著者

石井 武政 磯部 一洋 相原 輝雄

出版者

Japan Association for Quaternary Research

雑誌

第四紀研究 (ISSN:04182642)

巻号頁・発行日

vol.26, no.1, pp.85-92, 1987-05-31 (Released:2009-08-21)

参考文献数

18

被引用文献数

1 1

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Gentle slopes are distributed between uplands and lowlands in the Tsukuba Upland area, Ibaraki Prefecture, Japan. These slopes are dominant on the left bank side of streams flowing to the west and on the right bank side of streams flowing to the southeast. Opposite side slopes, in contrast, are generally steep. In other words, topography along the streams in upland areas shows an asymmetrical valley pattern.Drilling samples show that the upland is occupied, from top to baseward, by the Younger Loam (equivalent to the Tachikawa and Musashino Loams) and the Joso and Narita Formations of Quaternary age. The gentle slopes are underlain by slope deposits of poorly sorted silty sands and gravels 1-3 meters thick. The deposits consist of particles essentially derived from the Joso Formation.The slope deposits lie unconformably on the Joso and Narita Formations and are overlain by the upper Younger Loam which has AT (Aira-Tn Volcanic Ash) dated at 21, 000-22, 000y. B. P. in its basal part.This stratigraphic evidence indicates that the slope deposits were formed during the last glacial age. It is assumed, from the relation between the slope direction and the frequency of slope cryoturbation in the winter season, that the slope deposits were accumulated by refrigeration in the cold climates of the late Pleistocene.