山陰地方における鉄穴流しによる地形改変と平野形成

抄録

The landform of the Chugoku mountains was transformed considerably in some places by the unique mining method called Kannanagashi between about 1600 and 1923. Kannanagashi is the method by which workers cut out a large quantity of weathered rock and wash out with running water the small amount of iron sands contained therein.
This paper aims at discussing the landform transformation caused by Kannanagashi and the associated formation of the alluvial plains in the four river basins of the Sanin district.
It is fairly easy to distinguish a Kannanagashi site (stope) from other kinds of landforms because it is characterized by unnatural cliffs, ditches and artificially cut hills. Numerous stopes are seen in the middle and upper river basins. Many of them are, in particular, concentrated into several parallel belts extending in an ENE-WSW direction (Fig. 2). Such densely distributed areas correspond with the places having the following three natural conditions: where granitic rock with iron sands such as granodiorite or diorite is distributed; where a deep weathered crust on an erosion surface has developed; and where plenty of water for Kannanagashi is available.
The total area of the scopes is estimated to be 7.8×10⁷m². It occupies nearly 4 percent of the area of the middle and upper river basins. The approximate volume of the waste earth discarded as a result of Kannanagashi is estimated at between 3.8×10⁸m³ and 5.4×10⁸m³.
The outflow of a large volume of waste earth into the rivers has caused the expansion of the delta fronts in the Hii and Iinashi Rivers and the widening of the sand-bar in the Hino River. Six borings drilled in the fan-like delta of the Iinashi River reveal a sand-and-gravel bed nearly 4 meters thick covering a deep silt-and-clay bed containing organic matter and fossil shells throughout. The upper bed is mainly composed of angular granitic particles and a number of iron slags which were thrown away by the iron works and the blacksmiths. This facies shows that the fan-like delta of the Iinashi River has been formed mainly by sediments from waste earth discarded as a result of Kannanagashi.