River Terraces in Chichibu Basin

Chichibu Basin is situated at the north-eastern part of Kanto-Mts. In this basin river terraces are distributed along the River Ara and River Akahira. We have researched the terrace deposits, especially size and angularity, sorting of terrace gavels, altitude and thickness of gravel beds, from which we have studied on the environment of deposition and the process of formation of terrace plains. The main subjects are summarized as follows: (1) Terrace plains in this basin are classified into. three groups. (2) Upper terrace gravels which form a divide between two rivers are fanglomerates of the River Ara and the upper terrace plain is a remnant of dissected fan. (3) Middle and lower terrace gravels which are found a long two rivers are deposits of old flood plains. (4) The thickest gravel deposit on each terrace is found at the southern part of Chichibu City, and it suggests relative subsidence of this region during each stage of terrace building, related to the earth movement of this basin.