New Mexico Bureau of Mines & Mineral Resources Bulletin 117 Geology of Carlsbad Cavern and other caves in the Guadalupe Mountains, New Mexico and Texas

Most speleothems are composed of calcite or aragonite, CaCO₃. Together, these two minerals probably comprise over 95% of all speleothemic material in caves. In Guadalupe caves the carbonate mineralogy is more complex than that in most other caves due to the effect of the magnesium ion in solution. Besides calcite and aragonite, the magnesium minerals hydromagnesite, Mg₃(CO₃)₄(OH)₂. 4H₂O, huntite, CaMg₃(CO₃)₄, and dolomite, CaMg(CO₃)₂, are present. Thrailkill (1971) tabulated the distribution of these five minerals for various types of speleothems in Carlsbad Cavern (Table 30) and found that, for the samples analyzed, flowstone-type speleothems are composed exclusively of calcite and aragonite, whereas moonmilk is composed exclusively of aragonite and the magnesium carbonate minerals. Other speleothems such as popcorn or wall coatings may be composed of one or a combination of these minerals. Tankersley et al. (1983) did a spectrochemical analysis of major and trace elements in a piece of aragonite flowstone from Carlsbad Cavern (Table 31).

TABLE 30—Number of occurrences of minerals in speleothems, Carlsbad Cavern. After Thrailkill (1971).

(click on image for a PDF version)

TABLE 31.—Major and trace elements in aragonite flowstone, Carlsbad Cavern. After Tankersley et al. (1983).

The depositional fabric of minerals making up carbonate speleothems takes a variety of forms. Huntite, dolomite, and hydromagnesite always form as extremely fine-grained precipitates of cream-cheese consistency, whereas aragonite usually forms acicular crystals or feathery arrays of crystals. In flowstone and dripstone travertine, the layers are usually composed exclusively of aragonite and calcite (Table 30), the calcite forming either as clear, columnar, "palisade" crystals with small "crystallite" terminations, or as felted bundles resembling coconut meat. In one 3 cm thick flowstone sample collected from Carlsbad Cavern, Folk and Asserto (1976) found that the felted "coconut-meat" crystals occurred at the base of the specimen and appeared to be the first type of material deposited after halts in crystallization, possibly during times when the surface of the speleothem had dried out. The distinctive fabric of palisade calcite crystals is believed to be caused by precipitation from thin water films that flow over the growth surface of the speleothem (Kendall and Broughton, 1977, 1978).

Deposition

The five carbonate minerals found in Guadalupe caves derive from vadose seepage along joints, bedding planes, and connected pores in overlying bedrock units of limestone and dolomite. Incoming solutions are charged with carbonic acid derived from carbon dioxide in the air and in the soil zone above the cave. Carbonic acid dissolves the limestone and dolomite rock below the soil zone and water entering the caves is enriched in the ions of calcium, magnesium, and bicarbonate (Table 28). Upon entering an air-filled passage, these solutions lose their excess carbon dioxide and precipitate their accumulated load of carbonate material. Thrailkill (1965b) found that the carbon-dioxide content of incoming drip water in Carlsbad Cavern is over 20 times that of the cave air.

The above carbon-dioxide scheme is the exclusive mechanism of carbonate deposition in many or most caves, but in Guadalupe caves another, perhaps even more important, mechanism is at work causing precipitation of calcium- and magnesium-carbonate minerals. Humidity in Guadalupe caves ranges from 34 to 100%, with a mean value of around 85-90% for air equilibrated with the cave environment (Table 4, Fig. 21); therefore, in most cave passages, evaporation of incoming cave water is also a major influence on precipitation. Thrailkill (1965b) calculated that in one area of Carlsbad Cavern as much as 40% of the carbonate deposition was caused by evaporation. Hill (1978b) determined that evaporation was a controlling factor in the deposition of carbonate minerals in Ogle Cave, with respect to both the kind of mineral expected and the speleothem morphology and type. Hill estimated that in a cave of 88% humidity, 10 times more evaporation will occur than in a cave of 99% humidity (as is common for caves in the eastern United States, such as Flint-Mammoth, Kentucky).

Stability

Evaporation is an important control on the type of carbonate mineral deposited because, during evaporation, water which remains saturated with respect to calcite will shift toward a relatively higher magnesium concentration. The concentration of magnesium ion in solution is the reason why the usually rare hydromagnesite, huntite, and dolomite are relatively common in Guadalupe caves. The process can be visualized by examining the phase diagram in Fig. 96. Following the dashed line in the dolomite stability field, note how the mineral species changes with a loss of carbon dioxide and an increase in evaporation. As ground water entering the cave degasses carbon dioxide, calcite is precipitated first and the magnesium ion increases relative to the calcium ion (a move down on the vertical axis of the diagram). High-magnesium calcite (Mg-calcite) deposits next, then aragonite, then huntite, and finally hydromagnesite. A Ca++/Mg++ plot of vadose water in Carlsbad Cavern shows that most drip water and practically all poo1 water lie to the right of the Ca++ = Mg++ line (Fig. 97). This relationship suggests a late-stage evolution towards decreasing calcium content at approximately constant magnesium concentration, as would be expected to occur where calcite-saturated seepage is subject to evaporation.

FIGURE 96—Evolution of cave water in Guadalupe caves. As water loses carbon dioxide and as evaporation increases, the water moves down and then toward the left of the graph along the dashed line, depositing calcite, Mg-calcite, aragonite, huntite, and hydromagnesite, in that order. After Lippmann (1973).

FIGURE 97—Ca/Mg ratio in vadose water, Carlsbad Cavern. Data taken from Boyer (1964).

The geochemical shift toward water high in magnesium is reflected in the mineral trends of Guadalupe speleothems on both a small and a large scale. Low to high magnesium-mineral trends (e.g. calcite-aragonite, dolomite-huntite, aragonite-hydromagnesite) within individual speleothems have been reported by several investigators (Thrailkill, 1965b; Hill, 1973c; Folk and Asserto, 1976). Small-scale changes in mineralogical composition are especially pronounced in cave popcorn, where the general sequence of popcorn deposition is rhombohedral calcite surrounded by nodular over-growths of spicular aragonite capped by blebs of hydromagnesite moonmilk on the tips of the aragonite spicules (Fig. 107).

On a large scale, a transition from calcite dripstone to aragonite anthodites to hydromagnesite moonmilk can be followed in Carlsbad Cavern from the Big Room to the Lunch Room to Left Hand Tunnel, respectively. This trend may reflect changes in stratigraphy from reef to forereef (Fig. 18), or be a function of increased temperature and evaporation from the Big Room into Left Hand Tunnel (Fig. 19), or both. Another such transition in mineralogical composition can be seen in the Lake of the Clouds Passage where evaporation again seems to control mineralogy. Bushes of coralloidal calcite surround the Lake of the Clouds where the humidity is relatively high. Directly upslope from the Lake, the bushes grade into blunted aragonite spicules and then, higher upslope, delicate acicular-aragonite frostwork occurs (Fig. 93).

The effect of evaporation and carbon-dioxide loss on the various types of carbonate speleothems deposited can be understood by examining the carbon-oxygen-isotope values of speleothems (Fig. 98). (For an explanation of the carbon- and oxygen-isotope methods see Part I of this paper.) Most of the speleothems in Fig. 98 lie along Slope 1 which represents the deposition of speleothems due primarily to carbon-dioxide loss. Note that only popcorn and moonmilk lie on Slope 2, which represents the deposition of speleothems due primarily to evaporation. These findings agree with Thrailkill's (1971) mineral-occurrence chart (Table 30), where popcorn and moonmilk are composed of aragonite, hydromagnesite, huntite, and dolomite, minerals that are known to deposit by evaporation from solutions high in magnesium. Of some surprise is the occurrence of aragonite anthodites along Slope 1, which implies precipitation primarily by carbon-dioxide loss rather than by evaporation. Evidently, aragonite in Guadalupe caves can form with or without evaporation being directly responsible for its precipitation.

