Geomorphic Development of Tokai Village, Ibaraki Prefecture

Abstract

Tokai Village in the northeastern part of Kanto Plain is situated on the southern bank of the mouth of the Kuji River. In this area, the Kanto upland extends close to the Pacific coast and is fringed by a narrow coastal lowland covered by sand dunes. At the mouth of the Kuji River there is a narrow alluvial plain which extends upstream along the river valley.Analysing coring data, the suthor found a drowned valley cutting the upper tertiary formations under the alluvial plain of the Kuji, which is deeper than 50 meters under sea level. Along the coastal margin of the upland, furthermore, there is a subterranean flat surface, shallower than 15 meters, of the upper tertiary formations, covered by alluvial deposits. The author inferred that after the formation of the upland the regression took place and the Kuji River cut a valley under the present sea level, and that the latter might be an abrasion platform which was formed during the period of the transgression succeeding to the regression and has been buried by alluvial and coastal sedimentation owing to the transgression.On the coast of this area, thera develop two ridges of sand dunes nearly parallel to the coast line, covering a coastal part of the upland and the coastal lowland. The landward ridge of sand dunes was formed on a sand spits at the mouth of the Kuji when the transgression was maximum and the coastward ridge grew up on offshore bars after the formation of the former.