MECHANISM OF THE FORMATION OF A PROTALUS RAMPART AT THE BOTTOM OF THE SO-CALLED KURANOSUKE GLACIAL CIRQUE, JAPANESE ALPS

Abstract

The Kuranosuke Glacial Cirque locates in the southern part of the Tateyama-Tsurugidake Range in the Northern Japanese Alps, Central Japan. The debris mound in the Kuranosuke Glacial Cirque was first reported by Dr. Imamura (1940), concluding that it was a nivation moraine. Based on the gemorphological study of glacial cirque, Dr. Fukai (1968) reported that the debris was a glacial moraine without describing the reason. In the Japanese Alps today, a large amount of rain and snowfall are observed. Therefore, accumulation of post glacial debris has been intense at many cirque bottoms in the Japanese Alps. In the Kuranosuke Glacial Cirque, both snow drifting on the wind sheltered slope and amassment of snow by avalanches at its bottom in winter are important factors to nourish snow patches. On the east to north-east facing wall and at the bottom of the Kuranosuke Glacial Cirque which are sheltered from insolation, there are residual snow patches until late summer, and some of these snow patches remain until next year. According to the author's observation, the snow patch seems to play an important role in transporting debris which now forms the debris mound at the cirque bottom, since the surface of snow patch is smooth enough to slide down the debris caused by heavy rain. The main purposes of this paper are to make clear the mechanism of the accumulation of debris forming the protalus rampart, and to examine whether the debris mound at the cirque bottom is a so-called glacial moraine or protalus rampart. In July 1971, the cirque was entirely covered with the remaining snow. In September of the same year, deposition of new debris over the residual snow patch was observed. However, in October, it was entirely covered with new thin snow. The process of transport of this debris observed is shown in Fig. 4 as schematic profiles. It is certain that the debris had partly slided over the smooth surface of the snow patch. Air temperature of the Japanese Alps never fell below 0°C in this summer. As to the mechanism of formation of a protalus rampart, it is considered in Europe and America that a protalus rampart is to be created by frost shattering. However, as illustrated in Fig. 4, transport of the debris to the bottom of the slope is largely made by heavy rainfall in midsummer. Therefore, so far as the Japanese Alps, the role of heavy rain should highly be considered. As mentioned above, the formation of this debris mound was attributed to a glacial process (S. Fukai). The author utilizes “sedimentary fabric” in order to make clear the transported direction of the debris and its mechanism. Debris mounds in the studied area are classified into two types; (1) granltic debris mounds pushed out from the east-facing cirque wall which is smaller than granodioritic debris, and (2) granodioritic debris mounds pushed out from the north-facing wall of Mt. Fujino-Oritate, in which the diameter of some of them exceeds 5 meters. The pushed out directions of both types of debris are also demonstrated by analyzing relationship between the rock property of debris and the geology of Kuranosuke Glacial Cirque. A snow patch exists between the debris mounds at the cirque bottom and the cirque wall (Type 1). Because of depositing the debris mound at the foot of the cirque wall, neither the cirque bottom nor snow patch exist there (Type 2). From the above fact, the author wishes to conclude that the debris mound of the second type is not a glacial moraine, but a true protalus rampart. The author emphasizes the following fact as a conclusion that, in the Japanese Alps, the protalus ramparts being formed in recent time and other topographical characteristics are influenced not only by snow and ice but also by heavy rainfall.