FIGURE 98—Carbon-oxygen-isotope compositions of different speleothem types, Carlsbad Cavern (L. González's unpubl. data 1985, and this study). (click on image for a PDF version)

Speleothems

The number of carbonate minerals in Guadalupe caves is small, but the variety of mechanisms that control carbonate deposition causes a large number of speleothem types. Five basic hydrologic mechanisms, dripping water, flowing water, seeping water, pool water, and condensation water, influence the type of speleothem. Examples of primarily dripping-water speleothems are stalactites and stalagmites which are elongate in the vertical direction of dripping. Flowstone forms from thin films of water which uniformly flow over cave walls or floors and deposit thin layers of travertine. Helictites twist in every direction because they grow by water seeping through tiny, internal, capillary canals. Cave rafts are flat, planar speleothems because they form on the surfaces of cave pools, and rims are deposits which form where water is condensing on bedrock or speleothems. Compound speleothemic shapes result where two or more of these basic mechanisms are operating simultaneously. For example, a drapery speleothem is created when a drop of water flows down an inclined ceiling and then drips to the floor. Shelfstone is a morphological extension of a cave raft produced when water moves by capillary action onto the upper surface of the raft, thereby increasing the raft in thickness. Cave pearls form when water drips into a shallow pool. And so on.

Added to these five basic hydrologic mechanisms are other factors, such as bedding, limestone porosity, and evaporation, which influence speleothem type and shape. Bedding can influence speleothem deposition in a number of ways. In Flint-Mammoth Cave, Kentucky, an impermeable sandstone caps the caves so that speleothem-depositing water enters the system only where the sandstone is truncated by valley erosion. The silty Yates and Tansill Formations, on the other hand, are sufficiently fractured and bedded so that surface water easily moves downward along joints and dipping bedding planes. Vadose seepage thus enters the underground throughout the whole Guadalupe Mountain recharge area, the diffuse penetration accounting for the fact that in Guadalupe caves speleothems occur practically everywhere and in great profusion (Palmer, 1975).

Bedding and permeability factors can influence cave mineralogy in other ways. In Carlsbad Cavern the type of speleothem changes as one traverses from the bedded-backreef Tansill and Yates Formations at and near the cave entrance down into the unbedded Capitan Limestone (Hill, 1972). Speleothems found in the impermeable (0.01 millidarcies) Tansill Formation at the cave entrance are primarily travertine deposits—dripstone which drips from ceiling joints, or flowstone which issues forth along horizontal bedding planes and flows down along cave walls (Fig. 99). As one descends along the Main Corridor into the unbedded, more permeable (14 millidarcies) Capitan reef limestone (just beyond the Whale's Mouth), popcorn becomes increasingly abundant on cave walls. The pattern of vadose-water flow creating these different types of speleothems can be visualized as follows: water from the surface descends into Guadalupe caves mainly along vertical joints and deposits dripstone (rows of stalactites and stalagmites) wherever the joint intersects a cave passage; or, where water moves along a bedding plane, flowstone can form below where the bedding plane intersects the cave (Fig. 100). But, if solutions move down far enough to reach the massive Capitan Limestone, they no longer intersect bedding planes but instead diffuse through interconnected pores in the limestone and move toward the cave walls where they form popcorn-type speleothems.

FIGURE 99—Flowstone issuing from a bedding plane, Tansill Formation, Carlsbad Cavern. Photo Alan Hill.

FIGURE 100—Influence of bedding and permeability on speleothem type, Carlsbad Cavern. After Hill (1972).

Bedding and dip are factors which can control the position and quantity of travertine deposited. Along the joint passage connecting the Main Corridor with the New Mexico Room, a cascade of massive dripstone and flowstone covers the left (west) wall of the passage, whereas the right (east) wall is devoid of speleothems. This distribution of travertine is produced by the intersection of the passage with the eastward-dipping beds of the Bell Canyon(?) Formation. Water moving downdip along bedding planes intersects the west wall and deposits its load of calcium carbonate there. Along the east wall, where beds intersect the passage at an obtuse angle, the water supply is cut off from the wall by the passage void, resulting in absence of speleothems. A similar situation exists both along Left Hand Tunnel and the south end of the Big Room, where dipping forereef beds divert water away from the reef and produce an absence or an uneven distribution of travertine.

The types of carbonate speleothems in Guadalupe caves are discussed in alphabetical order. The classification scheme of Hill and Forti (1986) is used throughout for speleothem types and subtypes.

Anthodites

Anthodites (anthos = flower) are speleothems which consist of clusters of colorless to white, needle or quill-like, crystal sprays. "Frostwork" is the term used for the acicular, needle-like variety of aragonite anthodite that resembles cactus or thistle plants (Pl. 11B). Anthodites are found in practically every Guadalupe cave, but form particularly exquisite arrays in the Medusa Room of Spider Cave and in the Christmas Tree and Bifrost Rooms of Carlsbad Cavern (Hill, 1979, 1982a). In the Medusa Room, a spray of aragonite frostwork almost 0.3 m long rests on top of a ledge. In the Christmas Tree and Bifrost Rooms, aragonite stalagmites up to 1.2 m high look like real Christmas trees, complete with spiny branches and needles. When flashed with a strobe light, the Bifrost Room frostwork displays a green phosphorescence, especially on recently formed growth tips.

Anthodites are believed to form where slowly moving, thin films of water seep between and over the surface of individual crystal needles, thus increasing the speleothem in breadth and height. Stalagmites often take on spiny, Christmas tree-like shapes because drops of water in overhead stalactites precipitate their load of calcite first and become increasingly enriched in magnesium; as the overhead drops fall to the floor and stalagmites are built up, this enrichment favors the deposition of aragonite over calcite.

Balloons, Cave

Cave balloons are roundish, thin-walled speleothems with gas inside of a mineralized, bag-like pouch. The best balloon specimens in Guadalupe caves occur in the Left Hand Tunnel of Carlsbad Cavern. These balloons are composed of hydromagnesite, are up to 2.5 cm in diameter with an approximate wall thickness of 0.1 mm, and are white with a satiny or pearly luster (Hill, 1973b, d; Fig. 101). One of the hydromagnesite balloons in Left Hand Tunnel hangs from the tip of an acicular aragonite needle on a small Christmas tree stalagmite, itself fed by drops from a calcite stalactite. Other, less perfect, deflated or desiccated balloons have been observed on the northeast wall of the New Mexico Room (C. M. Seanor, pers. comm. 1985), in an area about 1 m² in the Grinder Passage, Virgin Cave (H. DuChene, pers. comm. 1985), and in a number of places in Lechuguilla Cave (Hill, 1986).

FIGURE 101—Hydromagnesite balloon 2.5 cm in diameter, Left Hand Tunnel, Carlsbad Cavern. Photo Pete Lindsley.

Cave balloons are believed to be an extremely short-lived speleothem, transient in the time it takes for their initial growth and also in the time it takes for them to desiccate and disintegrate. Balloons may possibly grow when incoming solutions under pressure inflate carbonate moonmilk material. Moonmilk is usually a cryptocrystalline substance which exhibits a high plasticity when it contains just the right amount of water (Bernasconi, 1961). The plasticity of moonmilk increases when the aqueous phase comprises about 35% of the total mass, attaining a maximum at 37-40% and then decreasing rapidly up to 65%. If water under pressure reaches moonmilk blebs on the tips of popcorn, the plastic moonmilk can blow up in exactly the same manner as does a rubber balloon. In a relatively low-humidity cave like Carlsbad or Virgin, cave balloons can quickly dry, crack, deflate, and change in luster. Between 1973 and 1981, a small hole developed in one of the hydromagnesite balloons in Left Hand Tunnel and, during this time, the balloon also changed from a translucent to a more opaque luster (Hill, 1973d, 1981b).

Boxwork

Boxwork is so named because of its resemblance to a maze of crisscrossing post-office boxes. Thin, compact blades of crystalline material stand out in relief as a network of intersecting fins or plates corresponding to fractures in bedrock. Boxwork is not a common speleothem type in the Guadalupe Mountains because of the general lack of bedrock fracturing in the caves, but one small exposure of boxwork fins a few millimeters in width has been found in Lower Devil's Den, Carlsbad Cavern, near the calcified siltstone-cave rafts. Also, a small amount of boxwork has been noted in Wind (Hicks) and Spider Caves, the latter occurrence with fins 3 cm long and 1 mm thick.

