 Geological interest extends underground, and there are many rock shelters and solution caves in the area.These include King Arthur's Cave and many others in the area of Symonds Yat and Slaughter Stream Cave near Berry Hill.At St Arvans, near Chepstow, the underground watercourses have carved out long cave systems, which exit at Otter Hole at the base of Piercefield cliffsthe only cave system in England or Wales which can only be reached through a tidal sump, making it a mecca for experienced cavers.