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

Topographic and geologic characteristics

Guadalupe caves differ from other cave systems in many ways. The following list categorizes these differences; it is based on the findings of Bretz (1949), Thrailkill (1964), Palmer (1975), Queen et al. (1977a, b), Jagnow (1977, 1979), Davis (1979a, 1980), Queen (1981), Wilson and Ash (1984b), and this study.

(1) Well-developed surface karst landforms such as towers and sinkholes are not prominent features in the Guadalupe Mountains.

(2) Caves lack a clear genetic relationship to surface topography. Intersections of caves with the land surface (entrances) are random and have no apparent relationship to recharge or resurgence points, ancient or modern.

(3) Few caves are known in the highest, southwestern part of the Guadalupe Mountains (e.g. the Guadalupe Peak region) and those are very small (e.g. Lower and Upper Sloth Caves, Dust Cave, and Williams Cave). Virgin Cave is the only extensive cave found west of Big Canyon (Fig. 5).

(4) Caves are located within 12km of the reef escarpment; most are within 4 km (Fig. 5).

(5) Guadalupe cave passages are often large, many exceeding 15 m in height and width. The Big Room of Carlsbad Cavern is 600 m long, 330 m wide, and 87 m high at its tallest point, the Top of the Cross.

(6) Large cave passages end abruptly. The south end of the Big Room, Carlsbad Cavern, and the Entrance Hall, Cottonwood Cave, terminate in unbroken rock, without breakdown collapse or major passage extensions. None of the caves contain extensive linear passageways in the style of Flint-Mammoth Cave, Kentucky.

(7) Walls of large passages are honeycombed with smaller passages called spongework or boneyard-interconnected, nontubular, solution cavities of varied size and irregular geometry arranged in an apparently random, three-dimensional, maze-like pattern (Fig. 15).

FIGURE 15—Crawling through boneyard, Carlsbad Cavern, Photo Cyndi Mosch Seanor.

(8) Guadalupe caves are deep, but not long. The caves descend via a series of drops of about 30-50 m each. The deepest known single drop in a Guadalupe cave is in the Cave of the Madonna, a total of 64 m. The Four O'clock Staircase, a slot-like series of connected drops in Virgin Cave, is 152 m deep. The deepest cave in the Guadalupe Mountains is Carlsbad Cavern, with a total depth of 312 m (Sheet 1).

(9) Large rooms and horizontal levels are connected or underlain by: (a) enlarged vertical fissures (e.g. Dean's Drop, Cave of the Madonna; Cable Slot, Carlsbad Cavern); (b) vertical tubular pits (e.g. Bottomless Pit, Carlsbad Cavern; the entrances to Deep and Ogle Caves); and (c) passages which descend at angles of 20-30° (e.g. Main Corridor, Mystery Room, Guadalupe Room, and Lake of the Clouds Passage, Carlsbad Cavern).

(10) Pits, tubes, and vertical fissures lack drains, and avenues for water escape seemingly disappear with depth. Vertical pits and fissures end in breakdown, fill, travertine, pinchouts, or unbroken bedrock (e.g. Nicholson's Pit, Carlsbad Cavern, both pinches out and is filled with break down).

(11) Caves reach a certain overall depth and then seem to descend no further even though the water table is below the lowest passage (e.g. the Lake of the Clouds, the lowest passage in Carlsbad Cavern, is about 30 m above the present water table in the Capitan reef aquifer; Sheet 1).

(12) Solutional features point to a non-vadose regime of cavern development. Vertical shafts, domepits, scallops, ripple marks, cutoffs, ceiling channels, flutes, horizontal grooves, and incised meanders are either rare or absent in Guadalupe caves.

(13) Cave sediment is sparse; where it does occur, it is almost always a coarse silt to fine-grained sand.

(14) Thick floor blocks of massive gypsum and thinner wall rinds of gypsum are conspicuous features in many Guadalupe Caves.

(15) Native sulfur occurs in Guadalupe caves either associated with massive gypsum or directly overlying bedrock and speleothems.