The association of the boxwork with calcified siltstone in Lower Devil's Den may not be a coincidence. The contact zone between the shelf facies and reef core has been an area of past weakness. The calcified siltstone-cave rafts are found along this contact, and breccia deposits are there as well (Figs. 30, 40). A zone of weakness is exactly where one would expect bedrock fractures to develop and boxwork fracture fillings to form.

Coatings and crusts

Perhaps the most non-descript of all speleothems are the light-colored carbonate coatings and crusts which line the walls, ceilings, floors, and pools of most Guadalupe caves. This type of speleothem is formed either subaerially or subaqueously.

Subaerial coatings and crusts—Subaerial coatings line cave walls and ceilings, acting as a backdrop for the more showy popcorn or helictite speleothems. Of mineralogical note are the dolomite coatings identified by Thrailkill (1968) in Carlsbad Cavern. The dolomite has acicular crystals of aragonite in the coating and, for this reason, Thrailkill speculated that the dolomite had formed from the aragonite in the presence of high-magnesium solutions.

Floor silt in Guadalupe caves is often covered with a crust of white carbonate material (Fig. 36). These crusts form when carbonate-bearing solutions seep up through the silt toward the dry cave passage and deposit their mineral load at the surface of the silt. The crusts covering the silt are a few centimeters to tens of centimeters thick. Perhaps the best display of this type of crust is in Lower Cave, Carlsbad Cavern, where it overlies some of the thick silt banks (Fig. 34).

Subaqueous coatings and crusts—In the underwater variety the carbonate material lines the sides and floors of cave pools. The most notable example of such pool linings are the "clouds" of the Lake of the Clouds Passage, Carlsbad Cavern (cover photo). The calcite of the cloud linings is fine-grained, having formed from numerous crystal nuclei under supersaturated conditions. Orange clouds also occur at the bottom of Lechuguilla Cave (D. Davis, pers comm. 1986).

Conulites

A conulite is a conical shell of carbonate material which visually resembles an ice-cream cone with its apex pointing down. Conulites are rare speleothems originating as drip tubes in soft sediment that becomes subsequently lined with calcite. When the sediment (usually mud or sand) is eroded away, the calcite lining is left exposed as a hollow, cone-shaped speleothem.

Two conulite localities are known in Carlsbad Cavern. The most unusual one is in Lower Cave, in the fissure passage leading from the bottom of Mabel's Room toward the Boneyard. This particular conulite is unique because it has formed in moonmilk rather than in sediment (Hill, 1984a). The conulite in moonmilk is shaped like a horn coral (Fig. 102); it is 5-12.5 cm high, 6-7.5 cm wide across its top, and has a wall thickness of less than 1 cm. A piece of light-green, waxy, endellite clay is attached to a lower, outside portion of the conulite.

FIGURE 102—A conulite in moonmilk, Lower Cave, Carlsbad Cavern. Conulite is 12.5 cm high. A mummified bat is in lower left of figure. Photo Cyndi Mosch Seanor.

More typical conulites occur in the Balcony of the New Mexico Room, in sandy floor just past the climb up into the first room. The largest of the four conulites is 25 cm deep and 15 cm across the top. Shifting of the overhead drip point has created vertical flutes on the insides of these conulites.

"Bird baths" are low-angle, bowl-shaped conulites which form in soft sediment. Tiny bird baths, 2 to 3 cm across and 2 cm high, have been found in floor silt at the bottom of the first pit in the Hall of the White Giant, New Section, Carlsbad Cavern.

Coralloids

"Coralloid" is a collective term used for a number of morphological varieties of nodular speleothems. This type of speleothem has been discussed above in Part I, where it was related to late-stage speleogenesis events. Coralloids also relate to other problems, such as air-flow direction (Fig. 24) and past climate. Like coatings and crusts, coralloid speleothems can be either subaerial or subaqueous in origin.

Subaerial coralloids—An estimated 95% of all coralloids in Guadalupe caves are subaerial. They assume a variety of unusual and fascinating shapes depending on their history of development. Most common are the grape-shaped (Fig. 103), coral-like (Fig. 104), or popcorn-shaped coralloids (Fig. 66, Pl. 11B). Four morphological variations of popcorn coralloids are button popcorn, flat-bottomed popcorn, palm-stem popcorn, and calcite blades.

FIGURE 103—Soda-straw stalactites and grape coralloids. Photo Kenrick Day.

FIGURE 104—A growing coral frond, Ogle Cave. Photo Pete Lindsley.

"Button" popcorn is a type of knobby popcorn with concentrically plicate surfaces which sometimes appear to be twisted or folded (Thrailkill, 1953). Good examples of this type of popcorn occur in Manhole, Virgin, Three Fingers, Mad Russian, Lechuguilla, and Wind (Hicks) Caves (Hill, 1976c).

"Flat-bottomed" popcorn consists of clusters of popcorn which terminate in flat, horizontal tiers. The popcorn in the clusters grows on limestone ceiling and wall pendants, and the clusters get continually thicker down to their horizontal terminations. Flat-bottomed popcorn can be observed in many places along Left Hand Tunnel, the Big Room, Lower Cave, and Secondary Stream Passage of Carlsbad Cavern. This form is best developed where a main passage meets a side passage (e.g. at the entrance to Billing Dove Tunnel, Big Room).

"Palm-stem" popcorn is similar to flat-bottomed popcorn except that the flat surfaces are tilted from the horizontal so as to resemble tiers of cut palm-stems. Palm-stem popcorn often extends out from the sides of walls or stalagmites at angles of approximately 60°. Two of the best places to observe this kind of popcorn are in the Big Room along the trail by the Lion's Tail and along the south wall of Jim White's Tunnel where the passage starts to descend at a steep angle. In the latter occurrence, the popcorn masses are tilted both in the horizontal and vertical direction.

"Calcite blades" are a type of monocrystalline popcorn composed of calcite rhombohedrons oriented at slightly different angles on the blade (Hill et al., 1972; Hill, 1978c; Fig. 106). In Guadalupe caves, "normal" nodular popcorn usually begins its growth as a monocrystalline rhomb (Fig. 105); then, as the shape evolves and becomes increasingly rounder, the popcorn assumes the rosette form of a calcite blade (Fig. 106), and finally the form evolves into a smooth, nodular shape. This gradual transition of form is beautifully displayed in Musk Ox Cave (Hill, 1976b) and in the Lunch Room of Carlsbad Cavern. In the Lunch Room there is, in addition to the first three transitions, an added sequence of aragonite needles overlying normal popcorn and blebs of hydromagnesite moonmilk on the tips of the aragonite needles (Fig. 107).

FIGURE 105—Monocrystalline rhombohedral popcorn, Appetite Hill, Carlsbad Cavern. Popcorn in the caves of the Guadalupe Mountains often begins its growth cycle as rhombohedral spar. Photo David Jagnow.

FIGURE 106—Facets of rosette-shaped rhombohedral calcite called "calcite blades," Appetite Hill, Carlsbad Cavern. Photo David Jagnow.

FIGURE 107—Diagrammatic presentation of five stages of popcorn growth in Guadalupe caves: (1) monocrystalline rhombohedrons of calcite; (2) calcite blades with rhombohedral crystal faces discernible; (3) "normal" rounded nodules of popcorn with microcrystalline, radial structure; (4) spicules of aragonite; (5) berry-like terminations of hydromagnesite moonmilk.

In Guadalupe caves, subaerial popcorn forms by (1) water seeping through wallrock or speleothems, (2) thin films of condensation water, (3) splash from overhead dripping, and (4) splash from a pool. All four mechanisms have one thing in common: they all involve thin films of water as the critical criterion for popcorn deposition. When thin films of water move over surface irregularities in the limestone, more carbon-dioxide loss and evaporation take place at the apex of the irregularity than at the base, and deposition thus occurs preferentially at the apex and a nodular shape eventually results (Thrailkill, 1965a).

