芦田川水系の河川争奪と天然記念物「久井・矢野の岩海」の形成

国の天然記念物である「久井・矢野の岩海」は巨礫が谷底に集積した特殊な地形である。花崗岩の風化とともに，細粒物質のみが侵食，除去されることにより，コアストーンが残留した「疑似岩塊流」とされる。コアストーンは花崗岩域で普遍的に生成されるが，岩海は約14km 離れた指定地付近に限られる。一方で，いずれの地点も争奪河川の分水界近くに位置する。本研究では，芦田川の河川争奪地形および久井の岩海の地形を記載し，それらの発達について考察した。その結果，いずれの岩海の指定地も，争奪河川の最上流に位置するものの，河川争奪が生じても数十ｍ程度のわずかに下刻が生じるような絶妙な地形環境であったと考えられた。

The boulder fields in Kui and Yano of Hiroshima prefecture, which are national natural treasures of Japan, have unique topographies where large granite boulders accumulate along valley bottoms. The Cretaceous granite had weathered since the Sera upland low-relief mountain surface was developed in the middle of the Miocene epoch. The fine-grained weathering material had been washed away from the valley, and only the core stones remained in the designated sites. These are known as “pseudo block streams” based on the process and cause of the topography. Core stones are universally generated by granite weathering. However, the boulder fields are distributed only in two designated areas, 14 km away from each other. Both sites are located near the watersheds of the Ashidagawa and Gonokawa river systems. In this study, we described the piracy topography of the Ashidagawa River and the Kui boulder field topography and examined these developments. The Ashidagawa River, downflowing to the Seto Inland Sea, captured the upper stream of the Gonokawa River system, probably due to the subsidence of the Seto Inland Sea. The designated sites are located within the rare topographical environment in the uppermost reaches of the piracy rivers, where slight erosion with a depth of several tens of meters has occurred.