A Reevaluation of the Origin of Blocky Landforms in Cirques in the Japanese Alps

Abstract

In the Japanese Alps, only a few rock glaciers associated with mountain permafrost have been identified, although the present lower limit of mountain permafrost is estimated to lie at 2, 500 to 2, 800m a. s. l. and permafrost is considered to have been distributed more widely during the previous cold periods than at present. The purposes of this paper are to reexamine the origin of blocky landforms in cirques, which have often been interpreted as moraines or protalus ramparts, and to reconstruct rock glacier distribution in the Japanese Alps. As a result of air-photo interpretation and field observations, many rock glaciers were identified in the Northern and Southern Japanese Alps. These landforms comprise transverse low ridges and furrows, a continuous marginal ridge, or a tongue or festoon-shaped frontal ridge located at the foot of talus. At one of these landforms, a two-layer structure, comprising a matrix-supported finer debris layer overlain by an open-work bouldery layer, was observed. These morphological and structural characteristics are similar to those of typical fossil rock glaciers, suggesting that the blocky landforms are fossil rock glaciers rather than moraines or protalus ramparts. Therefore attention should be paid to the origin of blocky landforms when past glacial distribution and the paleoenvironment are reconstructed using landforms.