Seeping water (mechanism 1) explains popcorn which has pushed gypsum rinds out from cave walls (Fig. 66). It is probably the most universal mechanism by which popcorn has formed in Guadalupe caves, although this mechanism may have been operative primarily in the past, during or just after the time when the water table descended out of a passage and the limestone was losing its interstitial water. This mechanism, and also especially mechanism (2), is probably responsible for much of the popcorn associated with the condensation-corrosion "popcorn line" of Hell Below Cave and the Left Hand Tunnel-Big Room of Carlsbad Cavern (see Part I of this paper for a discussion of the condensation-corrosion process). Acidic condensation water corrodes the limestone and speleothems on or near the ceilings where CO₂ levels are high (Table 5), and the water runs down the side of the wall to form popcorn below the area of corrosion. A good place to view actively forming condensation popcorn is in the Left Hand Tunnel near the second bridge, where water droplets are condensating on the eastward side of a limestone pendant, while flat-bottomed popcorn is growing on the bottom and westward side of the pendant.

Thrailkill (1965b) thought that most of the popcorn in Carlsbad Cavern had formed by mechanisms (3) or (4). It would have been more correct to say that most of the popcorn forming today grows by splash, primarily by mechanism (3), because mechanisms (1) and (2) are probably no longer operative on a large scale. Most actively growing popcorn occurs on the tops and sides of stalagmites where water is dripping from the roof and splashing onto stalagmites. Type (3) popcorn can be seen in the New Year's Eave Gallery of Hell Below Cave and in many places along the trail in Left Hand Tunnel of Carlsbad Cavern. It may also be seen in the Main Corridor by the Whale's Mouth, on the north wall, where dripping water has splashed on the apex of the stalagmite and has created a ring of popcorn on the wall concentric around the stalagmite's apex. Type (4) coralloids are the least common. They can be seen around and just above the edge of the pool at Devil's Spring in the Main Corridor and also in Lower Cave, in the "coral" alcove just below and beyond the Jumping Off Place.

Subaqueous coralloids—This type displays cauliflower- or grapefruit-like surfaces and shapes. Good examples are at Devil's Spring and about midway along Left Hand Tunnel, Carlsbad Cavern, and in Virgin Cave (Pl. 13B).

Tower coral is a variety of subaqueous coralloid which sticks straight up like so many toy towers (Fig. 108), except in places where constant dripping has caused coralloid growth to incline away from the vertical. Cross sections of tower coral exhibit a radial, layered structure characteristic of all coralloids, but the layers are elongate in the vertical direction rather than symmetrical as in most coralloids. Black (1956) referred to the tower coral in the Chinese Wall section of New Cave as "free, gravel-like accretions with rough granular surfaces." Hill (1978b) reported tower coral in Ogle Cave and also in the Black Forest section of New Cave (Sheet 7), and related both occurrences to high evaporation rates in these caves. According to Hill's interpretation, tower coral forms in shallow pools where the top part of the pool is supersaturated with calcite compared to the less saturated, lower part of the pool; precipitation is greatest along the top of the growing coralloid and a vertical shape is favored.

FIGURE 108—A "forest" of tower coral, Ogle Cave. Photo Pete Lindsley.

Popcorn growth and air flow—Oriented popcorn is popcorn which has grown on walls or speleothems in a direction which either points away from, or towards, the cave entrance or the direction of air flow in a passage. Hill (1978b) related entrance-oriented popcorn in Ogle Cave to differences in evaporation relative to the two cave entrances. Sides of speleothems facing the Ogle entrance and an inflow of dry, outside air have experienced greater evaporation, which in turn has favored slow-moving films of water and popcorn growth. Speleothem sides facing away from the Ogle entrance have experienced less evaporation loss and hence are covered with flowstone deposited from faster-flowing, thicker films of water. Queen (1981) likewise at tributed oriented popcorn to air flow and, based on the direction of popcorn growth in Carlsbad Cavern, constructed a map of atmospheric circulation in the cave (Fig. 24).

Flat-bottomed popcorn and palm-stem popcorn are varieties which are probably also related to air flow. The palm-stem popcorn in Jim White's Tunnel is located where air is ascending from a lower passage; the popcorn, in both its horizontal and vertical components, is oriented in the direction of ascending air. Air flow may move fine films of water to the far end of a growing popcorn nodule, thereby influencing its orientation commensurate with the direction of that flow. Flat-bottomed popcorn seems to be the most pronounced in areas of high air flow and evaporation, or at least in areas transitional between high and low air flow (such as the entrance to Billing Dove Tunnel). Martini (1986) attributed the growth of flat-bottomed popcorn ("trays" he called them) to evaporation and undersaturated solutions where growth is possible only upward and laterally along a rock pendant. In Carlsbad Cavern the undersaturated solutions are probably condensation water.

Popcorn growth and climate—Queen (1981) also noted the evolution of popcorn where rhombohedral calcite is overlain by nodular popcorn, acicular aragonite, and berry-like terminations of moonmilk (Fig. 107), and related these growth stages to past climatic events, the rhombohedrons corresponding to wet and humid conditions and the moonmilk blebs to relatively dry cave conditions. Monocrystalline rhombohedral crystals typically form in humid caves, and this first stage of popcorn growth may correspond to an earlier, wetter, climatic episode in the Pleistocene.

While Queen's hypothesis is probably valid, it needs to be tempered by other factors. The stage of evolution of a cave may influence the type of popcorn. In Fiume Vento Cave, Italy, where sulfuric-acid dissolution and condensation-corrosion are going on today, rhombohedral-popcorn growth is the norm, while the other, later types of popcorn have not yet developed (P. Forti, pers. comm. 1985). Also, there is, in real time, a change from high-calcium to high-magnesium concentration in the normal evolution of cave waters. Evaporation will cause chemistry changes in a drop of water so that it shifts from a calcium-precipitating solution to a magnesium-precipitating solution within a short time, producing first calcite and last hydromagnesite.

Coral pipes

Coral pipes are speleothems that outwardly resemble tower coral but differ from them in internal structure, occurrence, and origin. Coral pipes are thin, pipe-like structures which have a soft interior (usually mud) surrounded by a thin shell of calcite. They are essentially calcified pillars that develop where mud has been drilled by dripping water and where the outside calcitic shell has been deposited concurrently with erosion.

The only Guadalupe cave where coral pipes are known to occur is Carlsbad Cavern: in the Rim Room and New Mexico Room (C. M. Seanor, pers. comm. 1985), in the Big Room by the Bottomless Pit, and also in the Mystery Room near the Mabel's Room Overlook. The Mystery Room coral pipes are unusual in that they have formed in bat guano instead of sand or mud. These are 2-4 cm in diameter, about 20 cm tall, and possess a 3-4 mm thick shell of calcite surrounding a reddish interior of bat guano (Fig. 109).

FIGURE 109—Coral pipes in the Mystery Room, Carlsbad Cavern. Material in the center of the largest coral pipe (lower right) is bat guano. Photo Alan Hill.

Draperies

A drapery is a vertically folded, curtain-like speleothem that hangs from an inclined cave ceiling or wall (Fig. 110). When draperies are thin, translucent, and stained in bands, they take on the appearance of bacon; such "bacon" is especially beautiful when lighted from behind so that its translucency and color-banding are accentuated. Most draperies are composed of calcite, but one drapery in Pink Dragon Cave is reportedly composed of calcite and dolomite (B. Rogers, pers. comm. 1979). Draperies are essentially a composite dripstone-flowstone speleothem. When water flows down an inclined surface, it builds up ribbons of flowstone material along its sloping path of descent before it drips to the floor. Severely furled draperies form on steeply sloping walls, whereas fringed draperies form under gentler slopes where slow-moving droplets of water are held between individual serrations along the drapery's bottom edge. A good example of a fringed drapery occurs in the side passage off the Boulder Room in Ogle Cave.

FIGURE 110—A drapery formed along an inclined ceiling, Carlsbad Cavern. Photo Kenrick Day.

Practically every cave in the Guadalupe Mountains has drapery speleothems, the most notable specimens occurring at the Whale's Mouth and in the Dome and Mystery Rooms of Carlsbad Cavern, and also in Three Fingers, Hell Below, and Virgin Caves. In the Lake Room of Virgin Cave is a composite-drapery-column called the "Peppermint Tree." About a dozen draperies, stained in various shades of brown and tan, adorn the Peppermint Tree, looking like sticks of peppermint candy ready for the picking (Pl. 12). An exquisite lace-like drapery occurs in the very bottom of the Mystery Room, the lace effect having been created by alternating strips of crystalline calcite and moonmilk. When the moonmilk layers dried out and flaked to the floor, the strips of crystalline calcite were left, giving the speleothem the appearance of being edged with lace.

