Piracy of the Ashidagawa River System and Formation Mechanism of the Kui and Yano Boulder Fields

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  • 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

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Abstract

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.

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