Some Physio-geographical Considerations on the Volcanogenous Deposit, Akatsuchi

Abstract

The volcanogenous deposit, Akatsuchi of aeolian origin, which covers hills and uplands in the southern Kanto Lowland, is called the Kanto Loam Formation stratigraphically and is divided into four members, namely Tachikawa-, Musashino-, Shimosueyoshi- and Tama-Loam, from upper to lower. The author is going to report the result of his study, summarized in the following three problems.
Firstly, the boundary between the Tachikawa-Loam and the Musashino-Loamis discussed in relation to the two Sagamino Scoria layers sandwiched in these Loam members. There can be classified the Tachikawa Terrace group on which the Tachikawa-Loam covers, and the Musashino Terrace group on which the Tachikawa- and Musa shino-Loam lie, but the two terrace groups are considered to he a series formed in the same regression period probably caused by the glacial eustatic change of sea Level.
Secondly, buried soil bands interposed in the Akatsuchi are discussed. At many places, the one which makes the base of the Musashino-Loam has a colorless elluvial zone on a colorfull illuvial zone, and the fact suggests the formation of “podsolic” type of soil. The soil of the same type is seen in present Japan about eight degrees (latitude) to the north, so the buried soil band is supposed to have been formed under a colder climate than it is today, and it may be called a fossil soil.
Thirdly, the landslips in Yokohama and its surrounding area owing to two torrential rains on 26th Sept. 1958 and 28th June 1961 are investigated. Heavy rain, landform and geology are important among the factors relating to these landslips. As for the Akatsuchi, the Shimosueyoshi-Loam is inferior to the Tachikawa-Loam or the Musashino-Loam in terms of consistency and permeability, and the presence of the Shimosueyoshi-Loam is responsible to the collapses. But besides these factors, one cannot overlook the human activities which have ignored the characteristics of the Akatsuchi and other basal deposits which helped enlarge the disaster.