River Terraces and their Displacements along the Hirose River, Miyagi Prefecture

Abstract

The terraces along the Hirose river well-develop especially in the Ayashi basin, and in the urban area of Sendai. They, however, often disappear in the gorge between the two areas mentioned above. These terraces have been studied by several researchers. The present writer tried the re-examination of terrace classification and correlation especially from the viewpoint of the influence of the crustal movement on the terrace surfaces.
In addition to the tephrochronological method, the thickness of the weathering crust of terrace gravels is used as an index for terrace classification and correlation, as proposed by Akojima et al. (1971).
The present writer's terrace classification and correlation is shown in Fig. 1. The writer found displacements of the terrace surfaces in the Ayashi basin and in Sendai which suggest the continuous crustal movements (Fig. 2). It is considered that these displacements are due to flexures with axes of N-S direction in the Tertiaries. Such crustal movements are more remarkable in the upper stream area, and seem to have been continuing with uniform velocity.