The Ihara Group is found on the Iwabuchi, the Habuna and the Hoshiyama Hills around the lower Fuji River. This area is considered to be northern extention of the the Suruga Trough. The Ihara Group, whose age is assigned to be the Early to Middle Pleistocene, is divided into the Kanbara and the Iwabuchi Formations in ascending order. The Kanbara Formation is consisted of the Kanbara gravel and the Bessho gravel Members. The Iwabuchi Formation is subdivided into the Iwabuchi volcanic rock, the Minamimatsuno sand and gravel, the Senzui sand and gravel, Saginota gravel, the Koda gravel, the Mizunuma gravel and silt, the Kamihabuna sand and gravel and the Hoshiyama gravel Members. The Iwabuchi Hill is characterized by NNE-SSW-trending synclinal structure (the Umusegawa syncline). The northern part of the Habuna Hill is characterized by a series of ENE-WSW-trending fold. The Kanbara gravel Member was supplied with gravel through the Fuji River. On the other hand, the Bessho was supplied through the river from the Tanzawa Mountains or the area below Mt. Fuji. The Iwabuchi Formation is consisted of the Iwabuchi volcanic rock and the lagoonal sediments isolated from the outer sea by topographic high formed by the volcanic activity. The gravels of the Iwabuchi Formation were supplied through the Fuji River, however, the Kamihabuna sand and gravel Member in the northern part of the Habuna Hill was supplied through the Fuji River and also the river from the Tanzawa Mountains or the area below Mt. Fuji.
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