(16) Colorful, soapy or waxy clay composed of the minerals montmorillonite and endellite fills solution pockets in many Guadalupe caves.

(17) Travertine deposits are profuse. Guadalupe caves have the well-earned reputation of being among the most beautiful in the world.

(18) Cave limestone surfaces are often whitened, rounded, and eaten-away looking (e.g. Boneyard, Carlsbad Cavern). In some places the cave limestone is deeply etched and corroded (Figs. 16, 27).

FIGURE 16—Rillenkarren on limestone bedrock, Boneyard, Carlsbad Cavern. Photo Ronal Kerbo.

Caves studied

Caves visited during this investigation include: Big Door, Black, Carlsbad, Christmas Tree, Corkscrew, Cottonwood, Damn, Deep, Dry, Endless, Goat, Helen's, Hell Below, Hidden, Lechuguilla, Little Beauty, Little Sand, Madonna, Mad Russian (Hidden Chimney), McKittrick, Musk Ox, New, Ogle, Pink Dragon, Pink Fink Owlcove, Pink Palette, Pink Panther, Queen of the Guadalupes, Rainbow, Sand, Sentinel, Spider, Three Fingers, Virgin, and Wind (Hicks). Other caves mentioned in the text have been described by cavers or cited in the literature.

The primary focus of this study, Carlsbad Cavern, is depicted in two maps, a horizontal view (Sheet 2) and a profile (Sheet 3). Other maps included in this report are of Cottonwood, Ogle, New, and Dry Caves. Excellent maps and descriptions of many other Guadalupe caves have been given by Jagnow (1977, 1979).

Carlsbad Cavern

The entrance and upper levels of Carlsbad Cavern occupy a limestone ridge 240 m high and 2.4 km wide at the base. This ridge parallels the reef front and is truncated on its western side by Walnut Canyon and on its eastern side by the reef escarpment (Sheet 4, Fig. 17).

FIGURE 17—Diagrammatic presentation of levels and passages in Carlsbad Cavern with respect to surface topography in the Capitan reef and Gypsum Plain. (click on image for a PDF version)

Cave passages in Carlsbad Cavern trend in one of three directions (Sheet 2). Bat Cave, Left Hand Tunnel, Main Corridor, Guadalupe Room, Scenic Rooms, and Mystery Room all trend approximately N80°E or parallel to the reef front. Perpendicular to the reef, or trending about N15°W are Talcum Passage, Mabel's Room, the Cable Slot, Appetite Hill, the west end of Lower Cave, the west and north ends of the Big Room, and the Right and Left Hand Forks of Left Hand Tunnel. Oriented at approximately N40°E are Secondary Stream Passage, the New Mexico Room, parts of Lower Cave, and the middle section of the Big Room from the Salt Flats to the Polar Regions.

Four main levels of horizontal cavern development occur in Carlsbad Cavern: -60 m from the entrance (Bat Cave level); -120 m (New Section level); -230 m (Big Room level); and -260 m (Lower Cave level) (Sheet 3). Passages at different levels are connected by the Main Corridor and the Guadalupe Room—steeply dipping, parallel passages which both descend at an angle of 20-30°. The Bat Cave level corresponds to Spider Cave in altitude, and the Scenic Rooms correspond to the Lower Cave level (Table 3). By far the most dominant level in Carlsbad Cavern is the Big Room horizon which includes not only the Big Room but also the Mystery Room, New Mexico Room, Mabel's Room, Talcum Passage, Left Hand Tunnel, and Bell Cord Room.

