Abstract
Kurara-Seto is a narrow passage about 7 kilometers in width located between the Chikuzen O-Shima (Island) and the northern coast of Genkai-Machi, Pukuoka Prefecture.
There exist several islands and reefs in the strait and strong tidal currents is passing through them. Two striking ridges formed by linearly arranging islands and reefs cross the strait. These ridges are composed of hard rock called the Kanmon Group of Mesozoic Age. The basin between the two ridges has a smooth bottom with a depth of 10 to 20 meters. The basin is composed of soft rock belonging to the Munakata Group of Tertiary Age. Such features of the ridges and basin are originated from selective erosion of tidal currents acted on the Kanmon Group and the Munakata Group. Two caldrons develop on the outer sides of the ridges along the axis of the strongest tidal current. The caldron on the south side of the north ridge was formed by the erosion of the tidal currents acted on the Munakata Group. The caldron related to the south ridge is not a result of abrasion by tidal current but an old submerged valley which have been free from sedimentation by biolent action of tidal currents.
