 Exposed at low tide, the feature comprises some one thousand pentagonal and hexagonal rocks, each 1 to 1.5 metres in diameter, stretching fifty metres from north to south over a length of two hundred and fifty metres.It was formed during the Miocene period by the columnar jointing of andesitic lava as it cooled and contracted.The name is derived from the resemblance to a room of close-fitted tatami mats, while the feature is sometimes also likened to a turtle's carapace.