Abstract
The authors concluded a study of the depositional slopes distributed in the Chugoku Mountains. These depositional slopes are divided into four types: colluvial slopes, block streams, debris-man-tled smooth slopes, and rock fall talus. The colluvial slopes are distributed in the piedmont area, and are divided into six types: Surface I, II1, II2, III, IV and V, respectively. The block streams, which can be divided into the Upper and Lower Block Streams, and the debris-mantled smooth slopes are located around the summit. Rock fall talus is found at the flanks of the mountains. Several tephra, which are useful as time markers, are embedded in the sediments of these slopes.
The formative ages of the colluvial slopes were revealed to be as follows: Surfaces I, II1, II2, and III were formed prior to the last interglacial stage, during the first half of the last glacial age, during the second half of the last glacial age, and between the late glacial age andapproximately 6, 000 years ago, respectively. Then followed the formation of Surfaces IV and V. The Upper Block Streams took shape during the first half of the last glacial age, while the Lower Block Streams were formed during the second half of the last glacial age. The rock fall talus and the debris-man-tled smooth slopes were formed during the second half of the glacial age.
Each of the formative ages of Surfaces II1 and II2 the block streams, the rock fall talus and the debris-mantled smooth slopes is correlated with a cold period. This is because the rock fragments of the sediments were produced by frost shattering and then were shifted by various types of mass movements, including solifluction. This suggests that these formative processes are closely related to the periglacial environment.
