In Gotsu district, central part of Shimane Prefecture, highly weathered reddish plutonic rocks are developed. These reddish weathered crusts in part underlie the Plio-Pleistocene Tsunozu formation. Main mineral compositions of the reddish weathered crust are kaolin and vermiculitic mineral. Undoubtedly these reddish weathered materials must have been formed under much warmer climatic circumstances during late Pliocene judged from the stratigraphical relation of the weathered crust to the Tsunozu formation. Not only climatic condition and physiographic features but also chemical and physical properties of the rocks take part in the weathering processes. In this study, the writer proposes a “degree of the absolute chemical freshness” based on the following empirical formula: Degree of the absolute chemical freshness FeO+MnO+MgO+CaO+Na2O+K2O/Al2O3+Fe2O3+H2O (+) (Weathered rock) /FeO+MnO+MgO+CaO+Na2O+K2O/Al2O3+Fe2O3+H2O (+) (Fresh rock) This formula is applicable to the applied geological works.
The area where the landslides occurred is situated to the west of Atsushio in the Higashiaizu area, Fukushima prefecture. Geologically the area is occupied mainly by Neogene sediments and Quaternary deposits. The landslides in this area can be divided into three types, on the basis of geoglogic features, as follows: 1) Landeslides along the line of the fault zone. 2) Block glides down the bedding plane. 3) Rock fragment flows. Among them, the Block glide type causes the greatest disaster. The mechanism of the Block glide was pursued in our studies, and in our efforts to determine it the author carried out the stability calculation of the landslides and the detailed geological survey. And it was concluded that Block glides are due to the permeation of water into vertical crucks, which are controlled by the geologic structure and are caused to occur at intervals of 1.5 meters.