Abstract
On the south side of the River Usui, the erosion surface of Iwanoya Hill slopes from NE to SW, while the terrace plains of Usui slope from SW to NE, so that they form the “Hinge feature”. (Fig. 1, Fig. 2)
The writer researched the distribution of terrace gravels, from which he presumed that the River Usui probably flew on the south side, of the hill and r n into the valley of Kabura. (Fig. 3) As it is presumed that, the western part of Iwanoya Hill was as high as the eastern part, when the River Usui flew on the upper terrace plain afterr changing its course, so it seems that the formerr part suffered more denudation than the latter.
Moreover, the terrace plain which remains highest in the western area of this region, is covered by gravel beds, and the north-eastern part of the hill which also remains higher in that area is chiefly composed of conglomerates, while the lowerr western part is composed of soft mud stones and sand stones. From these facts the writer concludes that such “Hinge feature” may be interpreted by the difference of resistance to erosion which is effected by the distribution of gravels.
