MORPHOGENETIC APPROARCH TO MAN-MADE LANDFORMS DUE TO OPENCAST MINING OF PORCELAIN CLAYS AND SILICA SANDS, AROUND SETO CITY, AICHI PREFECTURE, CENTRAL JAPAN

Abstract

In spite of rapid expansion of man-made landforms after the 1960's in Japan, Japanese geographers, especially geomorphologists have paid little attention to this problem. As geomorphologists perform genetic approach to natural landforms by means of precise analysis of geomorphic processes, man-made landforms must be studied genetically by precise analysis of the processes of human actions. Present author offers the genetic approach to the man-made landforms by analyzing 1) the original geological and geomorphological conditions, 2) the purpose of human actions, 3) the method of transformation, 4) the characters of businesses acting on landforms, and 5) the regulations controlling landform transformation. From such a viewpoint of analysis, the author deals with the man-made landforms resulting from the opencast mining of porcelain clays and silica sands around Seto City, Aichi Prefecture. The studied areas are located in the northern part of Seto City and the Yakusa district of Toyota City, where many mines are densely concentrated because of thick deposits of high quality porcelain clays and silica sands. These areas are composed of gravel beds of the Yadagawa Formation, Plio-Pleistocene (the Mizuno Gravel Bed) where low relief hills with summit level of 180_??_190 m high spread over (Fig. 1). The Seto Forma-tion beneath the Mizuno Gravel Bed is composed of porcelain clays and silica sands which are just mined, and are subdivided into three members ; the Motoyama Kibushi Clay Bed (upper), the Mizuno Clay Bed (middle), and the Yadoko Silica Sand Bed (lower) (Figs. 2 and 3). Landforms of mining fields are generally characterized by disordered distribution of large mining hollows such as honeycombs. Each hollow consists of steep cliffs, step-like benches, . abandoned roads and settling basins in the bottom of hollows. Such features are common to almost all mining fields (Fig. 4, alos see stereo-pair air photographs). The Mizuno Gravel Bed which is called the “surface layer” by mine workers is evacuated and transported to the abandoned mining fields or the upper part of valleys to make flat or rolling surfaces. The dump of the evacuated surface layer, called “thane-yama”, is used to fill up: the abandoned mining hollows. The ceramic ware industry of the Seto district is one of the traditional localized industry of Japan which dates back to the 11th century. In general, scale of business is very small and the working system is old-fashioned. Such character also reflects on the mining works. About eighty percent of mining businesses have less than ten workers (Table 1), and the area of each mining field is also very limited (Figs. 5 and 6). This results in numerous disordered congregation of honeycombed mining hollows. Since the mining of porcelain clays and silica sands was provided under the Law of Mining Safety enacted in 1951, the mining method was gradually changed from pit mining to opencast mining for the sake of safety of workers by the administrative instruction of the Local Mining Safety Office of Nagoya. After the World War II, bulldozers were intro-duced as digging and excavating machine and motor trucks for transportation of mined minerals spread over this area, leading to the considerable increase of mining efficiency (Table 2). Moreover, especially after the 1960's, the rapid propagation of digging machines such as large bulldozers, power-shovels, loader shovels have greatly exaggerated the land form transformation. Present detailed features such as height and width of bench cliffs, benchs and roads of mining fields seem to be provided by the capacity and function of these heavy machines and dump cars. Mining regulation is a most important factor controlling the landform transformation. The Local Mining Safety Office of Nagoya guides mining businesses according to the guidline for the opencast mining of non-metal ores (Table 3) under the Law of Mining Safety.