Missouri Caves, Karst and Springs
Intro to Karst Topography
There are many kinds of karst. The name itself comes from the Karst region of Slovenia, along the Adriatic coast, where the landform was first noted. Karst is any terrain based on a layer of soluble bedrock, usually, though not always, of carbonate rocks. In the American Midwest, karst forms on limestones (calcium carbonate) and dolomites (magnesium calcium carbonate.) [Some areas of Texas, Oklahoma and New Mexico have gypsum karst. Pseudokarst is a term used for cave areas not formed primarily by solution, such as lava tubes, littoral (sea) caves, talus caves and suffosional (dirt pipe) caves.]
The precise erosional forms that the karst takes depends on many variables. The mechanical structure and chemical composition of the rock, the local climate and temperature range and the amount of vegetation and rainfall a region has all determine how fast a carbonate landscape erodes. Karst along a seaside is quite different than that inland, and tropical karst does not resemble karst in temperate or sub-Arctic zones. Landforms in zones with earthquake activity are quite different than that in quiet zones, or places where mountain-building have turned the layers of carbonate rock on edge. Some karst is formed as a result of sulfuric acid welling up from below instead of carbonic acid percolating down from above. Some places in the American West are a mix of the two processes.
But the karst of the Missouri Ozarks is almost textbook and is characterized by well eroded rolling hills, deep hollows, springs, caves, sinkholes, losing streams, natural bridges and tunnels. [The typical Missouri cave has a bluff entrance with a stream coming out. In other states the typical entrance is a sinkhole. Missouri has very few deep sinkholes, or pits, but many large caves and springs.]
Definitions
- A spring is a natural resurgence of groundwater, usually along a hillside or from a valley floor. [The term "effluent cave," used in Missouri, should be replaced with "resurgence," a more accepted term in karst geology and hydrology. Effluent usually connotes a polluted stream.]
- A cave is an air-filled underground void, large enough to be examined in some way by man. [Some state cave surveys have minimal length or depth definitions of cave, but they are not absolutely necessary.]
- A sinkhole or sink is a collapsed portion of bedrock above a void. Sinks may be a sheer vertical opening into a cave, or a shallow depression of many acres.
- A losing stream is one with a bed with allows water to flow directly into the groundwater system. There are many chert bottomed losing streams in the Ozarks.
- A natural bridge or tunnel is a void beneath still standing bedrock, usually of short extent, and allowing human passage from one end to the other, at least part of the time. A natural bridge is somewhat shorter than a tunnel and is more inclined to be air-filled than partly water-filled.
- [A rock shelter or shelter is usually an erosional cave in a bluff, often used as a campsite by native Americans. Some solutional caves have very large entrances that were suitable as campsites too. A rocks helter might be defined as a cavity that is wider than it is long.]
A Cave Factory
Missouri, especially south of the Missouri River, has all the natural resources to make (in the words of Jerry Vineyard, one of our best known geologists and cavers) a wonderful cave factory. In order for temperate karst to form, there must be sufficient layers of carbonate rock (in Missouri, anywhere from none to thousands of feet thick); adequate rainfall (about 45 inches annually); a reasonable vegetative cover to provide humus and carbon plant debris (oak-hickory forest and grassland over much of the state); suitable entrances to the bedrock (faulting and dipping from the Ozark uplifts and seismic activity); and a variable climate (bored with our weather? Just wait a few minutes.).
Karst is formed when rainwater picks up carbon dioxide from the air and dead plant debris in the soil, then percolates through cracks dissolving the rock. The bedrock becomes saturated with water at some level, and dissolving continues as the water moves sideways along bedding planes (horizontal cracks between rock layers) and joints (or fractures) in the rock itself. These conduits enlarge over time and move the water via a combination of gravity and hydraulic pressure, further enlarging the conduits through a combination of solution and abrasion of water on the surrounding rock.
Eventually, much of this water under pressure reaches the surface of the land as a spring. A spring may emerge high on a cliff, at the base of one or even forced upward from below the level of the surrounding surface streams, depending on nature of the surrounding rock and the altitude of the groundwater level, with respect to the base level of the controlling stream in a drainage area. Often in Missouri, springs have little relationship to surface drainage because so much of our water movement is actually groundwater movement. In some areas of the Ozarks, more than 70 percent of all water goes underground via karst processes.
As groundwater levels in an area drop, more and more of the underground passage becomes air-filled. When it is sufficiently air-filled, springs become cave entrances, passable by humans. Other voids never develop a natural opening and are intersected by drilling, notably of wells looking for water. At this point, due to changes in chemical equilibrium underground, the resulting caves begin to fill with dissolved mineral called cave deposits or speleothems. [These deposits, which take many forms, are usually precipitated by the release of carbon dioxide from groundwater, causing calcite or aragonite crystals (calcium carbonate) to form. Many other minerals have been reported from caves.] Caves may refill with water or continue to dry out, or even cycle several times as water levels change. [Missouri caves exhibit long geologic histories, with repeated periods of sedimentation followed by erosion. You can see evidence of this in old red clays stuck to high ceilings and hanging flowstones that used to coat clay banks that are now gone.]