Carlsbad Cavern is developed in the reef and forereef facies of the Capitan Limestone; the Tansill, Yates, and possibly the Seven Rivers Formations of the backreef Artesia Group; and possibly also the basinal Bell Canyon Formation (Figs. 17, 18). The Tansill crops out at the cave entrance and Tansill beds delineate the planar ceiling of Bat Cave. The Yates Formation crops out at many levels in the northern part of the cave, from the lower walls of Bat Cave down to a sandstone tongue at the top of the Guadalupe Room, or about 60 m in vertical extent. The Seven Rivers Formation may possibly extend below the Yates Formation in the area of the Lower Guadalupe Room and New Mexico Room (Wilson and Ash, 1984a). The white, clean sands of the Bell Canyon Formation possibly interfinger at the east end of the New Mexico Room and at the south end of the Big Room (Fig. 18).

FIGURE 18—Sedimentary-facies map, Carlsbad Cavern. Based on Thrailkill (1965b, 1971), Hart (1969), Queen (1981), Wilson and Ash (1984b), and this study. Cm = Capitan Limestone, massive member; Cb = Capitan Limestone, breccia member; Bc = Bell Canyon Formation; Sr = Seven Rivers Formation; Y = Yates Formation; T = Tansill Formation. (click on image for a PDF version)

The reef facies of the Capitan Limestone is the primary unit in which Carlsbad Cavern is developed. Passages terminate near the intersection of the reef limestone with forereef or backreef beds. Crude bedding planes of the forereef facies of the Capitan Limestone slope at an angle of about 30° away from the reef and can be seen intersecting the cave at the southern end of the Big Room and also along the Left Hand Tunnel at intermittent intervals all the way to the Bell Cord Room (Fig. 18). The southeast wall of the middle section of the Big Room and the southeast wall of Lower Cave are located along the contact of the reef-forereef facies, the horizontal offset of the passages at these two levels possibly being due to the progradation of the reef over the forereef, so that in a vertical plane the forereef facies underlies the reef facies at an angle (Figs. 4, 17).

Carlsbad Cavern is the main focus of this study because it is the largest and most extensive cave in the Guadalupe Mountains. It also contains all of the various types of cave deposits: gypsum blocks and rinds, cobble gravel, silt, breccia, calcified siltstone-cave rafts, chert, sulfur, montmorillonite-endellite, and travertine. All of the known events that have occurred in Guadalupe caves are represented by the deposits in Carlsbad Cavern.

Cottonwood Cave

Cottonwood Cave is located between Dark and Black Canyons in a narrow limestone ridge which lies along the crest of the Guadalupe Ridge anticline. The cave is developed primarily in the backreef Seven Rivers Formation which directly underlies the pyritic Yates Formation (Sheet 5). The Entrance Hall of Cottonwood Cave is 30-50 m high and 15-20 m wide, and trends N14°W or approximately perpendicular to the reef escarpment. The sometimes rectangular to square passage cross section of the Entrance Hall represents the intersection of a flat ceiling (caused by collapse along a Seven Rivers bedding plane) with vertical walls (caused by exfoliation of bedrock along a series of parallel, vertical joints). An area of maze passage along the west wall near the very end of the Entrance Hall connects the Hall with the Gypsum Passage, a narrower, parallel, side passage which contains gypsum blocks, native sulfur, and gypsum and epsomite speleothems.

Cottonwood Cave is perhaps the second most important cave in this study, both because of its relatively extensive gypsum and sulfur deposits and also because of the replacement textures exhibited in its gypsum blocks and rinds.

Slaughter Canyon caves

Two major caves, Ogle and New (Sheets 6, 7), plus a number of minor caves (Wen, Midnight Goat, Triangle, Helen's, Goat) are located in ridges intersected by Slaughter Canyon. All of these caves are developed at the contact of the massive and breccia members of the Capitan Limestone, and all are developed along joints perpendicular to the reef escarpment and parallel to the Huapache monocline, As with Carlsbad Cavern, the Slaughter Canyon caves exhibit specific horizontal levels of preferred development (Table 3).

Ogle Cave, the largest Slaughter Canyon cave, has a primarily rectangular cross section averaging 30 m in height and breadth (Sheet 6). DuChene (1966) speculated that a lower bedding plane of the Tansill Formation defines the flat ceiling of the cave, and Jagnow (1978) attributed the vertical walls to exfoliation along vertical joints. The cave has two entrances, the Ogle and the Rainbow. Prominent sandstone dikes of two types and possibly of two ages are exposed at the Ogle Cave entrance and elsewhere in the cave.

