抄録
Mt. Nagi apperes to present a subdued form, and on its southern side it goes down to the basin of Nihonbara as a scarp of the Nagisan fault. There are some low hills and dissected fans which seems to be of Tertiary age. A zone of soil of usual apperence extends in a W-E direction between the moutainland and the flat parts of the Tertiary beds and the alluvium, which the author thinks is colluvium. The term “colluvium” originally refered to a soil possesing certain petrographic characteristics. The author, however, used the word here for a certain soil accumlation based on geological and topographical characteristics.
The colluvium consists of angular gravels of diameters less than 10cm, and probably covers to a certain extent the fault plane of Mt. Nagi. The inclination of the colluvium, which is 100 to 200‰, is steeper than that of ordinally alluvial fans and elevated deltas, but more gradual than that of talus. If the breadth of the valley of the upper stream is wide, the colluvium is accumulated in the form of a fan. In a series of valleys of smaller breadth, the colluvium forms a continuous surface, which should be called colluvial base.
When the mountain at the back is of subdued form, that is, the elevation of the mountainland is proceeding at a very slow rate, and the conditions of the rock is suitable for the formation of the colluvium, it may be concluded that conditions are right for the deposition and accumulation of the colluvium.