Flowstone

Flowstone is one of the most common speleothem types, covering the floors, embankments, and walls of numerous Guadalupe caves (Fig. 111). Flowstone is usually composed of calcite or aragonite (Table 30), but rarely it can be composed of the magnesium carbonate minerals huntite and dolomite. In caves such as the Flint-Mammoth Cave, Kentucky, or Blanchard Springs Cavern, Arkansas, massive flowstone deposits form "frozen waterfalls"—cascades of travertine deposited from solutions which issue out along bedding planes and flow over bedrock ledges. In Guadalupe caves, waterfall-type flowstone is uncommon because the reef and near-reef limestones are usually unbedded or only crudely bedded. One notable exception to this rule is the Chocolate Drop in the New Mexico Room of Carlsbad Cavern, a 7 m high and wide flowstone cascade chocolate-mocha ice cream in color. Other exceptions are the 4-5 m high frozen waterfall of Lake Cave in Slaughter Canyon, the colorful 6 m high Temple of the Fire God in Three Fingers Cave (Pl. 16A), and the 15 m high flowstone cascade in Lechuguilla Cave (Hill, 1986).

FIGURE 111—A cascade of flowstone, Hell Below Cave. Photo Alan Hill.

Flowstone, as is more typically seen in Guadalupe caves, occurs as a sheet-like deposit associated with stalagmitic travertine, either covering the sides of stalagmites as protruding bulges or extending away from the base of stalagmites along the floor. In some cases, flowstone covers clastic silt or bat guano; when this material is eroded away from beneath the flowstone, a canopy may be left as a projecting travertine sheet. One example of this type of clastic-related canopy flowstone occurs in New Cave where protruding travertine has been undermined of bat guano. A canopy deposit that is not related to clastic material is the "baldacchino canopy," which forms when flowstone reaches the surface of a pool. Baldacchino canopies can be seen along the lower edge of the Crystal Springs Dome stalagmite, next to the trail in the Big Room, and also in Billing Dove Tunnel, Big Room, Carlsbad Cavern.

A prevailing type of canopy-flowstone deposit in Guadalupe caves is the "bell canopy," good examples of which are in New, Ogle, Three Fingers, Pink Panther, Christmas Tree, and Virgin Caves. Bell canopies are essentially flowstone modifications of dripstone stalagmites and columns which assume an odd assortment of shapes, the most common being a bell-like figure (Fig. 112). Bretz (1949, p. 456) was the first to recognize the peculiar bell-canopy form: "differential solution has largely spared the top while eating away the sides, so that the form produced suggests a gigantic canopy umbrella or toadstool, the underside of the cap and irregular stem showing the concentric growth structure." Bretz thought that bell canopies formed when the host stalagmite or column was partly or completely submerged under water, but Black (1954, p. 140) proposed another, non-flood origin for this type of travertine: "a formation alternately spreading and contracting showing beautifully the effect of the increase and diminuation of water falling on them." Black's hypothesis of origin of this type of flowstone speleothem is essentially correct. Hill (1973a) named the form and elaborated on Black's hypothesis, invoking flow rates which equal or just barely exceed preceding flow rates so that new travertine layers deposit directly over previously precipitated layers but not beyond them. In this manner, carbonate material builds up to form laterally protruding ledges or projections of travertine. Hill went on to explain the prevalence of bell canopies in Guadalupe caves as being due to the relatively low humidity and high rate of evaporation.

FIGURE 112—A bell canopy, Three Fingers Cave. Photo Kenrick Day.

Infrequently, huntite and dolomite also form as flowstone. The fine-grained, moonmilk-like consistency of these minerals causes the flowstone material to swell and buckle, and then to dry and crack, the whole mass finally resembling a pile of corn flakes or Chinese fortune cookies (Fig. 113). Such buckled-flowstone deposits are called "crinkle blisters" and are best displayed in Wind (Hicks) Cave, in side passages off of Left Hand Tunnel, and in the New Mexico Room and Bifrost Rooms of Carlsbad Cavern (Hill, 1973c, 1976c).

FIGURE 113—"Crinkle blisters," crinkled flowstone composed of huntite and dolomite, in a side passage off Left Hand Tunnel, Carlsbad Cavern. Photo Pete Lindsley.

Oddly shaped or textured flowstone may be produced by other factors. In some places, such as near the Chinese Wall of New Cave, flowstone surfaces are covered with microgours—tiny rimstone dams which crenulate the surface of the flowstone. Microgoured flowstone may result from pulsating or irregular patterns of incoming water flow rather than from continuous flow that produces smooth flowstone surfaces. A high proportion of silt in the calcite making up flowstone can produce sharply pointed edges somewhat like the teeth of a saw blade (Fig. 114). Examples of this type of silt-laden flowstone exist in the New Mexico Room at the point of descent into the East Annex, and also in Lechuguilla Cave on ledges along the 50 m drop (Hill, 1986).

FIGURE 114—Sawtooth-like flowstone containing a high proportion of silt, New Mexico Room, Carlsbad Cavern. Photo Cyndi Mosch Seanor.

Folia

Folia are speleothems which resemble inverted rimstone dams or interlocking wavy ribs. Named for their resemblance to the rumpled leaves of a book, folia project downward and outward from ceilings and overhanging walls at an angle averaging (with much variation) about 20° from the horizontal. The bottom edges of folia may be horizontal or inclined several degrees.

The origin of folia is uncertain, but is believed to be due to a fluctuating water level. In all known instances, folia coexist with clouds and/or cave rafts. At first, cave rafts form as a thin, scum-like layer on the surface of a body of water. If the water level fluctuates upward so that it reaches a wall overhang or cave ceiling, the "scum" attaches to the ceiling along surface irregularities. As the water level in the pool slowly subsides, the precipitate is continually supplied to the lower edge of the forming folia, creating its usually horizontal bottom ridge, and additional material is pulled up onto the growing ridges by surface tension. Water flowing from above may accentuate the lateral rib shape and can also produce a non-horizontal bottom edge.

Folia are best developed in the Christmas Tree Room of Carlsbad Cavern; less well-developed folia occur nearby, off of the Lake of the Clouds Passage (Davis, 1970). The folia of the Christmas Tree Room have ribs averaging 5.0-7.5 cm in length. They are associated with cave rafts stranded on wall ledges.

Helictites

A helictite is a speleothem that can twist in any direction. Unlike stalactites, helictites are formed by seeping rather than dripping water. In the center of each helictite is a tiny capillary tube through which water moves under hydrostatic pressure to the outer growth tip. The growth is therefore not controlled by gravity and can proceed in any direction.

Helictites can be divided into four groups based on size and shape: (1) a filiform variety consists of tiny hair-like filaments; (2) a beaded variety is shaped like a string of rosary beads; (3) a vermiform variety is worm-like in appearance; and (4) a twig- or antler-like variety is thick and has straight stems and bifurcating branches. Guadalupe caves have all four varieties. Aragonite alternating with calcite is responsible for the "beaded" or "seaweed" filamental forms; aragonite makes up the radiating, beaded part of the helictite and calcite makes up the straight-stemmed part. Good examples of beaded helictites occur in Musk Ox Cave (Hill, 1976b), in the New Mexico Room of Carlsbad Cavern (Fig. 115), and in the Southern Splendor Passage of the New Section, Carlsbad Cavern, near the Hall of the White Giant.

FIGURE 115—Beaded helictites in a side passage off the New Mexico Room, Carlsbad Cavern. Photo Alan Hill.

Vermiform helictites are found in many Guadalupe caves, the "snake dancers" of Hell Below and Virgin Caves being two of the finest examples of this kind of helictite (Pl. 13A, Fig. 116). Other exceptional displays of vermiform helictites are in the Queens Chamber of Carlsbad Cavern, where thousands of helictites interweave on the ceiling above the trail, and at the Wall of Medusa, Spider Cave, where a tangled web of helictites adorns a 12 m long wall (Hill, 1979). Near the Wall of Medusa is the "Plumber's Nightmare," an aggregate of soda straws and helictites so interconnected as to resemble the pipeline under one's kitchen sink.

