THE GEOMORPHOLOGICAL DEVELOPMENT OF THE HIDA MOUNTAINS AND THEIR CIRCUMJACENT REGION

Abstract

In order to understand the geomorphological development of the Hida Mountains and their circumjacent region (Fig.1), the writer finds it necessary to settle the following basic problems. They are: 1) The relation between volcanic activities and river terraces; 2) The relation between volcanic activities and glacial age; 3) The relation between glacial age and terraces; 4) The landform at preglacial age and the age when erosion surfaces in the Hida Mountains were formed, etc. In this paper, the writer has tried to discuss these fundamental problems. 1) The volcanic activities that exercised an influence upon the formation of the river terraces of the Jinzu River are characterized by acidic andesite. This eruption brought volumes of tuffucius andesitic sands and hornblende andesite gravels along the valley, and formed the high terrces but they are fragmentary down the stream. This originated in northern or north-eastern part of Norikura volcano group at the late Dl stage. After these volcanic accidents the transition of topographical formation was made down the Jinzu River (Fig.2) The river terraces which can be correlated to be the incipient activities of Tateyama volcano are unknown. The subsequent volcanic activities formed Midagahara lava plateau. Its lava is chiefly constituted of two pyroxine andesite and mud flow. The lava plateaus at Tengudaira and Goshikigahara formed by the later activities are characterized by hornblend andesite and mud flow. The gravels brought by these eruptions are to be found in the rivr terraces beds at Awasuno and Ashikuraji. (Fig. 5, 6). 2) At Mt. Tateyama, the hanging glaciated valley (Hida glacial II a) was formed eroding the late lava flow of Tateyama volcano. As a result of the survey of lower moraine at the glaciated valleys (Hida glacial I) and the climatic terraces of the adjoining ones, and besides the comparision between their altitude and that of the summit, the writer has concluded that Hida glacial age came at the early mature stage when the existing river system had been nearly shaped (Fig. 9). The lower erosion surfaces in the Hida mountains were formed by the Dl stage. 3) Kitamatadani river terraces continuous to the moraine at Shirouma-Daisekkei were formed at the glacial age (Fig.11) These terrace beds are thick and mainly composed of angular gravels, where were found gravels with striae. The writer has compared the condition of these deposits with the one of those at the westerm side of the Hida mountains. In Awasuno gravel beds are mixed a lot of gravels derived from the lava which one formed Midagahara. The height of Awasuno gravel beds is very near to the assumed forest limit at Hida glacial age (Fig. 12). From the above data, the writer has concluded that Awasuno gravel beds are the outcome of the both volcanic activities and the maximum expansion stage in Hida glacial I. Basing upon the fact and interpretations as stated above, the writer has made the chronological table of the geomorphological devlopment of the Hida mountains and their circumjacent region. (Fig. 16). The highest dissected fans in the western part of the Hida mountains were formed at the late Dl stage and the high eleveted fans at Dm stage. About that time Midagahara lava plateau was formed. At glacial age (Hida glacial I) these elevated fans were eroded and up the rivers deposition was continued till rejuvenation began. At the recessional stage of Hida glacial I, the water river became overflowing, and by the help of base elevation caused by crustal movement, transported gigantic boulders, forming here and there rock terraces. At Dm stage the Kurobe River formed a canyon, and later at Du stage it was more and more eroded which was greatly influenced by the joint of granite.