DOCUMENT RESUME

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AUTHOR Davies, W. E.; Morgan, I. M.TITLE Geology of Caves.INSTITUTION Geological Survey (Dept. of Interior), Reston, Va.PUB DATE 80NOTE 21p.; From the series "Popular Publications of the

U.S. Geological Survey." Photographs may notreproduce well.

PUB TYPE Reference Materials General (130)

EARS PRICE MF01/PC01 Plus Postage.DESCRIPTORS Earth Science; *Geology; Instructional Materials;

*Physical Geography; Secondary EducationIDENTIFIERS *Caves; PF Project; *Speleology; United States

ABSTRACTOne of a series of general interest publications on

science topics, the booklet provides those interested in the study ofcaves (speleology) with a nontechnical introduction to the subject.Separate sections examine types of caves, how caves form, cavefeatures, minerals found in caves, uses of caves, and caves asnatural underground laboratories. The final sections present a briefdescription about caves to explore in the United States (with a mapshowing locations of known caves) and precautions to take whenexploring newly discovered or unattended caves. (LE)

************************************************************************ Reproductions supplied by EARS are the best that can be made *

* from the original document. *

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Geology of Cavesby W. E. Davies and I. M. Morgan

A cave is a natural opening in the groundextending beyond the zone of light andlarge enough to permit the entry of man.Occurring in a wide variety of rock types andcaused by widely differing geological proc-esses, caves range in size from single smallrooms to interconnecting passages manymiles long. The scientific study of caves iscalled speleology (from the Greek wordsspelalon for cave and logos for study). It isa composite science based on geology,hydrology, biology, and archaeology, andthus holds special interest for earth scientistsof the U.S. Geological Survey.

Caves have been natural attractions sinceprehistoric times. Prolific evidence of earlyman's interest has been discovered in cavesscattered throughout the world. Fragmentsof skeletons of some of the earliest manlikecreatures (Australopithecines) have beendiscovered in cave deposits in South Africa,and the first evidence of primitive Neander-thal Man was found in a cave in the NeanderValley of Germany. Cro-Magnon Man createdhis remarkable murals on the walls of cavesin southern France and northern Spainwhere he took refuge more than 10,000 yearsago during the chill of the ice age

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Interest in caves has not dwindled.Although firm figures for cave visitors arenot available, in 1974 about 1.5 millionpeople toured Mammoth Cave in Kentucky,and more than 670,000 visited CarlsbadCaverns in New Mexico, two of the mostfamous caves in the United States.

Types of Caves

A simple classification of caves includesfour main types and several other relativelyless important types.

Solution caves are formed in carbonateand sulfate rocks such as limestone,dolomite, marble, and gypsum by theaction of slowly moving ground waterthat dissolves the rock to form tunnels,Irregular passages, and even largecaverns along joints and bedding planes.Most of the caves in the worldas wellas the largestare of this type.Lava caves are tunnels or tubes in lavaformed when the outer surface of alava flow cools and hardens while themolten lava within continues to flow andeventually drains out through the newlyformed tube.

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Sea caves are formed by the constantaction of waves which attacks theweaker portions of rocks lining theshores of oceans and large lakes. Suchcaves testify to the enormous pressuresexerted by waves and to the corrosivepower of wave-carried sand and gravel.Glacier caves are formed by melt waterwhich excavates drainage tunnelsthrough the ice.Of entirety different origin and not to be

included in the category of glacier cavesare so-called "ice caves," which usuallyare either solution caves or lava caveswithin which ice forms and persists throughall or most of the year.

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4. .,."), , .."..14'4% Iii.7*Wave-cul cave, Lake Superior, Grand Marais,Michigan (Photo by W. E. Davies).

In desert areas, some shallow caves maybe formed by the sandblasting effect of siltor fine sand being biown against a rock face.These eollan caves, some of which arespectacular in size, are surpassed in numberby caves of other origins in most deserts.More common even in the driest deserts aresandstone caves eroded in part by water.particularly if the sandstone is limy. Cavescommonly known as "wind caves," such asthe one in Wind Cave National Park in SouthDakota, are named not for the mode oforigin of the cave but for the strong aircurrents that alternately blow in or out of thecave as the atmospheric pressure changes.Most wind caves are, in fact, solution caves.

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How Caves Form

The melt-water streams draining outalong the floor of a glacier cave or thesurging, pounding waves at the mouth of asea cave offer immediate evidence of theorigin of these caves. Solution caves, how-ever, have always been a source of wonderto man. How do these extensive, complex,and in some places beautifully decoratedpassageways develop?