FIGURE 116—The "snake dancer" helictites of Virgin Cave. Photo Alan Hill.

Antler helictites have bifurcating branches up to 15 cm thick that resemble deer antlers. More often than not, the branches take off in an almost horizontal or vertical direction. The Scenic Rooms, Left Hand Tunnel, and Vegetable Room of the Lake of the Clouds area, Carlsbad Cavern, contain the best displays of this kind of helictite. Antler helictites seem to have a distribution similar to that of moonmilk and of deflected stalactites. The cause of this apparent connection is not clear. While some antler helictites (and deflected stalactites) contain moonmilk layers between crystalline layers, still others seem to be devoid of moonmilk. One cross-sectioned antler helictite was found to contain microscopic channels ("canalicules") radiating away from its central canal and towards the outer surface of the helictite, suggesting that perhaps capillary water seeps from the central canal through the canalicules to the outside of the helictite, thereby thickening it.

Moonmilk

This term describes aggregates of microcrystalline carbonate minerals of varying composition. Moonmilk is soft, plastic, and pasty when wet but crumbly and powdery when dry. Wet moonmilk looks and feels like white cream cheese when rubbed between the fingers, while dry moonmilk resembles talc. Texture, not composition, is implied by the term moonmilk. Nine mineral varieties of moonmilk have been described (Hill and Forti, 1986), but only aragonite, hydromagnesite, dolomite, and huntite have been identified in Guadalupe caves (Table 30; Hill, 1979).

Guadalupe caves possess an abundance of moonmilk. One walks through it, crawls through it, and finds it as tufts on the tips of popcorn or anthodites, as creamy-white flowstone, and even as cave pearls. Chalk-white flowstone "rivers" of moonmilk adorn the cave walls in Pink Dragon Cave. In Left Hand Tunnel of Carlsbad Cavern and in Hell Below Cave, moonmilk is so abundant in certain areas that one's boots inadvertently stick to it along the trail.

In Guadalupe caves, moonmilk seems to be most abundant in areas where the Capitan reef is interbedded with the more dolomitic backreef or forereef facies. A perfect example of this is Spider Cave, formed in dolomitic units of the Yates Formation. There piles of dolomite moonmilk are up to 2 m deep and stalactites are sometimes layered with moonmilk between and over layers of more crystalline calcite (Hill, 1979). Left Hand Tunnel, a passage which is developed along the reef-forereef-facies contact (Fig. 18), has such an abundance of moonmilk in reef sections that the trail was constructed with moonmilk in these areas.

Many investigators feel that moonmilk is the product of bacterial action. Gerundo and Schwartz (1949) found denitrifying bacteria in Carlsbad Cavern but did not relate these bacteria to moonmilk generation. G. Moore (pers. comm. 1970) identified possible organic filaments in hydromagnesite moonmilk collected in Left Hand Tunnel, Carlsbad Cavern. While some moonmilk production in Guadalupe caves may be aided by microorganisms, most of it is probably the result of direct chemical precipitation of the high-magnesium carbonate minerals such as hydromagnesite, huntite, and dolomite. It is the finely crystalline nature of these minerals that gives the moonmilk its pasty and plastic-like qualities.

Pearls, Cave

A cave pearl is a banded, pearl-like concretion that forms in shallow cave pools or floor depressions where water is dripping from above. Cave pearls can be spherical, cylindrical, or irregularly shaped, depending on the shape of their nucleus. They are usually polished to a high glow and their appearance is nearly identical to that of real pearls (Fig. 117). Cave pearls form around any nucleus, be it a grain of sand, a bat bone, or another speleothem (Hess, 1929; Baker, 1963). Black (1951a, 1952) speculated that spherical cave pearls require constant turning to maintain their roundness, dripping water being the most efficient turning mechanism. If a pearl cannot turn, then the lower side will grow flat while the upper side will become top-heavy with more precipitate matter. Pearls that are free to turn develop calcite "cups" or "nests" below them (Fig. 117).

FIGURE 117—A "nest" of cave pearls, Cave of the Madonna. Photo Kenrick Day.

Carlsbad Cavern is the classic locality for cave pearls. The Rookery of Lower Cave was lined with hundreds of cave pearl "nests" before the guides in the 1920's and 1930's handed most of the pearls out to visitors as souvenirs. Although the cave pearls in the Rookery are now but a meager remnant of what once used to be there, other occurrences remain undamaged, the most notable being the golf-ball-sized pearls in the Cave Pearl Room. Boyer (1964, p. 18) described these pearls as: "found at the foot of a large flowstone mass, up to 4 cm in diameter, remarkably spherical, and of a bluish-white color, the upper portion of each pearl generally being stained darker. Each one is free, but occupies its own facet, or 'nest,' amongst the others."

Other Guadalupe caves also contain cave pearls, but not in the same profusion as in Carlsbad Cavern. The Cave of the Madonna has its nests of cave pearls (Fig. 117), and so does Hell Below Cave. In Hell Below hundreds of marble-sized cave pearls used to adorn cave floors; the largest pearl was supposedly "as big as a billiard ball" (Nymeyer, 1938, p. 38). Cave pearls composed of aragonite moonmilk have been found in Able Goat Cave (Hill et al., 1972a), and white and dark gray cave pearls have been discovered in Lechuguilla Cave (Hill, 1986).

Rafts, Cave

Cave rafts are thin, planar deposits of crystalline calcite or aragonite that float on the surfaces of cave pools (Pl. 13B). Raft growth is oriented with the c-axes of calcite or aragonite crystals parallel to the water surface. Rafts grow on the surface of quiet cave pools until some force such as dripping water sinks them to the bottom of the pool. Repeated sinking of cave rafts at a drip point results in the rafts piling up on the pool bottom in conical masses called cave cones.

Cave rafts occur in many Guadalupe caves, either as Type I rafts in the cave walls, Type II rafts usually found in cave cones, or Type III rafts floating on the surfaces of small cave pools. The three types have different carbon-oxygen compositions (Fig. 98). Based on their carbon-oxygen signatures, Type I rafts are believed to have formed at the water-table surface during an early stage of speleogenesis (see Part I of this paper for the origin of the calcified siltstone-cave raft sequence). Type II rafts have the same oxygen composition as Type I, but have a higher σ¹³C value which suggests that when they formed the cave was experiencing more air flow and CO₂ loss. Type III rafts are different from Types I and II in both oxygen and carbon composition. Type III formed in pools of meteoric water (rather than at the water table) under conditions of increased air flow and passage dryness.

The best occurrence of Type III rafts is in the Left Hand Tunnel of Carlsbad Cavern. At the end of the Right Hand Fork, a large pool of water is almost completely covered with a sheet of calcite, looking much like a winter pond about to freeze over. Black (1953) reported another small pool with floating rafts of aragonite in Left Hand Tunnel.

Cones up to 4 m high and composed of Type II rafts are present on the Balcony overlooking the Lake of the Clouds, Carlsbad Cavern (Fig. 93). Cones up to 1.2 m high also occur in the East Annex of the New Mexico Room, and cones with exposed sides up to 0.5 m high occur in the Bell Cord Room. In all three instances, the cones are associated with corrosion features and are believed to have precipitated during the time when condensation-corrosion processes were active in these passages. In the Balcony of the Lake of the Clouds, aragonite anthodites are perched on the apices of some of the cones, having formed subaerially from the same drip points that formerly sank the cave rafts (Hill, 1981a). Volcano-like cones are present in the Bell Cord Room, at one locality as a cluster of about 35 separate cones and at another locality in a cluster of about 20 cones (Fig. 93). The cones in the Bell Cord Room are pastel yellow, peach, and brown, with individual rafts oriented at about a 45° angle to the vertical axes of the cones and with layers of dark-gray manganese alternating between calcite raft layers. These cones have drip tubes drilled down their apices. One hole in a 45 cm high volcano cone was plumbed at depth of 1.5 m, showing that the cave cone extends much lower than its visible base. Very large cave cones are known only from Carlsbad Cavern. One 12.5 cm high cone has been observed in a small pool in Three Fingers Cave (C. M. Seanor, pers. comm. 1985), and even smaller cave cones are known to occur in other Guadalupe cave pools.

