Tatami Ishi
The Henromichi (“Pilgrimage Path”), leading from the foot of Yashima to Yashimaji Temple, is dotted with Buddha statues and places that feature in local legends, but it also offers valuable insights into the geology of Yashima. One natural highlight is the Tatami Ishi (“tatami rock”), a rock formation named for large rectangular blocks reminiscent of the dimensions of tatami mats.
The distinctive joints (horizontal breaks) visible in the rock here are evidence of the geological processes that formed Yashima. Approximately 14 million years ago, a volcanic eruption covered the length of what was then a valley with lava. This lava solidified into a type of black rock called sanukite, which formed a kind of extremely hard lid on the valley, protecting it from erosion. As millions of years passed, the mountains around the valley were dissolved by rain and wind, transforming the sanukite-covered valley into the flat mountain we see today. The dark rock that makes up the Tatami Ishi is sanukite, and the joints in it were formed when lava flowed horizontally into the primeval valley and hardened.
The thin layer of clay soil on top of the rock is also volcanic in origin and supports several plants that grow only in places with this type of geological makeup. These plants include the iwashide (Korean hornbeam; Carpinus turczaninovii), a sturdy tree with serrated leaves that turn vivid shades of yellow and red in autumn, and the choji-gamazumi (Viburnum carlesii var. bitchiuense), a viburnum with fragrant pink blossoms.