Solution caves are formed in limestoneand similar rocks by the action of water; theycan be thought of as part of a huge sub-terraneah plumbing system. After a rain,water seeps into cracks and pores of soiland rock and percolates beneath the landsurface. Eventually some of the waterreaches a zone where all the cracks andpores in the rock are already filled withwater. The term water table refers to theupper surface of this saturated zone. Calcite(calcium carbonate), the main mineral oflimestone, is barely soluble in pure water.Rainwater, however, absorbs some carbondioxide as it passes through the atmosphereand even more as it drains through soil anddecaying vegetation. The water, combiningchemically with the carbon dioxide, formsa weak carbonic acid solution. This acidslowly dissolves calcite, forms solutioncavities, and excavates passageways. The re-sulting calcium bicarbonate solution is carriedoff in the underground drainage system.

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It was once believed that caves formednear the Earth's surfaceabove the sat-urated zonewhere the water moved down-ward through the cracks and pore spaces.This view, however, left many Cave featuresunexplained.

Why, for instance, are cave passagesnearly horizontal, in places crossing foldedor tilted rock structures? How would hori-zontal passages form at several different butpersistent levels? Recent studies of themovement and chemistry of ground waterhave shown that the first stage in cave devel-opment the dissolving of carbonate rocksand the formation of cavities and passage-waystakes place principally just below thewater table in the zone of saturation wherecontinuous mass movement of water occurs.

A second stage in cave developmentoccurs after a lowering of the water table(the water table normally sinks as the rivervalleys deepen). During this stage, the solu-tion cavities are stranded in the unsaturated

Horizontal orientation of cave network.

zone where air can enter. This leads to thedeposition of calcite, which forms a widevariety of dripstone features.

The chemical process causing deposi-tion of calcite is the reverse of the prccess ofsolution. Water in the unsaturated zone,which dissolved some calcite as it trickleddown through the limestone above the cave,is still enriched with carbon dioxide when itreaches the ventilated cave. The carbondioxide gas escapes from the water (just asit escapes from an opened bottle of sodapop). The acidity of the water is therebyreduced, the calcium bicarbonate cannotremain in solution, and calcite is depositedas dripstone.

Cave FeaturesThe decorative dripstone features are

called speleothems (from the Greek spelaionfor cave and theme for deposit). When thesestructures are highlighted by lanterns orelectric lights, they transform a cave into anatural wonderland.

The most familiar speleothems are sta-lactites and stalagmites. Stalactites hangdownward from the ceiling and are formed asdrop after drop of water slowly tricklesthrough cracks in the cave roof. As eachdrop of water hangs from the ceilinv., ,t losescarbon dioxide and deposits a film of calcite.Successive drops add ring below ring, thewater dripping through the hollow center ofthe rings, until a pendant cylinder forms,Tubular or "soda straw" stalactites grow inthis way; most are fragile and have thediameter of a drop of water, but some reacha length of perhaps a yard or more. The largecone-shaped stalactites begin as thesefragile tubes and then enlarge to cones whenenough water accumulates to flow along theoutside of the soda straws, Deposition ofcalcite on the outside of the tubes, most ofwhich are near the ceiling and taper down-ward, results in the familiar cone shapes.

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_ ..... )Columns up to 60 feet high in Carlsbad Caverns,New Mexico (photo by National Park Service).

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Stalagmites grow upward from the floorof the cave generally as a result of water drip-ping from overhanging stalactites. A columnfoms when a stalactite and a stalagmitegrow until they join. A curtain or draperybegins to form on an inclined ceiling whenthe drops of water trickle along a slope.Gradually a thin sheet of calcite grows down-ward from the ceiling and hangs in decora-tive folds like a drape. Sheets of calcite thatare deposited on the walls or floor by flowingwater are called llowstone. Rimstone damsare raised fencelike deposits of calcite onthe cave floor that form around pools ofwater.

Helictites are curious twisted or spiralingcylinders or needles. They apparentlydevelop when water seeps through theceiling so slowly that slight chemical orphysical changes can cause reorientation ofthe crystal structure of the calcite or gypsum.Cave corals, also formed by slowly seepingwater, are small clusters of individual knobs.

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Flowstone drapery in Jewel Cave, South Dakota(photo by W. E. Davies).

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Most cave passages contain deposits ofmaterial that have been washed into thecave. This material, known as cave fill, variesfrom sand and clay to stratified gravel. Thepebbles in these deposits often are highlypolished or frosted and sometimes are aslarge as 6 inches in diameter. Cave fills areparticularly noteworthy because they containmaterials that reflect a geologic history anda record of past climates of the surroundingarea.