Rims

A rim is a shell of material that lines bedrock or other speleothems and is scoured on the inside and rough and coralline on the outside. Rims may be vent-shaped, rimming aperatures where holes or small passages emerge into a larger passage or room (Fig. 118), or they may be projections of material rimming corroded speleothems (Fig. 63). Carbonate rims are known from a number of Guadalupe caves. The largest rim is about 1 m high and 6 cm thick at the base, and is located where the smallest of three ascending routes meets with the Cavernacle in Virgin Cave (D. Davis, pers. comm. 1985). In Carlsbad Cavern almost all of the carbonate rims are associated with corrosion features, an example of which is the rim on the stalagmite known as the Creeping Ear in the Lake of the Clouds Passage (Fig. 63). The rim lining the Creeping Ear stalagmite is 15 cm wide and 0.5 cm thick, and has faint growth layers parallel to its outer edge which appear to be composed of acicular aragonite. The stalagmite is corroded on the side facing the Lake of the Clouds and the rim projects outward from the corroded part, its smooth inner side being an extension of the corrosion. Several vent-shaped rims 3-4 cm high and 6 cm across occur high on the wall along the trail between Appetite Hill and the Boneyard (Fig. 118).

FIGURE 118—Vent-shaped rims on the wall, Appetite Hill, Carlsbad Cavern. Photo Cyndi Mosch Seanor.

Davis (1982b) proposed that rims deposit from condensation water where warm, moist air meets with cooler, drier air. Hill (1984b) discussed the factors of temperature, pressure, carbon-dioxide equilibrium, evaporation, and air flow as they relate to the origin of rims, and proposed a model of rim formation for the Lake of the Clouds area where rims line the flanks of corroded stalagmites such as the Creeping Ear. As warm, moist air moved up from the Lake of the Clouds, it collided with the Creeping Ear stalagmite, reached its dew point, and condensed on the surface of the stalagmite. This condensation water had a high dissolved content of CO₂ (Table 5) and, due to its acidic nature, corroded the part of the stalagmite facing the Lake of the Clouds. Air flow then moved this condensation water, now saturated with calcium carbonate, to the flanks of the stalagmite where the calcium carbonate precipitated as rim material.

Rimstone dams

Rimstone dams are barriers of calcite or aragonite that impound cave streams or shallow pools. Carbonate material builds up at the point where water overflows the dam and where turbulence and carbon-dioxide loss are the greatest. As dams grow upward, water always seeks the lowest outlet; hence dams build up evenly, with deposition alternating between shallow points on the dam.

Black (1951b) described the rimstone dams at Mirror Lake, Longfellow's Bathtub, and Devil's Spring, Carlsbad Cavern, and Hill (1977b, 1980a) described the 40 cm high rimstone dams in Damn Cave and the 1.8 m high dams in Hidden Cave (Fig. 119). The most impressive rimstone dam in any Guadalupe cave is the Chinese Wall of New Cave. The Chinese Wall is only about 10 cm tall, but it is unique in that it is severely convoluted and furled, some of the individual furls having curved all the way back on themselves so as to form vertical tubes (Fig. 120). If the furls in the Chinese Wall were stretched out straight, it is estimated that they would total about 10 m in length.

FIGURE 119—Rimstone dams in Hidden Cave. The height of the tallest dam is approximately 1.8 m. Photo Jeep Hardinge.

FIGURE 120—The "Chinese Wall" of New Cave. The dam is about 10 cm in height. Photo Jeep Hardinge.

The height and convolution of rimstone dams is related to the gradient of slope: the greater the slope, the higher the rimstone dams; the shallower the slope, the lower and more convoluted the dams. Steep dams curve gently length wise, corresponding to topographic contours, and they are convex at spill-over points. Shallow dams curve around and back on themselves much like the meanders of a low-gradient river. Where dams are subject to slightly aggressive water, individual furls may become isolated from the rest of the dam in a manner resembling an oxbow cutoff of a meandering river.

Shelfstone

Shelfstone is a flat carbonate deposit that is attached as a ledge or eave-like projection along the edge of a cave pool or other speleothems submerged in a cave pool. When shelfstone lines columns, candlestick-like forms result (Fig. 121); when it forms over the top of stalagmites, lily pad or coke table forms result. Shelfstone begins its growth when cave rafts or other material attach to the sides of a pool. These can build in thickness and width until they partially or completely cover the surface of the pool. Shelfstone ledges always denote a present or past pool level.

FIGURE 121—The "Candlestick," Cave of the Madonna. The shelfstone of the candlestick and the shelfstone along the wall mark a former pool level. Photo Kenrick Day.

Most small pools in Guadalupe caves contain at least some shelfstone. Good displays of shelfstone are in Sand Cave, in the bottom of the Cave of the Madonna, and in the Lake Passage of Cottonwood Cave. One coke table in the Lake Passage used to have a candle-shaped stalagmite in its center, but the stalagmite was vandalized; this was called the "Wine Table" (Hill, 1977a). Alternation of dark-gray manganese with white calcite layers in shelfstone in the Lake of the Clouds Passage, Carlsbad Cavern, shows that shelfstone growth starts evenly on both the bottom and top sides of the shelfstone, but as the deposit thickens, preferential growth occurs along the top side of the shelfstone.

A peculiar, crescent-shaped variety of shelfstone can be seen along the trail by Mirror Lake, Big Room, Carlsbad Cavern, and also in Cottonwood Cave. This type of shelfstone forms in areas where water drips into a shallow pool. The convex parts of the crescent shelfstone point consistently toward the location of the drip, and the crescent shapes are produced by "waves" or ripples of water radiating from the drip point and deflecting off the shelfstone.

Shields, Cave

Shields (sometimes called "palettes") are speleothems consisting of two parallel, spherical plates separated by a medial, planar crack. Viewed from above, a shield resembles a shield of armor; from below, it looks like a column without a top (Fig. 122). Massive flowstone usually hangs from the lower hemisphere of a shield, and if this travertine gets heavy enough, the shield will separate along its medial crack and its bottom part will crash to the floor.

FIGURE 122—A shield developed along a wall joint, Ogle Cave. Photo Jon Vinson.

Like helictites, shields grow by water moving under hydrostatic pressure through the speleothem's interior to its outer, growth surface. Unlike in helictites, the water does not move through a capillary tube, but instead seeps through a planar crack and so creates the shield-like shape of the speleothem. It used to be thought that shields were a very rare speleothem, but exploration in the Guadalupe Mountains over the last 20 years has substantiated that shields are common. Pink Palette (the cave derives its name from the shields present), Pink Dragon, New, Madonna, Ogle, Deep, Corkscrew, and Carlsbad are among a number of Guadalupe caves known to contain shields. In Deep Cave, two shields have formed along the cracks in a large stalagmite rather than along cracks in the wall; the shields, 2 and 0.3 m in diameter, are oriented perpendicular to each other (Hill, 1978a). In Pink Dragon Cave, five nearly vertical shields occur at the ceiling, four of which are aligned parallel to the passage direction (along a major joint trend) and one aligned perpendicular to this direction (Hill, 1977b). The Pink Dragon shields are spectacular in that they seem to "hang in thin air."

A "welt" is a nubin-like shield which forms along cracks in a column or a cave wall. Welts have been noted in Hidden Cave (Hill, 1980a), Ogle Cave (Hill, 1978b), and in the Guadalupe Room of Carlsbad Cavern. Welts and shields often grow next to each other, the welts being incipient shields formed along the same crack as the shields and by the same solutions under pressure. Usually the upper sides of welts and shields are covered with helictites.

