rate. Size can be misleading. The largest column in the cave may be younger than a two-inch long soda straw. Conditions can be very localized. Water can change the path it follows into the cave, so formations that are dry may someday be wet again. The amount of water dripping into Lehman Caves now does not seem to account for the large formations in rooms like the Gothic and Grand Palaces. Many formations now are dormant and probably grew more in the past when the climate was wetter, possibly during the Ice Ages.

When water seeps into the cave and drips from the ceiling, a stalactite is the result. The first stage is a soda straw stalactite. These are hollow. The water drop travels down the central canal of the soda straw and hangs on the end, depositing more calcite before dropping to the floor. When the hollow tube eventually plugs up, more water runs on the outside of the stalactite, making it thicker and forming a stalactite. Yet there is still some water moving internally through stalactites (hence the helictites that form on some and the recrystallization of the internal portions). If the drop of water that falls still contains calcite when it hits the ground, this may deposit as well to form a stalagmite. They tend to be squatter than stalactites. The longer the water drop hangs from the ceiling, the less likely it is to still contain calcite when it lands on the floor. Not all stalactites have stalagmites beneath them. Columns result when a stalactite and stalagmite join.

The horizontal ridges that developed on some formations in the cave, such as the stalactites over the path in the Grand Palace, are called crennulations. Crennulations tend to have a wavelength of about 1 cm, no matter what material they develop in. They are common on icicles as well. One theory says that they begin when water flows over a slight bump in the surface. The water film thins as it passes the bump, which increases evaporation and/or carbon dioxide loss. More calcite is deposited on the bump. Just below the bump the water pools up again, and below this small pool the water thins again due to surface tension, depositing more calcite to form another ridge. The whole cycle starts over again. Crennulations seem to be self-perpetuating below the initial bump. Microgours in flowstone and the saw-tooth edge of some draperies may also be self-perpetuating in the same way.

Draperies form when calcite-rich water runs down an overhanging wall. Draperies that are striped from varying amounts of impurities are also called cave bacon.

Flowstone forms if there is a thin film of water flowing down a sloping surface. The calcite is deposited layer upon layer, like coats of paint.

Cave popcorn (also called cave grapes or corraloids) is a catchall term describing a small, bumpy speleothem that usually does not have a central conduit, unlike stalactites. Although these formations are given the same name, they can form in very different ways depending on the location in the cave. Popcorn can be deposited on the walls of a standing pool of water (like on the rimstone dams in the Cypress Swamp and the small hollow along the West Room path). Most of the popcorn in Lehman Caves was not formed under water, however. Other ways popcorn can form is by a thin film of water flowing over an irregular surface, splashing water, and by water seeping through the bedrock. The seepage method is probably predominant at Lehman Caves. The water "sweats" through the wall and the crystals of the popcorn. The water then evaporates, or sometimes drips, and leaves calcite deposits.

Wall coating forms in a similar way to popcorn. It forms by water seeping from the bedrock and depositing calcite. The result is a calcite crust covering the wall.

Rimstone dams are located in the Cypress Swamp and in the Lodge Room. When active, each dam would hold a small pool of water behind it. Water flowing over the top of the dam deposits more calcite, increasing the height of the dam. They can grow much larger in some caves, up to a height of over 40 feet! Some of the formations (especially flowstone) in Lehman Caves also have a much smaller variation of rimstone dams on their surface, called microgours. These can look like tiny terraces in the flowstone or just like bumps. They build up as a thin film of water on the formation flows over irregularities on the surface.

Shields are the formation that Lehman Caves is best known for. They are not as rare as was once thought, and have been found in at least 80 caves in the United States. Caves that have shields often have them in large numbers. Lehman Caves has an unusually large concentration of shields, more than 300. Shields consist of two round or oval parallel plates with a thin medial crack between them. The medial crack is thought to be an important clue to their formation. Shields tend to form in caves with highly fractured limestone (like Lehman). Shields grow at all sorts of angles from the ceiling, wall, and floor of the cave. The most accepted theory for how shields form relates to fractures in the bedrock. Water under hydrostatic pressure moves through thin fractures in the limestone. As it enters the cave passage by means of capillary action, the water deposits calcite on either side of the crack, building plates of calcite with a thin, water-filled crack between them. Shields may be decorated with popcorn or helictites on the top and along the medial crack, and draperies and stalactites on the bottom. Sometimes the speleothems on the bottom plate get too heavy and pull the shield apart.

