Forming caves, cave formations, and their surroundings
Caves typically are found in special landscapes, usually called “karst”, that have certain special features. These karst landscapes give us beautiful parks, but cause major challenges for construction and drinking water.
Dissolving a cave
Mammoth Cave and its surrounding karst landscape, like the great majority of large caves, was dissolved in a rock type called limestone. The limestone was deposited in shallow seas during the Paleozoic Era (a few hundred million years ago), and comes from shells and other materials deposited by sea creatures. Mammoth Cave is so big in part because the limestone lies beneath a strong sandstone layer from old beaches, which provides a “roof” that does not collapse easily as the cave is dissolved into the limestone.
As we saw in discussing rock weathering to make muds for the Badlands back in Module 5, rainwater and soil water are weak acids. Chemically, the calcium carbonate that makes up the limestone is especially prone to attack by acid. (In fact, the usual test for limestone is to drip a little weak hydrochloric acid on a sample; limestone fizzes vigorously as the rock decomposes to free carbon dioxide gas, but most other rocks react much more slowly and do not fizz.) Where soil waters move through limestone, the rock dissolves and washes away. You wouldn’t see much change from year to year while a cave is forming, but the rock dissolves very rapidly compared to many geologic processes.
Not all limestones make big caves when they dissolve. If the limestone has lots and lots of cracks, the water may spread out into so many different paths through those cracks that not enough rock dissolves along any one crack to make a cave. But if the limestone has just a few cracks for water to flow through, all of the dissolution will be concentrated in those few places, and cave passageways may form. Caves usually form while filled with water, but nearby rivers then may cut downward, draining water from surrounding rocks and lowering the water table, as Kentucky’s Green River has done near Mammoth Cave. This can empty the water from all or part of a cave, letting it fill with air.
Forming cave formations
The beautiful stalaCtites (from the Ceiling), stalaGmites (on the Ground), and other cave formations that tourists love to see in caves can then develop. You might be surprised that nature first hollows out the cave and then starts to fill the cave again, but this really does make sense.

Many processes in soil, such as dead things decomposing and worms exhaling, release carbon dioxide, and some of that carbon dioxide is picked up by rainwater as it soaks in and becomes groundwater. Thus, groundwater is more effective than rainwater at dissolving limestone. Occasionally, a cave may be so isolated from the surface that dangerous levels of carbon dioxide build up in the cave’s air, but caves usually exchange enough air with the outside world to have near-normal levels of carbon dioxide so that you can go into them safely (presuming you have lights and warm clothing and watch out for floods and don’t fall into giant pits...).
When groundwater drips into such an air-filled cave that has a near-normal carbon-dioxide level, the groundwater loses some of its extra carbon dioxide to the air. The water then cannot hold all of the limestone it has dissolved, and some of that limestone is deposited to form the beautiful stone features we see.
Forming other karst features
Almost all rocks have cracks, called joints. The next time you can safely examine a cliff or road cut, you should be able to see these joints. They occur in many orientations, but some are generally near-vertical, often in intersecting sets as shown in the picture.
The rocks in the picture are not limestone, but limestones have similar joints, and where water soaks downward along intersecting cracks in limestone, the rock is often dissolved, leaving space that may make a low spot in the surface, or may partially or completely fill with mud. Such a hole, whether mud-filled or air-filled, is called a sinkhole. Sinkholes also form when the roof of a cave collapses, leaving a low spot in the surface. Somewhat confusingly, when a pipe breaks beneath a city street and a road falls into the space, people also call that a sinkhole. For this course, we will focus on the sinkholes that are especially formed by dissolution of limestone, and leave the collapsing sewer pipes in cities for a different course.

Sinkholes formed by downgoing waters are very common near Penn State’s University Park campus. The Geosciences Department is housed in the Deike Building, which required extra funding for special strengthening because the building has sinkholes beneath—a building can rest firmly on bedrock, but tends to fall into air-filled or mud-filled holes. Extra funds were similarly expended to strengthen the nearby Mt. Nittany Middle School, the runway extension at the airport, and other construction projects in the area. A newly constructed storm-water catch basin at the airport filled with water during its first big rainstorm, and the weight of the water blasted mud out of a buried cave passageway somewhere beneath, suddenly clogging nearby Spring Creek with trout-choking red mud.
Where sinkholes and caves are common, streams often disappear underground into swallow holes, only to re-emerge at springs. Spring Creek is aptly named—it is fed by a lot of Springs!—and many other similar features occur around central Pennsylvania, around Mammoth Cave, and in other such regions.

