三名含礫片岩, Shikoku Island Travel Guide

The Sanmyo Conglomeratic Schist (三名含礫片岩, Sanmyō Ganrekihekigan) is a fascinating geological site located in Yamashirocho Nishiu, Miyoshi, Tokushima Prefecture, Japan. This natural monument features a rare type of metamorphic rock known as conglomeratic schist, formed from ancient conglomerate rocks that have undergone intense pressure and heat. The most striking characteristic of this rock is the presence of well-rounded pebbles and gravels flattened and stretched within a sandy matrix, a testament to the immense forces at play during its formation.These schists belong to the Sambagawa metamorphic belt, a vast zone of metamorphic rocks stretching across Japan. The rocks in this area were originally formed from sediments deposited on the ocean floor, which were then subjected to high pressure and temperature during plate subduction. The Sanmyo Conglomeratic Schist is unique because it contains rock fragments of continental origin, making it a rare example of metamorphosed conglomerate within the Sambagawa belt. The pebbles within the schist are primarily composed of siliceous igneous rocks such as biotite quartz diorite, granite porphyry and quartz porphyry, along with some sandstone and mudstone.Visitors to the site can observe the schist's distinctive features, including the flattened pebbles and the alignment of minerals, which give the rock a layered appearance. The schist is exposed along the riverbed of the Iya River (祖谷川) and the Tonotani River (戸ノ谷川), downstream from their confluence, and along the prefectural road downstream from the Ichiu Power Plant. The Sanmyo Conglomeratic Schist can be seen in the riverbed of the Oboke Gorge (大歩危峡). The area around Oboke and nearby Koboke Gorge is known for its stunning natural beauty, with opportunities for rafting and enjoying the views.