The Influence of Iron Sand Mining (<i>Kanna</i>-<i>nagashi</i>) on the Formation of the Sotohama Beach Ridges in the Yumigahama Peninsula of South-Western Japan

In the weathered granitic rock areas of the Chugoku Mountains, from the beginning of 17th century to about 1932, a huge quantity of earth was excavated using the traditional surface-mining techniques called Kanna-nagashi. The purpose was to collect iron sand for the iron smelting industry. The earth, which was mainly thrown into the rivers afterwards, is said to have accelerated the extension of the coastal plains.<br> The aim of this paper is to establish the role of the sediments produced by Kanna-nagashi in the formation of beach ridges in the Yumigahama Peninsula. The study is based on the sedimentolo-gical analysis of the grain size distribution and lithological composition of 23 samples of surface sediments and 39 samples from four bore holes.<br> The landform of the Yumigahama Peninsula is divided into three belts of beach ridges: Sotohama, Nakahama and Uchihama (Fig. 1). The Sotohama beach ridges, which are the newest of the three, have a flat surface less than 5 meters above sea level. The distribution of clear swales is very limited, and distinct sand dunes are absent. Inn contrast, many swales and large sand dunes can be seen in the Nakahama and Uchihama beach ridges.<br> Samples from the surface sediments in the Sotohama beach ridges are mainly composed of coarse sands and granules (Table 1 and Fig. 3). Granitic rock fragments and mineral grains count for a high percentage of all samples. The proportion of volcanic rock fragments is less than about 20 percent. Iron slag grains derived from the iron smelting furnaces in the Hino river basin are present in a ratio of from 3 to 10 grains to every 10, 000 sediment grains in all samples. The presence of iron slag grains in the sediments suggests that there is a close relationship between the formation of the Sotohama beach ridges and Kanna-nagashi.<br> The surface sediment textures of the Nakahama and Uchihama beach ridges are very different from those of the Sotohama beach ridges. Sediment samples consist mainly of medium or fine sands. The proportion of volcanic rock fragments is more than 50 percent in many samples. The proportion of granitic rock fragments is very small, and iron slag grain is not present except in two samples from the Nakahama beach ridges.<br> The ten-meter-deep sediments of the Sotohama beach ridge are stratigraphically divided into three beds from the top down: UI, UII and L (Figs. 4 and 5). The UI and UII, beds, with a combined thickness between 6 meters and 9 meters, contain a high percentage of granitic rock fragments and mineral grains. Volcanic rock fragments make up only around 10 percent of the total. A certain volume of iron slag grains is always included in these beds. In bed L, with a thickness between 1 meter and 3 meters, the proportion of volcanic rock fragments increases to more than 20 percent, and there is no evidence of iron slag grains. The lithological composition and the existence of the iron slag grains show that the sediments of the beds UI and UII constitute the body of the Sotohama beach ridges.<br> Based on the above-mentioned results, we can conclude that the Sotohama beach ridges were formed rapidly by a large quantity of materials, such as granitic rock fragments and quartz grains, caused by Kanna-nagashi in the Hino river basin, whereas the beach ridges of Nakahama and Sotohama were formed mainly by volcanic rock fragments such as dacite and andesite transported from the Daisen volcano.<br> By comparing the proportion of volcanic rock fragments in the Sotohama beach ridges with that of the other two beach ridges, the proportion of the newly added sediments directly caused by Kanna-nagashi can be estimated to be about 75 percent of the total earth volume of the Sotohama beach ridges.