Downward erosion of river bed and upheaval movement at the maximum Wurm stage around the Niigata Plain, Central Japan

The lowest sea level at the maximum Wurm stage was estimated to be 140 meters lower than that at the present time around the Japanese Islands (MINATO, 1966). The strong downward erosin of river bed might have been progressed due to the lowering of the base level. The author investigate the upper limit of the above influence at the maximum Wurm stage, drawing the longitudinal profile of river bed along the middle and lower reaches of the river Shinano. It is presumed, from the investigation that, there might have occurred「knick point」or discontinuity of gradient of the river Shinano at nearby Ojiya city. It is consequently inferred that, direct downward erosion due to the lowering of the sea level along the river Shinano might have attained up to the vicinity Ojiya City. The discontinuity can be attributed mainly to the Neotectonic movement. The downward erosion of the upper stream above the discontinuity would be caused mainly to the upheaval movement. Namely, the mass movement activity along the river Shinano which occurred in upper stream area above Ojiya City would not be effected by the lowering sea level at the maximum Wurm stage, but might have caused mainly due to the upheaval movement