Geomorphological Research of the Tama Hill Area:The Dissection-Units and the Assymmetrical Ridge-Wings of Branch-Hills

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The Miura Group (Pliocene) of the Tama Hill area in the South-westernn vicinity of Tokyo is composed of the marine deposits on the whole. It uphe aved and became the land on the occassion of the Post-Tertiary revolution. The Narita Group (Pleistocene) had deposited in the drowned valleys dissected, on the Miura Grup in the eastern region of this area. The Kwnto Volcanic Ash overlies the above-mentioned groups all over the area. The geological structures of this area can be analized as follows; monoclinal structure dipped to the south-east in the western region, anticlinal structure with the Kamihoshikawa dome in the southern region, and synclinal structure running through from the north-east to the south-west between them as Fig. 1. It seems that this area dipped generally to the north-east in the earliest stage, and that the present structures had been formed by the regional move-ment untill the Post Narita revolution. The author thinks that this areaa was generally dissected untill that time, and that the present late mature form of this area had already been formed nearly before the accumulationn of the Kwanto Volcanic Ash. In the sandy area, the thickness of the Kwanto Volcanic Ash is 3 to 5 meters, and 1 to 3 meters in the area of sand and mud alternation, and less than 1 meter in the mudy area as is shown in Fig. 2. It seems that these differe-nces have been controled by the flat forms of sandy hill-summits and by the-triangular forms of muddy hill-summits, and moreover to be controled by the sheet erosion type of soil that are more active on the muddy hills than. on the sandy hills. The dimension of primary unit of dissection of this area is 29.83km2 on an average (Table 2 and 3). In this case the relation between the geology and the unit is unclear. The secondary units of dissection are distinctly larger in the sandy area than in the muddy area. Forr example, on an average, they are 0.70km2 in the area rich in sand within the alternation of sand and mud in the vicinity of Ilriu, Tama-mura; 0.47km2 in the area rich in mud within the alternation of the same kind in the vicinity of Kurogane-machi, Yokohama-shi; and 0.27km2 in the Muddy area in the vicinity of Miwa, Okagami-mura as Table 4 and 5. (cf. Fig. 5) The author calls thesecondary units of dissection "the branch hills." Most of the branch hills of this area are dissected assymmetrically. (Fig. 6). How these assymmetrical ridge wing were formed? In the case of muddy hill, the wing of geological dip direction is longer than the opposite wing because of the development of gulleys on the wings. (cf. Fig. 7-A). In the case of sandy hill, it relates to the development of the streames under the both side wings, that is, when these streame are young, the side wing of geological dip becomes longer; but when these streams grow and begin the lateral erosion, the opposite wing becomes longer. (cf. Fig. 7-B). In the case of hetrogeneous ridge, the wing slope is steeper on the sandy side than on the muddy side. (Fig. 7-C).
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Geomorphological Research of the Tama Hill Area:The Dissection-Units and the Assymmetrical Ridge-Wings of Branch-Hills

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