Abstract
There are many terrace surfaces along the middle and lower courses of the Oirase River, especially on the left side, which are classified into the upper (four surfaces), the middle (two surfaces) and the lower (two surfaces).
The Hokkiri and Akôbo surfaces of the upper were cut in Shichisyaku and Furumaki uplands suffered from downwarping and tilting to the east before Riss- Würm Interglacial. The Shibayama surface was cut at the highest sea level in Riss-Würm Intergalcial. The Takadate surface was cut at the early Würm Glacial.
The middle terraces were cut at the lower sea level of the Würm Glacial, and the lower terraces at the higher and lower sea level stages in the postglacial time respectively.
The submarine terrace (-120 m) is compared with the Akanuma surface and the submarine terrace (-50 m) with the Sanbongi surface. Thus, the terrace surfaces in the region have different geneses such as crustal movements, sea level fluctuations and recent volcanic activities, or their combination.
