The swamp sits at an altitude of 920 metres and lies between the main cone of Mount Taranaki and the Pouakai Range to the northwest, which reaches 1,400 metres. It was formed about 3,500 years ago when a lava extrusion and a later debris flow blocked the Stony River. This extrusion, which reaches 1,052 metres, lies to the west of the swamp and is known as The Dome. The outlet from the swamp forms the Bells Falls.