奥羽山脈中部の大規模 mass movement

抄録

The distribution of large-scale mass movement in the central area of Ou Mountains including southern Rikuzen Hills, which has an area of over 1ha per one movement is surveyed. The relation between largescale mass movement and erosional processes of this area is studied. Results are as follows.
When this part is divided into three geomorphological unit—volcanic mountainland, non-volcanic mountainland and hilly land—, large-scale landslides are most conspicuous in the volcanic zone and least in the hilly area.
In the volcanic zone an desitic lava covers tuffaceous Tertiary basement, and large-scale landslides result from such a geological setting. Kuwanuma landslide (Fig. 3) is the largest, that has an area of 720ha. Generally speaking, this type of landslide is common to the old volcanoes with gentle surfaces covered by cap rock layer. This is a different pattern of volcanic dissection against the type of linear erosion seen on a new conide.
In the non-volcanic zone, the landslides are of rapid type and in small scale as compared with volcanic zone, and landslides occurred on the mountain slope in early mature stage.
In the hilly area most hills overlaid by resistant rock are dissected through the following processes—(1) linear erosion by stream into the flat hill-top surface, (2) occurrence of large-scale landslide, (3) stream erosion in the sliding mass, (4) occurrence of secondary landslide, (5) washing away of the sliding mass and linear erosion by stream in the surface of rupture, (6) formation of the dendritic ridge pattern. The landform development of Rikuzen Hills will be explained by application of the above-mentioned processes.