McKittrick Hill caves

The caves of McKittrick Hill are developed at the contact of the Yates and Seven Rivers Formations (Smith, 1978b). The five McKittrick Hill caves—Dry (Sheet 8), Endless, McKittrick, Sand, and Little Sand—are all formed along the flanks of the McKittrick Hill anticline, part of the Carlsbad fold complex. The caves are primarily horizontal, having less than a 30 m elevation difference between their upper and lower levels. Lower levels often terminate in flat sandy floors defined by a Yates bedding plane. Most passages in the McKittrick Hill caves are maze-like, with the exception of the Expressway Passages in Dry and Endless Caves which are roughly rectangular in cross section. The majority of passages in Endless Cave are oriented parallel to the escarpment, whereas Dry Cave and Sand Cave have only a few passages trending in this direction. Most of the passages in Sand and McKittrick Caves trend parallel to the strike of bedding.

The McKittrick Hill caves are important to this study because they contain gypsum blocks which often display well-developed dissolution features.

Cave meteorology

Soil temperature, air temperature, relative humidity, air flow, carbon-dioxide levels, radon levels, and gamma-ray dosages have all been monitored in Carlsbad Cavern with sophisticated equipment. Measurements made in other, harder-to-reach, Guadalupe caves consist mainly of temperature and humidity readings recorded with a portable sling psychrometer (Table 4).

TABLE 4—Temperature and humidity of Guadalupe caves other than Carlsbad Cavern. *W. Calvin Welbourn, written comm. 1985.

McLean (1970, 1971, 1976) studied the climatology of Carlsbad Cavern and carefully recorded both the temperature and relative humidity (Figs. 19-21). Relative humidity varies between 69% and 95% for different parts of the cave; lower humidities prevail from November to April and higher humidities prevail from March to October. The relatively low humidity in Guadalupe caves compared to other cave systems accounts for the relatively high amount of evaporation therein. McLean (1971) reported a relative humidity of 88% for the Pump Room-Lunch Room section of Carlsbad Cavern (in October), and a corresponding evaporation loss of 4.18 x 10^-2 cm/day from a small evaporative pan. Hill (1978b) calculated that 10 times the amount of evaporation should occur in a cave with 88% humidity (e.g. Ogle Cave) than in a cave with 99% humidity (e.g. a cave in the eastern United States).

FIGURE 19.—Cave soil temperature, Carlsbad Cavern, in September 1969. After McLean (1971). (click on image for a PDF version)

FIGURE 20—Annual variation in air temperature, Lower Cave, Lunch Room, and Upper Devil's Den, Carlsbad Cavern. After McLean (1971). (click on image for a PDF version)

FIGURE 21—Annual variation in relative humidity, Lunch Room and Upper Devil's Den, Carlsbad Cavern. After McLean (1971). (click on image for a PDF version)

The temperature in Carlsbad Cavern varies from 12.4 to 19.6°C (Figs. 19, 20) and averages 13.3°C (56°F). The significantly higher temperatures along Left Hand Tunnel and in the Lake of the Clouds Passage may be partly due to a combination of the geothermal-gradient factor and lower air flow in this part of the cave. The geothermal gradient for the Eddy County, Carlsbad, area is reportedly between 0.073 and 0.0103 10^-30C/cm (Clark, 1966), and at the WIPP site in the evaporite rocks of the Gypsum Plain it averages 0.85°F/100 ft (M. Rider, pers. comm. 1986). Using the average of the first numbers, the temperature at the Lake of the Clouds, due to the geothermal-gradient factor alone, should be about 16°C. Using the second number, the calculated temperature should be about 18°C. The actual temperature at the Lake of the Clouds is 19.6°C; this value may reflect a greater geothermal gradient in the reef limestones than in the evaporite rocks of the Gypsum Plain, or it may be a reflection of other, unknown factors.