Spar

As discussed in Part I of this paper, spar crystals occur in many Guadalupe caves, either as dogtooth-shaped crystals (Fig. 60) or as nailhead-spar linings. Good localities of dogtooth spar are in Carlsbad Cavern, Cottonwood Cave, Geode Cave, Three Fingers Cave, Crystal Cave, Idono Crystal Cave, Virgin Cave, Frank's Cave, and Pink Fink Owlcove. In Crystal Cave, the walls, floors, and ceilings are covered with dogtooth spar up to 55 cm long, the crystals "bristling in every direction like a million spikes" (Nymeyer, 1938, p. 40). Dogtooth spar crystals up to 35 cm long (some of them twinned) have been reported lining cavities of Idono Crystal Cave (DuChene, 1967). In Geode Cave, spar crystals up to 23 cm long adorn the walls (J. Burke, pers. comm. 1982).

Nailhead-spar linings are best developed in the Nailhead Spar Lining Room, at the very bottom of the Mystery Room, Carlsbad Cavern. Columnar nailhead spar with blunted facets forms as 7.5-10 cm thick linings on the ceiling of this room. In places this lining is still intact, but in other places sections of it have fallen to the floor, leaving a linoleum-like pattern on the ceiling.

Most of the spar crystals and crystal linings in Guadalupe caves are phreatic spar; that is, they formed in the phreatic zone under saturated conditions. Other spar, called "chenille spar" forms in cave pools, usually directly underneath shelfstone. Chenille spar does not have well-developed, external crystal faces, but rather looks somewhat like a subaqueous coralloid. However, these are not layered structures like coralloids, but single crystals of calcite. Chenille spar can be seen in Carlsbad Cavern in the New Mexico Room by the Chocolate Drop, and in the Big Room along the trail in the Polar Regions and near the Top of the Cross. The Park guides call the chenille spar at the Top of the Cross "hula skirts" because of its palisade-like appearance (D. and A. Cordera, pers. comm. 1985).

Small dogtooth spar crystals also exist as surface features on speleothems, causing them to sparkle with a velvety luster. "Velvet" spar crystals are approximately 1 mm from base to tip and grow perpendicular to the surfaces of speleothems. One of the best occurrences of cave velvet in Guadalupe caves is in the Texas Pit Passage of Lower Cave, Carlsbad Cavern.

Stalactites

Stalactites are the most familiar of all speleothem types, readily remembered for their icicle-like pendant shape and the fact that they hang "tite" to the ceiling. Stalactites in Guadalupe caves differ from those in other caves in that they often assume war-club-like shapes where the bottom part of the stalactite is thicker than the top part, or in that they are often deflected from the vertical so as to appear curved at their tips.

All stalactites begin their growth as soda straws, thin walled tubular stalactites with hollow centers (Fig. 123). Water droplets saturated with calcium carbonate collect on cave ceilings and then as carbon dioxide is lost, a thin film of carbonate material precipitates over the surfaces of the water drops. As more water accumulates in each drop, the drop becomes heavier and begins to oscillate. This causes the calcite film to spin up toward the ceiling and to adhere there by surface tension. When the drop falls to the floor, the carbonate film is left on the ceiling as a round rim of material—the initial ring of a soda straw. Subsequent precipitation extends the straw's length and water continues to move through the hollow tube and to the tip of the growing straw. If the center of the tubular stalactite is blocked, solutions flow down on the outside of the straw and create the carrot shape of a "normal" stalactite (Pl. 11A).

FIGURE 123—Crystals growing on the tip of a soda-straw stalactite, Carlsbad Cavern. Photo Pete Lindsley.

When a stalactite becomes submerged, a war-club shape can result. The "war clubs" in the War Club Rooms of Endless Cave, Hidden Cave, Pink Dragon Cave, and Carlsbad Cavern are related to a back-up of pool water where subaqueous coralloids coat the surfaces of the submerged stalactites. "Spanish moss," a variation of war-club stalactite, is known only from Cottonwood Cave (Hill, 1977a). These moss-like speleothems are milk-chocolate brown, porous, and very fragile; internally, they are composed of an inner stalactitic core overlain by a porous war-club-like covering, itself overgrown by the "Spanish moss" (Pl. 14A).

Deflected stalactites can be seen in many parts of Carlsbad Cavern (the Scenic Rooms, Left Hand Tunnel, Lower Cave; Fig. 124) and also in other caves such as New Cave and Christmas Tree Cave. They start out as curved soda straws and continue growing in this curved mode. The precise cause of deflected-stalactite growth is not known. Queen (1981) suggested that deflected stalactites are related to air flow, but, as Sanchez (1964) pointed out, curvature and orientation may be uniform in a single stalactite, but may differ in adjoining stalactites (within tens of centimeters). If deflection is connected to air flow as Queen suggested, then this connection is very subtle and must be related to extremely localized air-flow patterns. A deflected stalactite thickens by water flowing through the stalactite rather than over its surface, otherwise the deflected bottom would have a drapery build-up of material on its lower end. Perhaps solutions which thicken the deflected stalactite move through the stalactite via tiny channels ("canalicules"), like they probably do in antler helictites.

FIGURE 124—A deflected stalactite in Lower Cave, Carlsbad Cavern. Photo David Jagnow.

Stalagmites

Stalagmites are convex floor deposits which form when water drips from a stalactite or the cave ceiling. When drops of water impact the floor, carbon dioxide is expelled and calcium carbonate is precipitated. Stalagmites assume a variety of shapes depending on their particular history of development: broomsticks, totem poles, fried eggs, toadstools, and Christmas trees are some forms that occur frequently enough to have earned their own special label. The shape of a stalagmite depends on the drip rate and also on the crystallizing mineral making up the stalagmite. Aragonite forms spiny "Christmas tree" anthodite stalagmites, whereas calcite forms typically smooth-sided stalagmites. Totem-pole or broomstick stalagmites have small or non-existent counterpart stalactites; these stalagmites build up where water drips rapidly into a cave. Good examples of the tall, skinny, totem-pole variety of stalagmite are in Three Fingers Cave, Carlsbad Cavern, and Deep Cave (Fig. 125). Fried-egg stalagmites topped on their apex with a yellow "yolk" surrounded by "white" crystalline calcite can be seen in Pink Dragon Cave, New Cave, and in the New Mexico Room of Carlsbad Cavern. Hollow stalagmites are those composed primarily of aragonite or popcorn. This type is found in areas of pronounced corrosion of bedrock and speleothems, and is believed to be the result of acidic solutions dripping from the ceiling onto the stalagmites.

FIGURE 125—"Broomstick" stalagmites, Deep Cave. Photo Kenrick Day.

Stalagmites in Guadalupe caves are often massive. The Monarch in New Cave, according to the Guiness Book of Records, is supposedly the tallest stalagmite in the world, being almost 36 m high. Bell-canopy flowstone can modify the basically convex shape of stalagmites into bizarre and interesting forms. The Christmas Trees of New Cave and Christmas Tree Cave, Snoopy of Ogle Cave (Fig. 126), and the Clansman of New Cave (Fig. 127) are examples of composite stalagmites and bell-canopy flowstone. Burnet (1938, p. 382) made the Clansman famous by describing his first impression of it in these words:

. . . the sight would strike terror into one with a faint heart. The white mantle shrouded an older formation of dark yellow and brown, the whole effect being that of a huge, cruel face beneath the white robe. Small stalactites hung from the bestial upper lip, giving the appearance of monstrous fangs. The jutting chin hangs slack, and stains at the side of the mouth make the figure appear to drool. There could be no doubt that this figure should be called the Clansman, for the white robe and whole general effect was a perfect caricature of that fanatic nightrider of the past.

FIGURE 126—The stalagmite known as "Snoopy," Ogle Cave. Photo Pete Lindsley.

FIGURE 127—The famous "Clansman" stalagmite of New Cave. Photo Jeep Hardinge.

A stalactite joined with its counterpart stalagmite is called a column (Pl. 14B). Columns in Guadalupe caves assume incredibly large dimensions—sometimes over 20 m in height and 7.5 m in diameter (Fig. 128). The largest columns are typically aligned along ceiling joints where the greatest amount of water drips into the cave. Hill (1978b) correlated the location of the most massive columns and stalagmites in the Sequoia Room of Ogle Cave with a valley depression on the surface overlying the cave (Sheet 6). The depression has been responsible for directing water in along a ceiling joint and the columns have formed along this same joint.

FIGURE 128—The massive columns of Ogle Cave. A caver (arrow) serves as a scale. Photo Pete Lindsley.