The paper deals with the relationship between the landform and crustal movement of the Ohyama River delta plain in the southwestern part of the Shonai Plain, and the depression zone on this delta plain. The results are summarized as follows:
The alluvial plain of the Ohyama River is composed mainly of delta plain. To the west this alluvial plain adjoins the Kamo moutains, and to the south Yakushidake and Sanbokura mountain blocks. The Akagawa floodplain, the eastern boundary of this alluvial plain, coincides with a tectonic line running from north to south, which divides the Shonai Plain into the eastern and the western parts. Judging from landform, alluvial geological structure, bedrock geological structure and gravity anomaly distribution, the author is certain that this delta plain corresponds to a trough between the tectonic line above and that at the east of Kamo mountains.
The trough forming movement has continued throughout the making of delta plain, and is partly reflected in the results of repeated precist levelling.
Moreover, the 10-15m contours on this plain show a depression zone in the same direction with the Kamo mountains.
Under this depression zone, a synclinal movement of the Kamigo formation, which comprises the eastern part of the Kamo mountains, has been continuing even after the formation of delta plain.
This movement has brought the undulation on the plain surface such as a down-warping with dimensions of 1-1.5km by about 9km.
According to the results of repeated precise levelling from 1956-1966, including the crustal movement caused by the Niigata earthquake, the bench mark located closest to this depression zone is still dropping.
Therefore, the author concluded that the depression movement of the Ohyama River basin is still going on in the recent geological time. The Ohdo River flows south-southwest to north-northeast on this depression zone.