McLean (1971) measured the carbon-dioxide content in the Lunch Room, Carlsbad Cavern, and found it to vary from 345 to 490 ppm in contrast to an outside carbon-dioxide content of 330 ppm (Fig. 22). In the Left Hand Tunnel and Lake of the Clouds Passage, the carbon-dioxide content of the air is as high as 1,000 ppm (Table 5). McLean attributed the relatively low concentration of atmospheric carbon dioxide in the Lunch Room to a high rate of air circulation. By the same reasoning, the much higher values of carbon dioxide in Left Hand Tunnel and Lake of the Clouds are probably due to a poorer air circulation in this part of the cave. It is interesting to note that the carbon-dioxide content near the ceiling along Left Hand Tunnel is higher than that near the floor (Table 5).

FIGURE 22—Annual variation in carbon-dioxide content of the air, Lunch Room, Carlsbad Cavern. After McLean (1971). (click on image for a PDF version)

TABLE 5—Temperature, humidity, and carbon-dioxide content of the air, October 1985, Left Hand Tunnel, Lake of the Clouds Passage, and New Section, Carlsbad Cavern. CO₂ measurements made with a Drager Multi-Gas Detector.

Air-flow velocity in Carlsbad Cavern has been determined by actual measurement, and air-flow direction has been inferred from oriented popcorn growth and radon levels. McLean (1971) reported an air-flow velocity of almost 50 cm/sec at the cave entrance (Fig. 23) and an air flow somewhat less than 5 cm/sec in the Lunch Room area. Wilkening and Watkins (1976) reported an air flow of 30-40 cm/sec for both inflow and outflow of air in the Devil's Spring area of the Main Corridor. Reportedly, the cave "breathes out" during stormy weather (which is associated with a falling barometer), and "inhales" during dry, fair weather (when the outside barometric pressure is high) (Boyer, 1964).

FIGURE 23—Air-flow velocity at the Natural Entrance of Carlsbad Cavern, 11 January 1970, 8:00 p.m. After McLean (1971).

Queen (1981) plotted the distribution of oriented popcorn in Carlsbad Cavern and constructed an air-flow map of the cave from this distribution (Fig. 24). Based on popcorn orientation relative to the two cave entrances in Ogle Cave, Hill (1978b) speculated that a "chimney effect" operates in the cave. In the winter, when temperatures are higher in the cave than on the surface, warm, moist air flows out the upper Rainbow entrance and cold, dry air flows into the lower Ogle entrance.

FIGURE 24—Air-flow direction (arrows) as inferred from the orientation of popcorn, Carlsbad Cavern. After Queen (1981). (click on image for a PDF version)

Atkinson et al. (1983) showed that natural circulation of o cave air can be related to radon levels. Radon-222 (²²²Rn), the daughter product of ²³⁸U, is produced from the natural radioactive decay of rocks and soils in the interior of a cave. Actual levels of ²²²Rn encountered in a cave are dependent upon the net exhalation of radon from the walls of the cave, the volume of the cave, and the degree of mixing of outside air with cave air. Wilkening and Watkins (1976) noted that at Devil's Spring, Carlsbad Cavern, the concentration of radon gas in warm air moving out of the cave averaged about twice that of incoming air. They also noted that the radon concentration in Carlsbad Cavern averages 48 pCi/l in the summer but only 15 pCi/l in the winter. This difference is due to the mixing of cave air with outside air having radon concentrations of only 0.2 pCi/l (Table 6).

TABLE 6—Radon and gamma-ray measurements, Carlsbad Cavern.

Air-flow direction as inferred by radon levels is essentially the same as proposed by Queen (1981) using oriented popcorn growth, with the exception of Lower Cave and Left Hand Tunnel (compare Figs. 24 and 25). The highest radon levels measured in Carlsbad Cavern (99 pCi/l in November; Beckman et al., 1975) were in Lower Cave in the Naturalist Room. High radon levels in the Naturalist Room may relate to the relatively closed-off nature of this room, or possibly to the large amount of montmorillonite clay in Lower Cave. According to R. Kerbo (pers. comm. 1984) the highest radon measurements made in Lower Cave were associated with exposure to the gray-green clay.

