A PRELIMINARY STUDY OF PATTERNED GROUND ON SOME MOUNTAINOUS REGIONS OF CENTRAL AND NORTHERN JAPAN

Abstract

There is a stringency about the existance of periglacial activities in some districts of Japan from the last glacial stage to recent. Some amounts of studies on periglacial pheno-mena relative to both fossil and recent types have already been made, and a few attempts were carried out to outline the palaeoclimatic zones or palaeogeographical features of Japanese Islands during the Wurm stage using some data indicating palaeoclimates by field observations about fossil patterned ground and other features distinguished as periglacial phenomena. While the concept that patterned ground indicates the palaeoclimate or palaeogeography has been accepted, there remained some confusions between terminology and genetical ideas about patterned ground in Japan. So, it was hoped that from the viewpoint of genetical ideas, re-checking of patterned grounds might provide profound data for palaeogeography, if the works would be carried out without confusions. In this preliminary study, the authors did on-the-spot investigations at such mountains as Mt. Kamikochi (2803m) and Mt. Chausu (2600m) in the Southern Alps and Mt. Kaun (1954m) of Taisetsu Volcano in Central Hokkaido. After these field works, we analysed the observed data and materials, and discussed on genetical ideas and environment, under which these patterned grounds develop. Conclusions are summarized as follows: (1) According to Washburn's classification of patterned ground, the patterned ground we observed is classified as “sorted circles” from its morphological feature, and there are two groups of different sizes, diameter 10-15cm and 100-150cm. (2) The places where sorted circles develop are fiat or gentle slopes covered with weathe-red debris without vegetation covers, or some depressions formed by snow patch erosion and dammed up shallow valleys. Larger size sorted circles develop at the bottom of depressions, which are sometimes filled with snow-melted water during summer and also with weathered materials from bedrock. The altitudes are usually around 2, 500m in the Southern Alps, and around 1, 600m on Mt. Kaun in Hokkaido. In these areas, the altitude is above the timber-line and nearly the some with the zone of pines montana. (3) The observations of soil profiles of sorted circles show two stratified layers, coarse sand and debris for the upper, fine well sorted sand and silt for the lower. The boundary of these two layers is sharp and not horizontal but waved. Careful taking off of the upper layer enables one to observe the three dimensional forms of the boundary, and it shows the exi-stance of mound-like forms. There is an obvious relationship between this mound form and sorted surface features. The center of sorted circles consisted of fine materials coincides with the top of the peak part of mound form. The fringed contour of sorted circles consisted of coarse materials concides with the outskirts of mound. So this mound form controls the size of sorted circles. (4) There are no fissures and no sand wedges which cut deep the fringed contours of sorted circles. But the mound form of boundary shows the process that the lower fine sand and silt layer extrudes into the upper coarse sand and debris layer without developing fissures. (5) The features of sorted circles mentioned above support the idea by A. E. Corte, that is, by the multicycles of freeze-thaw, the lower fine material layers extrude into the upper layers, and this differential movement of particles forms the mound form. The depth of lower fine layers, termed as active layer, controls the size of sorted circles. By other mecha-nical sorting processes, the surface sorted circles will develop. Of course, it is hard to explain all processes and mechanisms of sorted circles by restricted genetical ideas, but the further advanced fieldworks and laboratory methods are hoped to be carried out in some areas distinguished as periglacial regionss in Japan.