木曽山脈主稜部における周氷河性岩屑生産

抄録

The purpose of this paper is to clarify the present and past periglacial debris-production in the alpine zone of Japanese high mountains. The studied areas are on the main ridge and its surroundings of Mt. Hinokiodake (2, 728m) and Mt. Nigorizawaôtake (2, 724m) located in the central part of the Central Japan Alps which is composed of granodiorite. Forest line runs at about 2, 650m. The results are summarized as follows:
1. Debris-production is closely related to the density of joint developed in the base rock. Joints are classified into 3 classes (A-C), according to their shape and size. Both A and B class joints are systematic, but with respect to intervals the former is not less than 1m, whereas the latter is about 40-60cm. C class joints are nonsystematic, and their intervals are about 8-12cm or less.
2. C class joints are mostly open fissures. The places where these joints are developed coincide with fault-crushed zones and show high joint density of 20-35/m. These places are now frost-splitted and supply a small amount of fresh debris. The existence of debris fields on the ridge and taluses can be regarded as the result of such supply (Fig. 2).
3. The places where A and B class joints are developed show low joint density of 2-10/m. Both class joints are tight in general and now stable, because frost-splitting along these large joints is presumably impossible under the present climatic conditions. Therefore fresh debris are scarcely produced in such places.
4. Possibly during the Last Glacial Age, however, all three class joints were opened by active frost action under the cold climate and a large amount of debris containing massive blocks of 1-3m in diameter was produced from bedrock. To the west of the main ridge, these debris was transported by active solifluction and formed block fields. Debris-covered gentle slope was developed through these processes. This slope is now stabilized and covered with vegetation.