The author investigated periglacial smooth slopes in and around Mt. Yakushidake (2, 926m), the Northern Japan Alps, to discuss geomorphic altitudinal zonation since the later Last Glacial stage.
Present geomorphic altitudinal zonation can be regarded as follows:
Zone I (none): block field zone
Zone II (above 2, 700m): deep reaching and free solifluction zone (periglacial rubble slope zone)
Zone III (2, 400-2, 700m): shallow and partly bound solifluction zone
Slope materials of present periglacial smooth slopes higher than 2, 700m above sea level and adjacent fossil ones have various characteristics, that is, surface openwork rubble layers, structures of particle sorting and/or multi-layers of debris, as well as platy or bladed layers, silt cap and sorted free grain accumulation under boulders. These are assumed to be due to freeze-thaw processes or advances of solifluction lobes. Therefore they can be regarded as good indicators of fossil periglacial slope deposits.
In view of the distribution of fossil periglacial smooth slopes and characteristics of slope materials, geomorphic altitudinal zonation during the later Last Glacial stage is assumed to have been as follows:
Zone I (block field zone): above 2, 500m
Zone II (deep reaching and free solifluction zone; periglacial rubble slope zone): 2, 100 or 2, 200-2, 500m
Zone III (shallow and partly bound solifluction zone): 1, 700-2, 100 or 2, 200m
Though periglacial smooth slopes were formed through the Zone I-II and III, in the Zone III freeze-thaw processes were not so dominant but slope wash and/or alpine debris flows played major roles in their development. Scars of collapses, as well as periglacial smooth slopes, were formed predominantly in the Zone ifi during the Glacial stage.
Periglacial rubble slopes were wider during the period after 4, 500 BP than those in the previous time and in the present. During this period the lower boundary of the periglacial rubble slope zone (Zone II) is assumed to have been placed at about 2, 600m. Taking a broad viewpoint, it coincides with the global cold period after the Hypsithermal (the Neoglaciation).
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