New peculiar cave ceiling forms from Carlsbad Caverns (New Mexico, USA): The zenithal ceiling tube-holes Jose-Maria Calaforra a, ⁎, Jo De Waele b a Water Resources and Environmental Geology, University of Almeria, Cañada de San Urbano s/n, 04120 Almeria, Spain b Italian Institute of Speleology, University of Bologna, Via Zamboni 67-40127 Bologna, Italy abstract article info Article history: Received 19 July 2010 Received in revised form 21 February 2011 Accepted 25 February 2011 Available online 23 March 2011 Keywords: Sulphuric acid cave Cave karren morphology Cave ceiling forms Speleogenesis During a trip to the Hall of the White Giant, Carlsbad Caverns (NM, USA) cigar-shaped vertically upward developing holes were observed on the ceiling at different heights of the passages. They have a circular cross- section with diameters of 1 to some centimetres and taper out towards their upper end. Their walls are smooth and their bottom edges are sharp, while their length can reach several decimetres. Sometimes gypsum can be found inside. They often occur randomly distributed in groups and their development is not necessarily controlled by fractures or other bedrock structures. We name these peculiar karren-like cave microforms “zenithal ceiling tube-holes” because of their origin by H 2 S environment corrosion processes and their vertical (zenithal) upward growth in ceilings. A comparison is made between zenithal ceiling tube-holes and other karstic or non karstic similar forms such as bell holes, oxidation vents, snailholes, Korrosionskolke (mixture-solution hollows) or pockets, röhrenkarren, light- oriented photokarren, borings of (often marine) organisms and negative stalactites. Zenithal ceiling tube-holes are created by the corrosive effect of sulphuric acid. H 2 S(g) dissolves in water giving rise to widespread sulphuric acid corrosion. When H 2 S bubbles are trapped underneath overhanging surfaces or ceilings and water level rises steadily the corrosive effect is concentrated vertically upwards, drilling vertical holes that can also completely pass overhanging rock ledges. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Carlsbad Caverns is a world renowned show cave located in the Guadalupe Mountains (Chihuahua desert, New Mexico) and managed by the United States National Park Service. The cave is hosted in the Permian Capitan Reef limestones that border the Delaware basin (Fig. 1). It consists of elongated passages connecting wide rooms that extend to a depth of almost 300 m beneath the natural cave entrance, almost 200 m below the floor of nearby Walnut Canyon (Hill, 1987). As most caves of the Guadalupe mountains it has formed by rising hydrogen sulphide deriving from underlying oil and gas deposits, producing sulphuric acid by mixing with fresh water which corroded the limestone into unusually large chambers (Hill, 1990). This H 2 SO 4 speleogenesis has produced gypsum that is present under various forms. Carlsbad Caverns has been widely studied by generations of cave scientists, and research has especially focalised on geology (Hill, 1995; Harwood and Kendall, 1999), speleothems (Hill and Forti, 1997; Melim et al., 2006), mineralogy (Polyak and Guven, 1996, 2000), water chemistry (Ingraham et al., 1990; Chapman et al., 1992), speleogenesis (Hill, 1995, 2000; Polyak et al., 1998), cave meteorology (Cheng et al., 1997), cave fauna (Northup and Crawford, 1992; Geluso, 2008; Hristov et al., 2010) and, mostly in the last decennia, microbiology (Northup et al., 2000; Barton et al., 2007; Snider et al., 2009). This has resulted in a large number of publications in many scientific journals, making Carlsbad Caverns one of the best studied caves of the world. In the squeezing crawlways that connect the main Corridor to the Sand Passage and the further lying Hall of the White Giant, several cylindrical centimetre-sized vertical holes have been observed on the ceiling at different levels (Fig. 2). These holes, here named zenithal ceiling tube-holes, are the subject of this paper. After a review of similar vertical cave ceiling forms, the genesis of this new form is explained. 2. Morphology 2.1. Zenithal ceiling tube-holes Zenithal ceiling tube-holes are centimetre to decimetre long conical holes that perforate the ceilings of cave passages in a perfect vertical upward direction (Fig. 3A). The host rock is made out of limestone of the Yates Formation (Hill, 1987). Their cross-section is circular with diameter ranging between a centimetre up to 5 cm (most are 2–3 cm wide). They are cigar-shaped, tapering out towards their upper ends (Fig. 3B), and they sometimes completely perforate horizontal over- hanging rock slabs (Fig. 3C). Their walls are smooth and their edges are Geomorphology 134 (2011) 43–48 ⁎ Corresponding author. E-mail addresses: jmcalaforra@ual.es (J.-M. Calaforra), jo.dewaele@unibo.it (J. De Waele). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.02.032 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph