Entrance to a cave near Marble Gap in the Marble Mountain Wilderness west of Fort Jones, California.  This particular region contains a vast region of marble karst (except for the summit block), w/ many channels & sinks of sculpted rock in solution leading down to vertical pits & cave entrances.  There are at least 150 known cave entrances within the karst, the largest is Bigfoot Cave w/ about 12.4 miles of known passages & a depth of 1,205 feet, which is more than Oregon Caves to the north.  The geology of the Klamath Mountains is among the most bizarre & intriguing in North America.  Most caves in the area require ropes as they contain vertical pit entrances, this was one of the few where it had a level entrance though it only went about 100 feet in, being relatively dry & lacking many formations.