There are several outcrops visible on the west side of Tobishima Island and in the northern Hōki area, where gentle slopes lie along the uplifted marine terrace. In these places, you can see a layer of small rocks (sediments) between layers of organic soil. These outcrops are 5—8 m above sea level, and the total thickness of the sediments is about 1 m.

Since there are no large rivers on Tobishima Island, these small rocks had to come here from the sea. However, they were carried here not by stormy waves or strong winter winds, but by tsunamis.
Radiocarbon dating of Haji pottery and Sue pottery fragments found within the sediments dates them around the 9th or 10th century. It is possible that this was the tsunami caused by the earthquake in Dewa Province in 850.
The topmost layer of sediment, closest to the surface, is thought to have originated in the tsunami from the Shōnai offshore earthquake in 1833.
Tobishima Island is known to experience a tsunami approximately once every 1,000 years, making this site a place where you can both learn about nature and feel a sense of reverence toward it.