北海道奧尻島沿岸の奇岩の地質環境とその伝説?伝承

摘要

Various oddly-shaped rocks on the coast of Okushiri Island in southwestern Hokkaido are examined based upon field geology and folklore literature. Characteristic twelve geosites of the island are described with their geology and chronostratigraphy, and determined geological attributions of oddly-shaped rocks. Based on chronostratigraphy, oddly-shaped rocks of Okushiri Island are divided into three horizons, i.e. Cretaceous, middle Miocene and lower Pliocene, in ascending order. The Cretaceous horizon is characterized by Mottate Iwa Rock (granodiorite) after block weathering and wave erosion. The Miocene horizon is represented by Cape Inaho and Benten, which are made of intrusions of volcanic rocks in the Tsurikake Formation after differential weathering and erosion. The Pliocene horizon is characterized by Nabetsuru Iwa, Hoya Ishi and Kabuto Iwa Rocks, which are composed of intrusions of volcanic rocks in the Hotokezawa and Yoneoka Formations after differential weathering and erosion. Folklore of oddly-shaped rocks in Okushiri Island are mainly local origin, but some folklore is related to the legend of Yoshitsune.