THE BARE HILLS SURROUNDING THE OKAYAMA DISTRICT (1)

Abstract

In Japan, soil erosion on hillsides is wel-l-demonstrated in the Okayama District along the Setouchi Coast. After a study of this district, the following conclusion were reached 1. Bare hillsides are found mainly along the ridges and adrept slopes of granitic hills. Other hills, of Liparite, Chichibu system, or Tertiary rocks, are not as bare as the granites. As a result of experiment, it was found that as much as 20 or 30 tons of soil are eroded, even on the surface of the Chichibu Palaeozoic rocks, when vegetation is lacking. 2. From macroscopic observation, it may be said that the bare lands are not generally found in mountains or thinly populated regions but, rather, in regions of dense population and thriving economic activity similar in topography and geology. This indicates the cause of erosion to be man's destruction of vegetation and soils. 3. Many of the bare hills surrounding the Okayama Plain have been abandoned and eroded. since the beginning of the Mei ji Era (1869-1912), a period of general disorder in forestry. Following this came the so-called “forest eater” stage of destructive erploitation of forest resources; streams and brooks built up their beds for 50 to 60 years, raising them to 5 or 10 meters above the adjacent fields. These rivers are termed “Tenjo-gawa” or “Flying rivers.” 4. Records and documents concerning the forests and plains in this region date back to pre-Meiji days. Archives indicate the three centuries prior to 1896. may be divided into three distinct periods: A-The third period: At the beginning of the 18th century, bare hills appear mainly on the Kojinma Peninsula distributed approximately as they are now (Figure 1.), differing only in detail, For example, the center of distribution was in the center of the peninsula instead of near Tainano as at present. B-The second period: For the second, half of the 17th century there are no clear records of bare land in the archives. In this period it seems likely that the hills were covered by pine forests even more thickly than previously. C-The first period: For this period documents are scarce, but it is clear thet a vast plain was utilized for grazing of cattle with a portion of it eroded. In summary, it appears that there were two periods of accelerating soil erosion separated by an era of comriparative stability (Table 1).