Nivation Hollows on the Southeast Slope of Mt. Onishi, Iide Mountains, Northeast Japan

Abstract

Many well-developed nivation hollows, 100-200 meters in diameter, are found in two large hollows (glacial cirques?) on the southeast slope of Mt. Onishi, central part of the Iide Mountains. In this paper the writer observed geomorphological characteristics and soil profiles of the nivation hollows, monthly distributions of snow patches and zonations of plant communities in and around the hollows (cf. Fig. 3-7). He also attempted to explain nivation processes forming the hollows from the above-mentioned observations and the result of an experiment.
Within a nivation hollow silt, sand and small gravel are transported mostly by rill-wash of rain water and snow meltwater. Average annual transport distances of gravel (∅1-5cm) have been known to range between 1.3 and 7.8cm from deformations of four experimental lines of painted gravel. Movements by solifluction and/or frost creep have not been observed from deformation patterns of these lines. Partial creeping of snow due to formation of crevasse in snow patch drags coarse materials, and it breaks down and shaves basement rocks. Both removal of sediment by rill-wash and denudation by creeping snow are relatively active on the core area of the hollow where lack of vegetation increases mobility of debris, while peripheral areas are protected against erosion by vegetation.
However, weathered granitic bedrock under thin debris in the hollow suggests that mechanical denudation working is slight at present. Nivation hollows are thought to have been formed in the past when nivation was much more vigorous. Many relict nivation hollows covered with vegetation and those dissected by gullies are scattered in the Tide Mountains.