GEOMORPHOLOGY OF THE ASHIGARA PLAIN IN KANTO DISTRICT : WITH SPECIAL REFERENCE TO THE RECENT TERRACES

Abstract

The Sakawa river, 45km long,flows from Mt. Fuji down to the Sagami Bay. The lower reaches of this river forms the Ashigara alluvial plain. There exist two kinds of Recent terraces in the plain. The author conducted a geomorphological survey of the distribution and the structure of these Recent terraces in order to show that the formation of an alluvial plain can be affected by volcanic activity and crustal movement. The greater part of the plain is occupied by a fan which was formed by the Sagami river. In addition, there are many small scale landforms such as sand dunes along the shoreline, small fans by the Oiso hill, and valley plains at the foot of Mt. Hakone (Fig. 2 ). The Recent terrace I surface, which is distributed fragmentarily in the gorge of the middle reaches and stands about 30m above the present river bed, corresponds to the Gotenba volcanic mudflow surface at the foot of Mt. Fuji. According to Machida, the volcanic mudflow occurred about 2, 300 years ago. In the Ashigara plain the distribution of the Recent terrace I surface is limited to the fringe of the Sekimoto hill and Mt. Hakone. The relative height of the terrace cliff between the Recent terrace I surface and the present river bed decreases rapidly towards the lower reaches of the plain: 25m at the lower exit of the gorge, 10m at Komagata-Shinjuku, 2m at Numata and so on. Consequently, the Recent terrace I surface is underlain by the present river bed. The Recent terrace II surface is either erosional surface or contemporaneous depositionl surface. Erosional surface is distributed in the gorge and about 10m lower than the terrace I surface. Depositional surface is preserved broadly around Kamonomiya on the left bank along the lower reaches in the plain. The terrace cliff of the II surface increases progressively from O. 5m at Nobukiyo to 4m near the seashore in its height (Fig. 3 , 4). The deposits constituting the Recent terrace I surface are basaltic sand and gravel. Most of the gravels are sub-angular, and laminae are well preserved in the bed. It is considered that these deposits were formed by rapid deposition of short duration. On the other hand, deposits constituting the Recent terrace II surface in the plain can be classified into three beds, Iower C_2, S, and upper C_3. The C_2 bed consists of clay, silt and humus. The S bed consists of the basaltic sands O. 5 to 3. 5m in thickness. The C_3 bed consists of clay, silt and sand with irregular thickness. The C_3 bed forms the present micro-relief of the ground (Fig. 5 ) (Tab. I ). The Recent terrace H around Kamonomiya is uplifted by the crustal movement. The amount of uplift becomes larger to the south and the west. It is considered that an uplift has amounted to ovcr 3m since 2, 300 years when the basaltic sand which corresponds to the Gotenba volcanic mudflow was deposited.