A HYPOTHESIS ON THE DEVELOPMENT OF VALLEYS DISSECTING THE SHIMOSA UPLANDS, LANTO DISTRICT, JAPAN

概要

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The author made a geomorphological survey of the valleys dissecting the Shirnosa Upland, one o the Pleistocene uplands developed in the area surrounding Tokyo. In the studied area, the Shimosa Upland has an elevation of 15-20 meters, and it consists of volcanic ash formations, the so-called Kanto Loam, and sand layers named the Narita Layer. The valleys are insequent with a dendritic pattern. The valley slopes are classified into two categories; steep slopes (20-45°) which are supposedly formed by landslides and gentle slopes (not more than 10°) which are considered to be erosional surfaces. The author investigated the distribution of the two kinds of slopes by field surveys and photo inter-pretation. The development of the valleys on the Shimosa Upland can be generalized as follows; (1) In the earlier stage, a valley is rapidly lengthened by headward erosion by landslides. (2) In the intermediate stage, as headward erosion becomes less active, gentle slopes appear at the head of a valley and subsequently widen the area downstream. (3) In the later stage, no landslides at the head occur any longer nor is a valley lengthened. Gentle slopes develop widely on the head and the sides of a valley.
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