Use of Rock Weathering-rind Thickness to Redate Fossil Periglacial Slope in Mt. Kisokomagatake, the Central Japanese Alps

Abstract

In the alpine areas of Japanese high mountains there are extensively distributed block slopes called fossil periglacial slope or “Glatthang” in German. The period of formation of these slopes, however, has not yet been specified, though it is roughly guessed to be in the last glacial age. In this study we tried to make clear the period of debris production and debris movement on these slopes, using the weathering-rind thickness on blocks as a measure. Our study area is a westfacing windward slope located back to back with Nogaike Cirque on Mt. Kisokomagatake, the Central Japanese Alps. Slope deposits are composed of coarse blocks and the greater part of this slope is covered with Pinus pumila scrubs.To gain the standard values of the weathering-rind thickness, we examined at first the younger morain which deposited at the lower margin of the Nogaike Cique. This morain are composed of large blocks and considered to have been constructed during the maximum glacial age (about 20, 000y. B. P.). There, we gained 5.66mm thick weathering-rind in average.On a fossil periglacial block slope two values of weathering-rind thickness were obtained; 5.85mm in the upper part and 5.64mm in the middle part. Though these values are slightly bigger than that of morain deposits, these three values are considered similar to one another. Judging from these data, fossil periglacial slope in this area is estimated to be formed in the last maximum glacial.