Terrace Topography in the Nozawa Basin and its Vicinity

Abstract

The accumulation terraces consisting of pumiceous materials from the Tadami Valley are distributed in the valley along the upper course of the Aagano River, making a noticeable feature of the valley. The author tried to make clear the following facts on the relations between the accumulation terraces and terraces of other origin from his survey.(1) In this region the terraces are classified into three groups; the higher, the middle and the lower terraces. The surfaces of the middle terraces are subdivided into four groups; P, P', M and M' Surfaces. P and P' Surfaces are the surfaces of the accumulation terraces consiting of pumiceous materials ejected from Numazawa Volcano. The accumulation of the pumiceous materials in the different parts of the previous valley floor occurred separately, making the narrow gorges its temporary base-levels. Consequently the longitudinal profile of the P Surface is discontinuous at the gorges and intersects the profile of the M Surfaces.(2) The structures of the terraces can be devided into four types (Fig. 3). The surfaces of the lower terraces at A and B types reappear at the previous valley bottom. The surfaces of the lower terraces are lower than the bottom of the valley at the C type.(3) Through the structural study of the terraces the author considers that the tectonic movement influenced the construction of the terraces in this region.