Rock material produced by the collapseof the ceiling or walls of a cave is calledbreakdown and may range in size from platesand chips to massive blocks. Most break-down present in caves today appears to haveoccurred thousands of years ago. It is gen-erally associated with the early history ofcave development.

The size and depth of many caves in theUnited States are impressive. Seven caveshave more than 15 oassage miles. Thelongest is the Flint-Mammoth Cave system in

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Cave fill, Laurel Creek Cave, West Virginia (photo byW. E. Davies).

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Kentucky with more than 169 miles. Theother six are Jewel Cave in South Dakota(54.4 miles), Organ Cave in West Virginia(32 miles), Wind Cave in South Dakota (28.7miles), Cumberland Caverns in Tennessee(23.2 miles), Sloan Valley Cave system inKentucky (22.4 miles), and Crevice Cave inMissouri (20.8 miles).

The deepest cave in the United States isNeff Canyon in Utah. There, a depth of 1,189feet below the entrance is reached along asteeply sloping 1,700-foot passage. Thesecond deepest cave is Carlsbad Caverns inNew Mexico; its lowest point is 1,022 feetbelow the entrance. Ellisons Cave system inGeorgia, a close rival of Carlsbad, has adepth of nearly 1,000 feet.

The largest cave room Is in CarlsbadCaverns, where the Big Room covers 14acres. This room is 1,800 feet long andranges up to 1,100 feet wide. The maximumheight of the ceiling is 225 feet. The size ofthe Big Room, the length of the caverns (14.9miles, the 11th longest in the United States),and the depth probably make Carlsbad thebiggest cave in the United States.

Minerals Found in CavesMany interesting minerals are found in

caves in addition to the calcite which formsthe major features. Aragonite, a calciumcarbona'e mineral similar to calcite but notas common, often occurs in intricate neediesknown as anthodites. Gypsum (calcium sul-fate) and related calcium sulfate mineralsare next to calcite in abundance. Somecaves, although they are developed in lime-stone, have extensive passages lined withfine, curling growtheof gypsum flowers. Inother caves, selenite (a less common varietyof gypsum) forms long transparent rods ornests of fibrous crystals. Sulfates of sodiumand magnesium are also found in caves,although they are less conspicuous thangypsum. Iron minerals in the form of oxides(limonite) and hydroxide (goethite) occur in

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caves and in some places form stalactites.Manganese minerals in caves are commonlypresent as thin, sooty coatings on walls andceilings and in earth fills. Nitrocalcite (cal-cium nitrate) is abundant in earth fills inmany caves, but individual fragments aregenerally microscopic. Barite (barium sulfate)and celestite (strontium sulfate) also occurin earth fills. In some solution caves, clayminerals exist in relatively pure forms; theseinclude the less common varieties attapulgiteand endellita.

In deep caves encountered during min-ing operations, a number of ore minerals havebeen found in the decorative wall draperies.Most common are azurite and malachite(forms of copper carbonate). About 50 otherminerals also have been reported in cavedeposits.

7k . ..Crystals of dogtooth spar (calcite) from Jewel Cave.South Dakota (Photo by W. E. Davies).

Uses of Caves

Today, caves it the United States areused mostly for recreation by sightseers andexplorers or spelunkers (from the Latin wordspelunca, which means cave), but the mostenduring use of caves, from prehistoric timesto the present, has ueen for shelter.

Recently, a number of large, dry, andeasily accessible caves throughout theUnited States were designated as fallout

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shelters, and a large cave in southwesternVirginia is used as a natural tunnel by theSouthern Railway. Studies are underway inEurope to extend the use of caves for domes-tic cold storage. air conditioning, and water-supply purposes.

From the early 19th century through theCiA War, caves In KAntucky, Tennessee. Vir-ginia, West Virginia, Alabama. Georgia.Arkansas. and Missouri were import 4sources of nitrates, an essential ingredientof gun powder. Surface or near-surface ac-cumulations of nitrate salts form coatings onrock walls, fill cracks and crevices, and min-gle with cave earth. The origin of the nitratesalts is not clearly understood, but the saltsare believed to result from the action ofnitrifying bacteria on organic matter orhumus. Although no accurate records ofproduction were kept. it has been estimatedthat over 15,000 tons of niter earth (producing200 tons of potassium nitrate) were removedfrom Mammoth Cave in Kentucky between1811 and 1814.

Caves have also been a source of batguano, a material mined as a phosphatefertilizer in the Southern United States andMexico. In general. the ':., gest deposits haveoccurred in limestone caves within thi flightrange of the Mexican free-tailed bat.

