Carlsbad Caverns have formed within the uppermost Capitan reef and, where the cavern margins have not been coated with cave formations, allow examination of three-dimensional transects through the reef and adjacent facies. This study was made within Left Hand Tunnel, which is developed mostly on one stratigraphic-level within the reef facies (Fig. 1a). Although in situ reef is developed both at the entrance to the tunnel and further along the tunnel, mapping has identified an area, commencing near the western end of the tunnel, where large rotated blocks of reef facies sediments are present (Fig. 1b). A further, smaller area of rotated reef facies blocks is present near the eastern end of the tunnel (Fig. 1b). These areas are interpreted as paleochannels which developed within the reef. The western paleochannel is some 230 m wide by over 25 m deep. The size of the eastern paleochannel cannot be determined as the surrounding areas are covered by cave formations.