FIGURE 25—Air-flow direction (arrows) as inferred from radon concentration, Carlsbad Cavern. Radon-daughter concentration (pCi/l) was measured by August by Beckman and Rapp (1976), and radon-daughter concentration* was measured in November by Beckman et al. (1975). (click on image for a PDF version)

The relatively low ²²²Rn values in Left Hand Tunnel are not consistent with radon levels for other parts of the cave, Theoretically, the profile of radon activity in a long, perfectly tubular, cave passage should continually increase from the entrance and down the tube unless there is a tributary current of air entering the cave from the outside, where upon radon levels suddenly drop. The inconsistent radon levels in Left Hand Tunnel may therefore possibly reflect outside air entering the cave somewhere along the Right Hand Fork or in the "blowing" maze off the passage leading into the Bell Cord Room, Gamma-ray-exposure levels are also lower in the area from the Lunch Room along Left Hand Tunnel to the Lake of the Clouds, the average of 15 readings being 7.5 uR/hr, whereas the average of 59 readings for other parts of the cave being 20.2 uR/hr (Moore, 1984; Table 6). Curiously, bats have been noted to regularly fly in and out along Left Hand Tunnel, another indication that an entrance might exist in this part of the cave.

Cave geomorphology

Cave geomorphology is the description and interpretation of cave patterns and forms in the underground which are due to solution, erosion, corrosion, and breakdown. Geomorphic forms can be classified into two groups: (1) cave-passage forms, which embrace the smallest cavity up to the largest cavern voids and the most elaborately branched cave systems; and (2) cave-karren forms, which originate on the surface of the cave bedrock. The first form defines the shape of the cave passage itself; the second form represents a modification of that cave passage. Both forms attest to the conditions under which caves dissolve and evolve.

There are two main regimes of geomorphic development in caves. The "phreatic" regime is where cave passages or features form beneath the water table by water under pressure, and the "vadose" regime is where cave passages or features are formed above the water table by free-flowing water. Cave-passage forms in the Guadalupe Mountains are all phreatic, while cave-karren forms may be either phreatic or vadose. In the following discussion of geomorphic forms, the terminology of Sweeting (1973) and/or Bogli (1980) is used, except in cases where new geomorphic forms are being introduced.

Passage forms

Spongework—A three-dimensional, honeycombed, "Swiss cheese" network of cavities a few centimeters or so in diameter and depth. Spongework voids are the result of diffuse circulation of water within pore spaces in bedrock. Every cave in the Guadalupe Mountains is surrounded by bedrock which exhibits a matrix or framework of sponge-work.

Boneyard—A large form of spongework which is separated by stretches of unaffected wall. "Boneyard" is so named because it looks like a jumble of bones (Fig. 15). This integration of space and rock may be a few meters deep and wide, or it may continue for a distance of many meters. A boneyard type of passage form is evidence of non-differential phreatic attack where cave ceilings are subjected to as much solution as cave walls. An example of a boneyard is the Boneyard, Carlsbad Cavern.

Spring shafts—Tubular pits created by upwelling water under pressure. They are distinct from "collapse shafts," which are created by the collapse of underground cavities and are characterized by breakdown on their floors. They are also distinct from vadose, dome pit, "vertical shafts," which are characterized by vertical wall flutes and floor drains. Spring shafts may constitute a "phreatic loop" between two cave levels (Fig. 14) or may lead to the surface. Examples of spring shafts in Guadalupe caves are the tubular entrance to Deep Cave and the Bottomless Pit and Pit Survey Shafts, Carlsbad Cavern.

Joint chimneys—Fissure-shaped canyons rather than tube-shaped shafts. They may connect two cave levels or may lead to the surface. One example of a joint chimney is in the Cave of the Madonna, where Dean's Drop connects the New Year's Eve Room with the Lower Passage.

