SAND DUNES SITUATED ON THE MARINE TERRACES IN NISHI-TSUGARU REGION, AOMORI PREFECTURE

Abstract

In Nishi-Tsugaru region along the coast of the Japan Sea, there develop typical marine terraces which are partly covered by sand dunes. The author surveyed the disribution, geomorphic features and composition of the sand dunes. The results are summarized as follows: 1) Sand dunes orerlie the A horizon of soil formed at the uppermost part of terrace gravel beds along the outer margins of the second terrace plains of which the altitude is about 50-60 m, the third terrace plains (30-40m) and the fourth terrace plains (5-10m) . The geomorphic features of the sand dunes situated on the older (higher) terraces are not so typical as those situated on the younger (lower) terraces, although on the first terrace, there is no sand dune. (Fig. 1 and Photos. 1-5). 2) The distribution of the sand dunes in this region is conditioned dy the exposure of the coast against the prevailing winds, and they are distributed on the coast of the direction from NE to SW facing winds from WNW in winter. (Fig. 1 and Tab. 1) 3) Moreover, the formation of the sand dunes is controlled by the amount of sand supply, which is conditioned by the nature of bed rock of the coast. In this region, the distribution of the sand dunes is limited to the area composed of Tertiary sedimentary rocks which are easily decomposed into a large amount of sands. But, in the coast to the south cf Fukaura, is composed of volcanic rocks which are more resistant to weathering than the former, neither beach nor sand dune can be formed. (Fig. 1) 4) The sand dunes are usually composed of well sorted medium or fine sands (smaller, than 1 mm in diameter), and the grain size of the older sand dunes is somewhat smaller than that of the younger ones (Tab. 2). It seems to be a result of subaerial weathering after the formation of the sand dunes. Because such a thendency corresponds to modificatin of the geomorphic features of the sand dunes. 5) The sand dunes modify the terrace topography. They give the terrace plains small reliefs. The terrace scraps facing the prevailing wind of winter are covered by sands, and their slopes are very gentle. On the other hand, the terrace scarps free from sand dunes preserve their original steep slopes (Tab. 3).