Geological Survey Investigations

Scientists value caves as natural under-ground laboratories. Of paramount impor-tance is the fact that caves and other solutioncavities in limestone have a direct bearingon the underground water system. Cavernouslimestone strata are among the most produc-tive aquifers (water-bearing beds) in theUnited State., dnd are therefore importantsources of water. Because of this, U.S.Geological Survey research programs con-cerned with limestone regions commonlyinclude studies of the path, rate of flow.amount, and quality of water circulatingthrough caves and hidden passageways.

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Saltpeter vats from Wer of 1512, Mammoth Cave,Kentucky (photo by National Park Sorykol.

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Stream passage, Piercys Milt Cave, West Virginia(photo by W. E. bevies).

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Geological engineers and others concernedwith ground stability are aware that regionsunderlain by cavernous limestone presentspecial construction problems. Studies of thesubsurface conditions are especially impor-tant in areas of limestone and gypsum be-cause of the danger of ground failure andsubsidence.

Caves To ExploreThere are about 17,000 known caves in

the United States. They occur in every State'except Rhode Island and Louisiana. About125 caves have been opened to the publicfor study and enjoyment. Of these, 15 are innational parks or monuments, and 30 are In

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Cavern areas in the United States.

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State parks. The remainder are privatelyowned and operated. Most of these cavesare in the Appalachian Mountains, the OzarkMountains, the Black Hills, and the limestoneregions of Kentucky, Tennessee, and Indiana.

CautionExploring newly discovered or unat-

tended caves can be extremely dangerous!Through experience, a set of safety rules hasevolved thatif observedmay prevent ac-cidents. If you plan to go cave exploring:

Always tell someone where you are goingand when you can be expected to return;obtain permission from ti-,e1 owner of thecave for the visit.Respect gates, whether they are in thefield or at the cave entrance.Never enter a cave alone.

Always carry several sources of light; donot depend solely on flashlights.

Make sure you have proper equipment ingood working condition.Never go beyond your physical and tech-nical capabilities.For the sake of conservation, keep visitsto a minimum.

Better yet, meet with knowledgeable andexperienced cavers. Association with a groupof experienced spelunkers is the best safetyinsurance that you can have.

Caves are natural features and should beprotected, but many have been vandalized bycareless visitors or damaged by poorlyplanned commercial development. Somecavestave been stripped of speleothemswhich took thousands of years to form andin many places will not form again. All shouldtry to prevent this random destruction ofthese natural wonderlands. Follow the foot-prints of others; look but don't touch; bringaway only photographs: leave no evidenceof your visit.

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Selected References

Folsom, Franklin, 1962, Exploring AmericanCavesTheir History, Geology, Lore, andLocation; A Spelunker's Guide (rev. ed.):Collier Books, New York, 319 p.

Harrison, D. L., 1970, The World of AmericanCaves: Reilly and Lee Books (Division ofHenry Regnery Co.), Chicago, Illinois.152 p.

Mohr, C. E., and Sloane, H. N., eds., 1955,Celebrated American Caves: RutgersUniversity Press, New Brunswick, NewJersey, 339 p.Descriptions of about 20 caves or caveareas.

Moore, G. W., and Su Ilivan,G.N ., 1978.SpeleologyThe Study of Caves: ZephyrusPress, Teaneck, New Jersey, 2nd edition,150 p.

Sloane, H. N., and Gurnees, R. H., 1966,Visiting American Caves: Crown Pub-lishers, Inc., New York, 246 p.Listing and data on commercial caves.

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This oublicatron is one or a serms of general interest publicationsPredated by the U S Geologtat Survey to provide information aboutthe earth sciences. natural ge3OutCeS. an0 the environment To obtaina catalog 01 athin1011111Ine, In the Sete03 Popular Puhhe31$0113 Of

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Branch of Distribution Or Branch of DrsurbotionU S Geotogrcal Surveil, U S. Geological Survey1200 South Cads Sheol Dos 25286, Federal CenterArlington, VA 22202 Denver. CO e0225

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As IS* Nation's principal conservation spumy. the Departmentof the Interior hes responsibility for most Of Our nationally ownedpublic lends and natural resources. This includes fostatirl thewisest use of our land and water resources. protecting our flab andwildlife, preserving ths environmental and cultural voices of ournational parks and histOricat Plates. and providing for the enjoymint of life through Outdoor recreation. The Department assessesOur energy and mineral resources and wort* to assure that theirdevelopment is in the best interests Of all our people. The Diminomen' siso has a wislor responsibility for American Indian reservationcommunities and for people who live in Island Territories under

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