FOSSIL PERIGLACIAL SLOPES ON THE CHICHIBU MOUNTAINS, CENTRAL JAPAN

Abstract

This paper describes fossil periglacial slopes on the Chichibu Mountains with the purpose of reconstruction of periglacial area during the Last Glacial Age. Smooth slopes covered with blocks (about 0.5 to 5m in diameter) are distributed around the main ridge of the Chichibu Mountains (Fig. 1). The thickness of the block slope deposits is about 1 to 10m. Formative stages of such block slopes can be divided into three: Kumotori-stage and Kinpu-stage I, and Kinpu-stage II, with Kinpu-stage I being older than Kinpu-stage II. The block layers of Kinpu-stages I and II consist of blocks without matrix, while those of Kumotori-stage with sandy silt matrix. The Kinpu-stage I layer rests upon the Kumotori-stage layer (Photo. 1). This fact indicates a change in a sedimentary environment between Kumotori-stage and Kinpu-stage I. At present, the block slopes of Kumotori-stage and Kinpu-stage I are covered with subalpine coniferous forests, and those of Kinpu-stage II with Pinus pumila or without vegetation. The pumice fall deposit “Pm-I” (80, 000 yr B. P.) is found underneath the block layer of Kumotori-stage (Photo. 2), and the volcanic glasses of “Akahoya ash” (6, 000 yr B. P.) in they humic soil resting on the block of Kinpu-stage II. Therefore, the block slopes are confirmed to have been formed under the periglacial condition during the Last Glacial Age. The outermost rim of these block slopes indicates the limit of periglacial area that has experienced intense frost action during the Last Glacial Age. The block slopes were most widely distributed during Kumotori-stage which is tentatively correlated to the early stage of the Last Glacial Age. They are distributed on the slopes higher than 1, 800 m in the western part, and around summits about 2, 000 m high in the eastern part of the Chichibu Mountains (Figs. 1 and 2). Kinpu-stage I is tentatively correlated to the maximum stage of the Last Glacial Age. The extent of block slopes during Kinpu-stage I has not been mapped yet because of lack of outcrops. Distribution of the block slopes of Kinpu-stage II is limited to slopes higher than 2 300 m around Mt. Kinpu-san (Figs. 1 and 2). This block slopes are presumed to have been formed during the latest stages of the Last Glacial Age. Such younger slope deposits have halted forest-growing on the slopes (Suzuki and Shimizu, 1982). From the existence of inthese three stages in the block slope development, it is suggested that the lower limit of periglacial area has inter-mittently risen during the Last Glacial Age to the post-Glacial Age (Fig. 7).