FOSSIL PERIGLACIAL SMOOTH SLOPES AND FOSSIL NIVATION HOLLOWS ALONG THE MAIN RIDGE IN THE MIKUNI MOUNTAINS, CENTRAL JAPAN

In this paper the author discusses thee vertical changes in the distribution of morphocli-matic zones from the Last Glacial Stage to the Post-Glacial Age on the basis of the fossil periglacial slopes and nivation hollows observed in the Mikuni Mountains, Central Japan.<br> The fossil periglacial smooth` slopes and nivation hollows were formed during the period from ; the Last Glacial Stage to the Post-Glacial Age. The former are convexo-concavo slopes not having a free face at their head unlike a talus. The latter are shallow depressions often located on gentle slopes near: ridges. The lower limits of their distribution are in general 1, 300-1, 400m a. s. 1. The slope mater ial mainly consists of angular-subangular gravel with a fine matrix covered with kuroboku and/or peaty soil. Its texture is characterized by vertical sorting and a fabric in which the long axis of individual gravels tends to be aligned in the same direction as local gradient: of surface slopes but less steeply inclined. These features suggest that they are solifluction deposits. In details, cross laminations are sometimes observed in the slope material beneath the fossil nivation hollows, indicating that rapid solifluction and slopewash operated on its movement.<br> At some sites of the fossil periglacial slopes and nivation hollows, angular-subangular gravel beds with openwork texture intervene in the slope material. The long axes of the gravel show slightly isotropic and/or oblique orientation to local gradient of surface slopes. This fact indicates that the gravel are almost autochthonous or that small-scale rockfall and rolling down operated on their movement.<br> The kuroboku and peaty soil usually contain Kikai -Akahoya ash-fall deposit (Ah, ca. 6, 300 y. B. P.) even at the summits (ca. 2, 000m a. s. 1.) . At the area lower than 1, 600 1, 750m a s. 1., the slope material and bedrock are often overlain by Tsumagoi pumice fall deposit (YPk, ca. 11, 000 y. B. P.) and the fine matrix of the slope material contains Aira-Tanzawa ash-fall deposit (AT, ca. 21, 500 y. B. P.). But the matrix of the slope material often contains a great number of “YPk” pumice grains in the area higher than 1, 600-1, 750m a, s.1. From these observations the succession of slope development in the Mikuni Mountains from the Last Glacial Stage to the Post-Glacial Age is inferred as follows;<br> During the Last Glacial Stage before the fall of “YPk”, periglacial smooth slopes and nivation hollows had been formed zonally in the area higher than ca. 1, 300-1, 400m a. s. 1. The vegetation was very poor at that time and it was much colder than the present.<br> The lower limits of the area covered with perigalcial smooth slopes and nivation hollows had risen since the late stage of the Last Glacial Age. Periglacial smooth slopes and nivation hollows were restricted in the area higher than 1, 600-1, 750m a. s. 1. around the age of “YPk” fall (ca. 11, 000 y. B. P.).<br> At the age of “Ah” fall (ca. 6, 300 y. B. P.), the formation of periglacial smooth slopes and nivation hollows ceased in the whole area of the mountains, even on the summits. Since then most of periglacial smooth slopes and nivation hollows have been stabilized being covered with kuroboku and/or peaty soil.