Solution domes—Semicircular to circular arches in cave ceilings which are a few meters to tens of meters in breadth and depth. In non-bedded rock they display solutional surfaces, but in bedded rock they may exhibit angular surfaces if modified by collapse. Collapse domes are vadose features, but solution domes are created by phreatic water lifting under pressure. Well-developed solution domes (partly modified by collapse) are in Sand Passage, Carlsbad Cavern.

Solution pockets—Spherical cavities in cave walls or ceilings that are tens of centimeters to tens of meters wide and deep. Ceiling solution pockets are approximately equidimensional, or they may be linearly developed along joints. Sometimes ceiling pockets form in a vertical series, where multiple concavities increase in circumference. Wall solution pockets may be approximately equidimensional, or they may be linearly developed along a bedding plane. Some wall solution pockets in Guadalupe caves appear almost scallop-like, but they are not discernibly directional as are scallops, a cave-karren form. Bogli (1980) classified ceiling- and wall-solution pockets as phreatic features, ones which form under low-velocity flow conditions. Equidimensional, linear, and multiple ceiling pockets and equidimensional wall pockets may be seen in the Lunch Room area, Carlsbad Cavern; linear wall pockets can be seen in bedded rock at the top of the Main Corridor.

Karren forms

Scallops—Shell-shaped solutional concavities on cave walls and ceilings which are ridged or crested on their upstream sides and more gently sloping on their downstream sides. Scallops are indicators of past current direction and velocity, the distance between scallop crests being inversely proportional to the flow velocity (Curl, 1974). Scallops are usually small features, not more than a few centimeters long, produced by fast-flowing vadose streams; however, they may also be larger features, indicating the past presence of slower-flowing, sub-water-table streams.

The walls of Guadalupe caves often exhibit scallop-like markings, but these are usually not developed well enough to determine the direction and velocity of past flow. An exception to this rule are the scallop marks in the entrance area of Bat Cave, Carlsbad Cavern. These have scallop lengths of 1.5-5 m and a skewness which indicates that past flow was eastward, along Bat Cave. Using Curl's formulas, this corresponds to a past flow velocity of about 0.5-2 cm/sec, or, for a passage cross section of 30 x 20 m, a flow rate of 3-12 m³/sec. The size of these scallops suggests that they are of phreatic origin. Two other possible scallop areas are along the south wall of the Main Corridor just before its connection with Appetite Hill, and along the west wall of Appetite Hill. The south-wall scallops appear to point straight up; the west-wall scallops appear to be oriented at about a 30° angle.

Air scallops—Smooth, concave pockets in cave walls and ceilings which resemble water-formed scallops in some respects, but are much larger and exhibit blunt, rather than sharp, crests. Air scallops often truncate travertine material as well as bedrock. They are believed to be condensation-corrosion features where corrosive air flow scallops bedrock and speleothems concordant with each other. It is entirely possible that air scallops are air-corrosion modifications of previously formed phreatic solution pockets. A particularly well-developed air scallop 3 m in diameter can be seen just below the base of two directionally corroded shields about 20 m before reaching the Lake of the Clouds, Carlsbad Cavern (Fig. 93). Other air scallops in Carlsbad Cavern occur at Devil's Spring where they cut across drapery speleothems and bedrock.

Rillenkarren—Grooves with rounded troughs and sharp ridges. They are a common surface geomorphic form in many karst terrains and can also be a cave-karren form. Rillenkarren are created by vadose drippage in areas where water is highly charged with carbon dioxide. In Carlsbad Cavern, rillenkarren are present on bedrock walls and floors in the Left Hand Tunnel, Bell Cord Room, Boneyard (Fig. 16), New Mexico Room, and Cable Slot (Fig. 26). One of the best examples is in the Bell Cord Room, where grooves are up to 30 cm deep, 10 cm wide, and 2 m long. These occur in bedrock and breakdown and also in subaerially formed flowstone (Fig. 27). The rillenkarren on breakdown blocks in the Bell Cord Room originated at semicircular drip points on horizontal slopes and then proceeded to form deep channels on vertical slopes (Fig. 28).