Welts are thought to be similar to shields. They are like scars of calcite. They grow along a fracture in the bedrock or in a speleothem that is opening very slowly. A sudden break will not form a welt. A column may pull apart because of settling of underlying sediment. As the crack grows, water seeps out and deposits calcite.

Some researchers believe that bulbous stalactites are related to shields and welts, but it is not known exactly how. Some researchers believe that turnip-shaped bulbous stalactites may be only a variation of a welt or shield. It has been observed that caves with this shape bulbous stalactites also have shields and welts, so there may be a connection. Another theory for the origin of bulbous stalactites has to do with intermittent flow of water. Usually, a growing soda straw stalactite has water flowing both internally and externally. If the internal flow stops, calcite may plug the internal channel. Then when the internal flow resumes, the internal pressure may cause the stalactite to rupture. If the water oozes out in small quantities, helictites form. If there is more water, a bulbous form results. However, no one really knows the real method that bulbous stalactites form by.

Soda straws in the Lodge Room area have been observed with bubbles on their tips during wet periods in early spring. Bubble-blowing stalactites are thought to be caused when internal flow in a soda straw is temporarily interrupted while external flow continues. This can draw water and air into the straw. When internal flow resumes, the result will be a bubble on the end of the soda straw with water dripping from the bubble. This theory of intermittent flow sounds similar to the one stated above for bulbous stalactites. Yet if this theory is correct, why the apparent correlation between bulbous stalactites and shields?

Moonmilk is a white formation that looks like powder when dry or cottage cheese when wet. There is a lot of it in the Rocky Road and on the ceiling in the Inscription Room. Moonmilk can be a combination of different (mostly carbonate) minerals. Some common minerals composing moonmilk are calcite, aragonite and hydromagnesite. Humans have used moonmilk as medicine to stop bleeding, induce a mother's milk, and for ulcers. There are several theories for moonmilk. One is that bacteria play a role in its origins. Another is that the moonmilk is deposited directly from water the same way other speleothems are, but for some reason the crystals never grow large or connected.

Gypsum formations form in a similar way to calcite speleothems. Gypsum is a mineral, with the chemical formula CaSO4· 2H2O. Basically, it is calcium sulfate with some water attached. Gypsum can take forms known by cavers as snow, flowers, crust, needles, and hairs, depending on the shape. There are no gypsum formations along the tour route, but there are many in the Gypsum Annex. They tend to be white (or colorless), small, and very fragile.

Calcite in its pure form is colorless, yet most of the speleothems in Lehman Caves are colored. The color comes from impurities in the calcite. White color can be caused by inclusions of water. Most of the brown to yellow to red color in the cave is caused by iron oxides (natural rust) or organics. One exception to this is the orange-pink color on the ceiling in the Inscription Room, which is caused by a bacteria, but no one knows what type.

Some of the formations in Lehman Caves have been redissolved. A good example of this is the Eagle’s Wing in the Lodge Room. The cave drained and some formations grew, then the cave refilled with water some time later. This water dissolved part of the formations. The water drained again, and decoration of the cave continued. It is also possible that some dissolution of formations happens in the air. The moisture in the air that condenses on formations and the bedrock may be able to dissolve some calcite. This is most apparent with draperies. Draperies can look heavily corroded and have holes in them. Draperies have a high surface area for water to condense onto and are very thin, so even a small amount of dissolution would be very noticeable. Some researchers believe that bacteria may play a role in condensation corrosion.

Flowing water caused the scalloping in some parts of the cave. The water (with a little acid in it) moved past the wall from the gradually inclined side towards the steep side of the scallop. As the water moved over the ridge, it swirled down on the backside of the scallop, enlarging it. The erosion was caused by acid in the water, not gravel and particles. It is possible to determine the speed of the water by measuring the length of the sides and the angle between them. The direction is determined by the steep and gradual sides. The steep side in each pocket is upstream (in other words, on the downstream side of the ridge). It is possible to map flow direction in a cave using the scallops. In Lehman Caves, there are places where the scallops in the same room indicate different flow directions. There were probably eddies off the main flow that caused this. The path of the water was generally from the Grand Palace towards the exit.

