荒川中流域における下位段丘の形成過程

Abstract

Fluvial terraces are well developed along the middle reach of the Ara river in Kanto district, Japan. These terraces can be roughly classified into upper, middle, and lower terraces. The authors investigated the lower terraces in order to clarify the influence of tectonic movements and climatic changes in the formation of fluvial terraces during the last glacial age.The lower terraces are subdivided into three levels-filltop terraces, fill-strath terraces, and non-cyclic terraces-based on topographic features and terrace deposits. The development of lower terraces after the last interglacial age is divided into four stages: “formation of buried valley under valley fills, ” “accumulation of valley fills and formation of filltop terrace, ” “slight down cutting which formed fill-strath terraces” and “rapid downcutting” (Fig. 10).The longitudinal profiles of the filltop terrace and fill-strath terraces are characterized by steeper gradient and less curvature than those of the present river bed (Fig. 3). On the other hand, the longitudinal profile of the base of valley fills is similar to the present river bed in its larger curvature. These profiles suggest that the fluvial process when the buried valley under valley fills was formed is similar to that at present, so the age of initiatial deposition of the valley fills may be the last interglacial. The authors estimate that the relative height of these two profiles indicates the amount of uplift since the last interglacial. The profiles of the filltop and fillstrath terraces, which are characterized by steeper gradient and less curvature, may have resulted from changes in fluvial process during the last glacial age, possibly from a decrease in discharge as represented by a decrease in gravel size of the valley fills.