Late Pleistocene Glaciation and Terrace Topography in the Ina Valley, Central Japan

Abstract

The Ina valley has been well-known as one of the typical areas of the river terrace topography in Japan. In this paper, the classification of the geomorphic surfaces and the history of terrace formation in the area are examined from the view point of volcano-stratigraphy. The stratigraphical relation between the tephra units (Shinshû Loam) and the glacial morphology of the Kiso mountains affords a good explanation for the glaciation and the terrace formation during Late Pleistocene in the Ina valley. Moreover, by tracing some tephra units from this intermontane region to the coastal area of the South Kantô plain, it is ascertained that the maximal extension of the glaciers in the Kiso mountains was contemporaneous with the lowest sea-level. In connection with this, the peculiar feature of the river terrace crossing topography is explained as the product of the maximal glaciation of c. a. 30, 000 yrs. B.P.