(a) Topographic map of Okayama Prefecture and the surrounding area in SW Japan. The area has been divided topographically into three zones, (1) Chugoku backbone range, where the main peaks are higher than 1,000 m, (2) Kibi Plateau consisting of a low-relief surface (200–600 m) and inselberg and (3) Setouchi hillock and delta zone of the three main rivers of the Takahashi, Asahi and Yoshii. (b) Photo of the Kibi Plateau taken from the point (b: the elevation is ca. 300 m) marked in (a) to the westward, showing the low-relief surface (200–600 m) and the inselberg. The low-relief surface slopes down gently to the south from the north. The distance to the inselberg (Tenjin Mt. of 777 m height that is highest in the Kibi Plateau) with an arrow from the point (b) is 54 km. The Jinseki Plateau in the eastern part of Hiroshima Prefecture behind Tenjin Mt. is visible. It is ca. 70 km far from the view point. (c) Photo of the U-shaped valley of the Yoshii river, taken from the point (c) marked in (a) to the northward. The width of the Yoshii river at the point is 220 m. Difference in elevation between the Yoshii river and the left-hand side peak is 390 m. (d) The cross-section of the Yoshi river at the location (d) shown in (a). It shows the form of the “U” and is accurately approximated by a parabola (The blue dotted line shows Y = a X2).