Some features in caves show definite directional characteristics. A good example in Lehman Caves is the directional popcorn found in the West Room and Inscription Room. Only one side of the formations has popcorn, while the other side is undecorated. This is probably because as water moved through the cave, it deposited the popcorn in the eddy behind the formations. It is also possible that this may have happened in an air-filled passage by similar means.

Speleologists have theorized that at one time there may have been rising warm or hot water in the cave. This water would only need to be at least 1° warmer than the water in the cave to make a difference chemically. In some places, especially in the Rocky Road, there are ceiling half tubes. These are felt to be places where warmer water entered the cave. The other evidence is the increased metamorphism of the marble along the Rocky Road fault. Not much research has been done to test this theory. Cracks in the bedrock (joints or faults) have been a major avenue for water (warm or cool) to enter the cave.

The location of faults controlled some of the cave passages. Cave passages tend to follow a fault for a short distance, then go off of it. Faults may be open or closed, meaning there may be space between the moving walls or not. Cave passages may develop along open faults because there is a space for the water to enter. If a fault is only open for a short distance, a cave passage may develop in that section, but the passage will not extend into the closed section of the fault. The Rocky Road, the Music Room, and the Royal Gorge (passage between Sunken Gardens and Talus Room) are examples of passages that developed along faults.

Any mention of faults usually results in questions about earthquakes. It does not seem that the faults in the cave have been active for a long time. If there were movement along one of them, it would probably cause some shifting in the cave. Earthquakes generated by movement along faults further away don’t seem to cause damage in the cave, even when they shake the surface. Earthquakes generate three different types of waves, which travel in different ways. The undulating motion waves travel along the surface and can cause a lot of damage. Sound waves generated by earthquakes can travel at deeper depths. It is sometimes possible to hear an earthquake while in a cave. The shear number of formations in Lehman Caves (and the size of the formations in the Gothic and Grand Palaces) is evidence that not much shifting has happened in most of the cave for a long time. The one exception is the Talus Room.

The rockfalls in the Talus Room may or may not have been triggered by earthquakes. Most collapse in caves happens either when the water first drains out of the cave or when the surface finally erodes to close to the roof of the cave. Not much has happened to weaken the strength of the bedrock since the water table lowered. Yet the water that filled the cave was also giving a little support for the weight of the ceiling. The Talus Room probably started to collapse when the water drained from the cave. It has not reached a stable point yet, probably because of the room’s size, although the fault zones have played a role as well. There are probably old cave passages buried beneath the rubble in the Talus Room.

There is airflow at the "ends" of the cave, beyond the Talus Room. Airflow in Lehman Caves is caused by a chimney effect, air moving between entrances. However, the air may be travelling through very small cracks and vents explorers would never be able to fit through.

Weather in caves tends to be very consistent, compared to the surface conditions. Lehman Caves is 50° F year round. The relative humidity varies between 90 and 100%. The closer a room is to an entrance, the more variation in temperature and humidity it will have. Most sizeable caves do have a nearly constant temperature. The airflow is not sufficient to change the temperature of the cave. Air entering the cave is either warmed or cooled by contact with the bedrock in the cave. The temperature of the bedrock controls the temperature of the air in the cave. The temperature of the bedrock is often just about the average surface temperature. Seasonal temperatur variations at the surface do not affect the bedrock at the depth of most caves.

The near-constant conditions in many caves is the main reason that change can be very slow in some caves. There are few large animals entering the passages in Lehman Caves. There are no rivers. It never rains. Airflow at its strongest is a slight breeze. Most of the cave is too deep to be invaded and changed by plant roots. The speleothems grow on a time scale that is difficult for humans to observe. This means that changes humans cause in caves will last much longer than might be expected by extrapolating from surface conditions. Footprints may last for centuries.

Lehman Caves present an interesting geologic puzzle. The geologists believe that they have figured out many of the mysteries of the cave, but there are many more questions to answer. There is a bacteria in the cave that should be studied, along with potential bacterial causes of moonmilk, colorations, and condensation corrosion. A paleonotological dig, especially in the Lost River Passage, could turn up some very interesting results and could help determine if and when there might have been another entrance in that part of the cave. Shields and bulbous stalactites are not well enough understood. The Gypsum Annex is an interesting part of the cave because it is so incredibly different from the main cave. Why? It might be possible to learn more about past climatic information by studying the speleothems. Many opportunities for discovery and learning are still in Lehman Caves. The mysteries continue, and the answers to these questions will only lead us to more questions.

Written: December 1999, Abigail Wines