著者

吉永 秀一郎 武内 和彦

出版者

東北地理学会

雑誌

東北地理 (ISSN:03872777)

巻号頁・発行日

vol.38, no.1, pp.1-15, 1986

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The Tama Hills, located in the western edge of the Kanto Plain, are mainly composed of semi- or unconsoidated shallow marine sediments of Plio-Pleistocene age, which, named the Kazusa Group, consist of several clay-silt, sand and gravel beds, On the hilltop gentle slopes in the western part of the Tama Hills, the mid-Pleistocene Gotentoge Gravel Bed rests unconformably upon the Kazusa Group. And the Hills are covered with airborne tephras named the Kanto Loam. The lower part of the Kanto Loam is full-weathered and rich in clay. The purpose of the present study is to clarify the distribution and formation of detailed- to micro-scale landforms composing small tributary basins, in connection with the hydro-geologic conditions in the western part of the Tama Hills. In valley-head areas, head hollow and head floor defined by Tamura(1974) are distributed and their location seems to be controlled by the hydro-geologic conditions. The maximum height of each head hollow and the position of springlet correspond well to the height of bedding plane of clay-silt bed in the Kazusa Group or the lower part of the Kanto Loam, both of which have less permiability. Head floor develops on the surface of less-permeable beds. Because of the less permiability, surface and sub-surface flow often occurs and marsh is sometimes formed on this slope unit. Locations of valley-head slopes on the hillsides are also affected by the distribution of lesspermeable beds. Here the development of the slopes seems to have been initiated by the landslides. Present head hollows were formed by slide-type failure. Each mass movement occurs along the bedding plane upon the less-permeable beds. Valley-head slopes are considered to have been formed by surface failures occurred during heavy storms. In addition to the occurrence of pipeflow and returnflow which are also related to hydro-geologic condiions, groundwater itself seems to pllay an important role in landforming process accompanied by surface failure. It is concluded that the distribution and formation of slope units in the Tama Hills are strongly controlled by the hydro-geologic conditions.