Erosion continues underground, and eventually a cave hollows enough for the roof to thin, and the cave collapses. Such cave collapse may actually unroof the cave if it is near enough to the surface, or simply form a slump in the level of the land. In either example, a sink forms. Natural bridges and tunnels can be formed as resistant remnants of a cave collapse, or independently, if a block of bedrock becomes cut off from the main land mass, and it is hollowed out by wind, ice wedging and rain.
Many karst areas have poor soil, and do not retain water easily, allowing it to go directly underground. Sinks also act as "swallow holes" or "swallets" for rainwater. [Some Missourians call these "influent caves," but that is not a mainstream term in karst geology and hydrology.] Some sinks take water under certain conditions, and resurge it at others. These reversible sinks, called estavelles, are among the curiosities of karst. Some springs in the Ozarks are periodic, or "ebb and flow" springs, whose discharge can be measured to rise and fall independent of local rainfall. Many theories, but no one knows why, for sure. Another oddity of Missouri karst is the karst window, where one may look into a cave or water filled sink below, but getting down there is another matter entirely.
Karst makes for beautiful scenery, but it is very vulnerable to groundwater pollution, due to ease of water flow. Natural filtration is nearly nonexistent in karst. To make matters worse, the use of cave conduits as natural sewer lines and sinkholes as garbage dumps in small towns and rural areas puts the local drinking water supplies at risk. It is only recently that these problems are being addressed. Urban expansion in karst areas often means the building of houses on land which cannot support them and problems with septic tanks, underground pipeline breaks and landfills.
Some of the most beautiful sites in Missouri are a result of karst processes, as are some of our most pressing land use problems. Public education about our karst jewels and our karst nightmares is one way the MSS tries to eradicate the problems by being part of the solution.
Some Easily Accessible Examples of Karst
Some of the best places to look at Missouri Karst are on public land, or have been developed, or set aside with access to visitors.
- Karst Landscape. Grand Gulf State Park, Hahatonka State Park.
- Springs. Big, Alley, Round--all National Park Service sites in the Ozark National Scenic Riverways. Also Greer Spring, (U.S. Forest Service, Winona Ranger District) Blue Spring (Shannon County--Missouri Dept. of Conservation) and Maramec Spring (St. James--The James Foundation).
- Caves. Bridal Cave, Camdenton, Mo., Fantastic Caverns, Springfield, Mo., Onondaga Cave State Park, Leasburg, Mo., Mark Twain Cave, Hannibal, Mo., Meramec Caverns, Stanton, Mo., Round Spring Caverns, Round Spring, Mo. These commercial caves (some private, some public) present a good cross-section of caves typically found in the state. Consult the commercial caves list for details on them.
- Sinks. The main feature at Grand Gulf State Park is the sinkhole collapse. Slaughter Sink and Conical Sink in Phelps County are impressive--Conical is visible from a public road, and Slaughter is on Forest Service land. Driving the public roads of Perry County, Mo., or Sinks Rd. in north St. Louis County, Mo., will give the observer a great look at a sinkhole plain. Sinks and sinkholes abound in the Ozarks.
- Losing Streams. Many intermittent Ozark creeks go underground, and re-emerge further down their course. Sinkin Creek in Shannon County and Hurricane Creek in Oregon County are well known losing streams, but there is really nothing to "see" at these places. Watch for signs of sinking creeks and losing streams as you hike throughout the Ozarks.
- Natural Bridges and Tunnels. Rockbridge State Park. As the name implies, there is a really nice natural bridge here, as well as much karst, and one of the longest caves in the state, toured by prior arrangement only during the winter, due to gray bats. Hootentown Natural Arch in Stone County, a local picnic spot, is one of the largest in the state.
- Estavelles. Ball Mill Resurgence near Brewer in Perry County, is a sinkhole which resurges after heavy rains, with the added feature that it becomes a "ball mill"--it tumbles stones in its basin quite violently and noisily under those conditions.
- Karst Windows. Devil's Well near Akers, in Shannon County on the Ozark National Scenic Riverways, and Schnurbusch Karst Window (SKW) in Apple Creek, Perry County, on the Catholic church grounds. Devil's Well lets the visitor look down another 80 feet from a viewing platform at the bottom of a sinkhole onto an underground lake. The SKW is a rock grotto where water emerges from one end of a cave, flows about a hundred feet, then goes back underground for several miles.
For more ideas on where to go, write to the public agencies with lands in Missouri, or go to the library and look up these books:
- Geologic Wonders and Curiosities of Missouri, by Thomas R. Beveridge, 1978, second edition revised by Jerry D. Vineyard, 1990, Missouri Dept. of Natural Resources, DGLS, Rolla, MO 65401.
- Springs of Missouri, Gerald Feder and Jerry D. Vineyard, 1974, 1982, MDNR, DGLS, Rolla, MO 65401.
- Exploring Missouri's Legacy: State Parks and Historic Sites Edited by Susan Flader, 1992, University of Missouri Press, Columbia and London.
- Cave Minerals of the World, Second Edition, by Carol Hill and Paolo Forti, 1997. National Speleological Society.
Adapted from the Ozark Caving website (see below) by Joel Laws, joel_laws@umsl.edu, with a few additions in brackets by William R. Elliott, October 1998