FIGURE 26—Distribution of rillenkarren in Carlsbad Cavern (stippled areas). After Queen (1981). (click on image for a PDF version)

FIGURE 27—Rillenkarren on breakdown (center) and on flowstone (lower left corner), Bell Cord Room, Carlsbad Cavern. Photo Alan Hill.

FIGURE 28—Rillenkarren formed on the upper surface of a piece of breakdown, Bell Cord Room, Carlsbad Cavern. Note that the rillenkarren grooves begin at semicircular drip points (lower left of figure). Photo Alan Hill.

Spitzkarren—A variation on the rillenkarren form is spitzkarren, sharply spiked solutional features in bedrock or breakdown which have been corroded down to sharp-slivered edges or pointed crests (Fig. 29). The only known spitzkarren locality in any Guadalupe cave is at the beginning of the Big Room, Carlsbad Cavern, where the historic trail descends from a high breakdown pile toward the Lunch Room. Here, spitzkarren and rillenkarren occur together, with secondary calcite crusts sometimes covering the surface of the spitzkarren. Where the spitzkarren is not calcite-covered, textural features such as fossils and clay-residue patterns are displayed in relief in the corroded bedrock.

FIGURE 29—Spitzkarren on breakdown near the junction of the Big Room with the Lunch Room, Carlsbad Cavern. The tallest spitzkarren is about 1 m in height. Photo Alan Hill.

Corrosion channels—Indentations in a cave ceiling or wall which are believed to have been formed by corrosive air. At the very top of the Lake of the Clouds Passage, a curving channel 1.5-3 m wide and 0.6-1 m deep can be followed along the ceiling to where the passage ascends toward the Bell Cord Room (Fig. 94). Stalactites along this channel are an opaque chalk-white and highly corroded. Corrosion channels also occur at the top of the ascent leading toward the Bell Cord Room where chalk-white calcite "lines" remain of once-present dripstone which has been corrosively attacked down into ceiling joints.

Punk rock—Bedrock which has been so highly corroded that it is soft and flaky rather than hard and crystalline. Upon corrosive weathering under air-filled conditions, the residue of the bedrock is exposed, causing the surface of the rock to appear dark brown (P1. 11A). Where the punk-rock residue has flaked off the wall, it forms debris piles of dark silt on the floor.

Cave hydrology

Water is noticeably absent in the caves of the Guadalupe Mountains. The water table is below all known cave passages (Sheet 1). Flowing vadose streams do not exist in any of the caves, with the exception of Vanishing River which is located in the bottom of a ravine and captures water during heavy rainfall. The only water in the caves is drip and pool water, vadose meteoric water which slowly seeped down from the surface along joints, bedding planes, and interconnected pores.

After a heavy rainfall, meteoric water is temporarily stored in the subcutaneous zone (upper weathered layer of rock beneath the soil) and then, after some delay, finds its way into the lower vadose zone. McLean (1976), using monthly rainfall and infiltration totals, found no correlation between rainfall and meteoric water entering Carlsbad Cavern. Williams (1983), however, using weekly totals, reported a response lag of 2-14 weeks between rainfall and infiltration. The lack of a clear relationship between lag time and depth in Carlsbad Cavern indicated to Williams that percolation routes through the vadose zone are highly variable, often independent of each other, and subject to "capillary barriers"—capillary-like channels which diminish the pressure pulse effect of storage recharge following rainfall.

Hydrograph measurements of cave-pool levels were made by McLean (1976) for 11 pools in Carlsbad Cavern and these fluctuations were correlated with evaporation rates. In addition, T. Rohrer (pers. comm. 1986) measured the drop in water level in the Lake of the Clouds: from 1966 to 1986 the lake lowered by 31.27 cm.