The Balliny closed depression is formed along a N-S line of weakness and lies within an extensive limestone plateau on the north-west side of Slieve Elva. Prior to the last glacial advance the area had a thin shale covering and the depression served as the major sink for the region. During late glacial times the shales were removed and the sink became choked with sediments. The depression became a seasonal lake and overflowed directly to the sea. The post-glacial cave Pollballiny has, in the lower part of its course, been captured by an older cave associated with this depression. The terminal choke of Pollballiny, whose position has been confirmed by radio-location, lies on th esame N-S line of weakness as the depression. It is likely that the further parts of this cave turn north along this line before heading west to the sea.