Tatami-ishi 畳石 (Tatami-Rocks) is a Nationally Designated Natural Monument located on the south coast of Oujima 奥æ¦å³¶ (Ou Island).
These pentagonal and hexagonal shaped rocks, formed by andesite magma cooled under a geological process called columnar jointing, are called Tatami-Rocks because of the resemblance to the tatami mats of traditional Japanese home design.
Water erosion has leveled and smoothed the tops of the rock structures.
