1987 年 26 巻 1 号 p. 27-45
In this paper mass movement and slope formation in the central Kitakami Mountains are discussed with special reference to their periods. Many tephra layers of the late Quaternary found in the study area enable to study the periods of mass movement and slope formation tephrochronologically (Fig. 2).
The slopes are classified as follows (Fig. 3). (1) gentle slopes on the summits, (2) piedmont gentle slopes, (3) fan-like gentle slopes, (4) smooth crest slopes, (5) upper head hollow slopes (continuing smoothly from the surrounding slopes), (6) lower head hollow slopes (smaller than (5), surrounded by clear breaks in slopes), (7) talus and alluvial cone, (8) other slopes. In the study area two periods of mass movement, chiefly by solifluction, are confirmed during the Last Glacial.
The first period was in the early Last Glacial Stage, perhaps around 50, 000y.B.P., and the second was in the late Glacial Stage, between 30, 000 and 10, 000y.B.P..
These periods of mass movement correspond to those of the involutions under periglacial climate in the Northern Kitakami Lowland area (Endo, 1977) (Fig. 7).
In the study area not only were well-jointed bedrocks such as shale, slate, and schist susceptible to frost shatterin, bnt also fallen volcanic ash and soil produced from deep weathered bedrock were also susceptible to solifluction.
Gentle slopes on the summits and smooth crest slopes have been formed by bedrock frost shattering and solifluction in these periods of mass movement by surface processes. Gentle piedmont slopes have been formed by solifluction. Upper head hollow slopes were developed as smoothly concave profiles by debris accumulation. Fan-like gentle slopes were developed chiefly by slope wash (partly by solifluction in the Last Glacial Stage) at the same time.
Most of the piedmont gentle slopes and fan-like gentle slopes began to form in the early Last Glacial Stage or in the cold period before the Last Interglacial Stage, and the deposits of the late last Glacial Stage, which is generally recognized as the maximum period of the last Glacial Stage in Japan, are only 0.5-2.0m thick.
During the warming period from the latest Pleistocene to Holocene, landslides have formed lower head hollow slopes and alluvial cones have been formed at the outlet of valleys with small river basins.