Corn cobs once were dumped in a sinkhole behind a cannery at Old Fort east of Penn State’s University Park campus, and after a rain would pop out of a spring in Spring Mills, a few miles away. A stream flowing off nearby Mount Nittany goes down a swallow hole in the town of Pleasant Gap and then comes back out in a spring a mile or so away… and once, a basketball that had washed down the swallow hole came out in the spring! Regions with sinkholes, caves, springs, swallow holes, etc., are referred to as karst, after a region in Slovenia with many such features. Karst features are present across 20% of the Earth’s surface, and roughly 40% of the US population obtain at least some of their drinking water from karst, according to the National Park Service.

In the past, people often threw trash into sinkholes. Big pieces would sink into the mud or fall into cave passages beneath, “disappearing.” When Dr. Alley was in high school and went to Sloan’s Valley, Kentucky to go wild caving (spelunking), one of the cave entrances was known as the Garbage Pit, which led into the Tetanus Tunnel. A commercial cave near Mammoth Cave was forced to close in the 1940s because of the stench from sewage draining in.
Slowly, we are learning just how stupid it is to dump things in sinkholes. A test conducted by the great Penn State hydrogeologist Richard Parizek during the building of the Nittany Mall east of Penn State’s University Park campus showed that a little harmless dye dumped in a sinkhole near the mall came out in a nearby trout stream in a day or two. It should be evident that anything else dumped in a sinkhole near the Nittany Mall (or many, many other sinkholes in the region and in other karst regions) would show up very quickly in the water used by people and wildlife.

Dr. Alley lives in a house served by a local water company well-known for its fine water from deep wells (with locations that were identified by Penn Staters Richard and Byron Parizek). But many years ago, before a reorganization of the water company and before the help from the Penn Staters, the intestinal parasite Giardia showed up in the local well water. Giardia causes intense and possibly dangerous stomach problems. Giardia usually is restricted to surface water; the spaces in most rocks are small enough to filter out the Giardia cysts before they reach a water well, or the water takes so long to go from the surface to the well through the small spaces that the cysts die of old age on the way. At the long-ago community meeting to discuss the water contamination, company officials noted that they had installed well filters to remove sticks, leaves, etc., that came out of the wells with the water. In karst country, surface water can become groundwater and return to the surface in hours or days. Whole streams go down and up, and if sticks can go through, microscopic cysts can, too. Clearly, contaminants dumped somewhere today can be poisoning someone tomorrow.
In some other regions, the groundwater-contamination problems are quite different. In sandstones, for example, the water moves slowly, pore-by-pore, through the rocks. In some places, the water can be shown to have first entered the ground during the ice age, more than 20,000 years ago, or even earlier. Contaminants dumped in such rocks may not bother people for a while. But, when the contaminants do start to bother people, clean-up can be very difficult.
Try this experiment. Squirt a little food coloring dye on a sponge, and squeeze the sponge a few times to distribute the dye well. The sponge is our rock, and the dye is the contaminant. Now, wet the sponge, hold it up, and squeeze it. Colored water will come out. Wet the sponge again, squeeze it again, and more dye comes out. Repeat, and repeat, and repeat. You may need ten or more times to remove enough dye that you no longer see it, and sensitive instruments would detect the dye through dozens or even hundreds of additional washings. Now, suppose that instead of edible food-coloring dye we had used a chemical that causes cancer in humans. If the water in the rocks naturally is hundreds or thousands or more years old, then nature takes a long time to wash out the rocks once, and washing them out ten or one hundred times will take much longer than all of human history.
There are things that can be done about groundwater pollution. You can pump clean water in and dirty water out to speed up the washing, or pump steam or hot water in and out to wash even faster (and then try to figure out how to clean the dirty water or steam once you have them back on the surface). People are experimenting with installing filters so that polluted water will flow through them, sometimes using large masses of iron filings to react with and break down some organic chemicals in groundwater. Geomicrobiologists are searching in heavily polluted sites for microbes that “like” to eat pollutants, and then trying to introduce those microorganisms into other polluted sites to break down harmful chemicals, while other biologists are trying to design pollutant-eating microbes. But, such techniques usually are very expensive and not very effective. Most people who have thought about it agree very strongly that the best way to handle groundwater pollution is to keep the chemicals out of the ground in the first place. A whole lot of money has been spent on clean-up because we did not learn that lesson soon enough—and there are days when it appears that we have not yet learned that lesson.