The Tsuyama basin, situated between the Backbone Ridge of the Chugoku mountains and the Kibi Highland, is an erosional basin where the Tertiary bed is subjected to the selective erosion. In and around the basin, erosion surfaces having 6 to 7 stairs are distributed. The distribution and poperties of these surfaces may be summerized as follows: 1. "Upper erosion surface" in the Chugoku mountains is distributed around the peaks of the Backbone Ridge with the heights of 900-1300 meters, and confined to the northern part of the "Mimasaka thrust fault". 2. "Middle erosion surface" in the Chugoku mountains is chiefly found on the hills of the basin, the altitude of which 400-600 meters high. The border line between the above "Upper surface" and this "Middle surface" does not always accord with the Mimasaka thrust fault line, and runs in the northern part off the Mimasaka thrust excepting the foot of the Mt. Nagi. In addition, the shallow valleys which belong to "Middle surface" stretch into the Backbone Ridge. Consequently, the period of formation of "Middle surace" may be safely said to be considerably later than that of "Upper surface". 3 "Nihonbara surface" is the oldest surface among Tertiary beds in the Basin. It is protected by the outcrop of hard rocks which are older than the Tertiary bed. The surface of Nihon-genya is typical. 4. In the northern part of Nihon-genya, there are large-scale taluses. 5. Around and among "Nihonbara surface" there are erosional surfaces with 3 to 4 stairs, that is, upper terrace (terrace I ), middle terrace (terrace II ), lower terrace (terrace III), and flood plain. It may be fittingly said that these surfaces were formed through the degradation of Tertiary beds, caused by inter mittent upheaval of the area since the formation of "Nihorbara surface".
