Geomorphological Problems in the Inter-mount Basin, in Nagano Pref

Abstract

KISO and INA horse mountains are situated along the southern border of the Inner Zone of Japanese Are, on the W side of the Median Line which is one of the main geotectonic lines in the Japanese Islands. INA intermount basin is located between these two horsts, and is irrigated by Tenryuriver that flows through the basin from north to south. In this paper the writer discussed on some geomorphological problems of the basin. The main subjects are summarized as follows: 1. There are some outcrops of younger faults along the geomorphological faultlines at the base of the above named two horst mountains on the margin of Ina Basin. 2. In the INA Basin, Y. Ichinose studied on the terraces, and classified them into 6 horizons in Ryusei, or the western reach of Tenryu, while 5 horizons were distinguished in Ryuto, or the eastern side of the river. Thereafter T. Shikama and K. Kobayashi classified the terraces and erosion surfaces in the basin into 9 stages. Ichinose concluded that No. 2 Terrace in the Ryusei were correlated to No. I in the Ryuto. The writer's conclusion through his study of these terraces are that No. 4 terrace in the Ryusei is comparable to No. 2 and 3 in the Ryuto. 3. At the higherr positions above No. 4 Terrace, the Hara topographies cited firstly in this paper have remarkable development around the basin. The Hara is a conception newly introduced by the writer in this paper, which means a kind of compound topography that combines alluvial fan with alluvial cone. Namely in its upper part it is formed by some or less flat plain, like the slope of alluvial fan, while its side slope is steep like alluvial cone. The development of this topography seemes to have been promoted by repeated faulting along the higher back fault-scarp, because the younger Hera deposits are piled. overr the older. 4. During the formation of the Hara at the level above No. 4 Terrace, lower terraces were developing below the level. of No. 4 Terrace.