TUDIES ON THE COASTAL DUNE FORMATION PERIODSIN THE REGION OF THE WESTERN EXTREMITY OF HONSHU
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概要
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The aim of this investigation is to trace systematiaclly, through the historical sites and relics (cultural fossils) involved in the alluvial ruins and the alluvial pile structures, the coastal dune formation periods in the regions specifically chosen for that purpose. Viewed, however, from the entire process of the writer's, study, he has consistently been continuing on vertical removes of residence belts and their prime movers which the theme of this research consists in the topographical or geological study of the land itself form d ing the bases upon which the place for habitations and life regions depend. The investigation will be summarized as follows; 1. The accumulating period of the sand and the gravel stratum which formed an independent alluvial dune in the far western district of the Honshu island is divided into four: (I) Jomon Pre-early, (II) First half of Jomon Late, (III) Kofun Late and (IV) Modern Ages.2. Now in some place or another there may be seen one to six rows of dune belts, their main constituents consisting of three rows or less of dunes of beach ridge or one row of flying sand dune. The deposit of Jomon Pre-early chiefly consists of sand and gravel, and makes the lowest part of stratum later accumulated, and buried at the bottom of dune. Some coastal terraces are covered by a very thin stratum of sand drifted by wind. The deposit in the first half of Jomon Late consists of pebbles containing sand and tiny gravel and in some places contains a great many pieces of pumice stone, forming the first and the second dune of beach ridge. The desposit of Kofun Late consists of tinier bits of sand, sand and tinier bits of gravel, and in a dune of beach ridge in three rows it covers the front part of the second beach ridge, and forms the third beach ridge ahead. The deposit of dune accumulated between the Modern Ages and present day is tinier bits of sand forming a flying sand dune. On the eastern coast of Hibikinada it covers the slanting surface of terraces and piedmonts behind the first beach ridge and covers part of the first and the second beach ridge, while on the northern coast of Suo-snada such is scarcely to be found. 3. Some facts clarified in the process fo this study will be mentioned the following. 1) It has been found that there are regional differences in respect of the structure to accumulation of sand and gravel forming the alluvial dune, the number of dune belts and their scope.(a) The way of accumulation of cultural fossils in the sand stratum making a dune is roughly divided into I) Rocking type and II) Burying type, the former being sub-divided into a) Accumulation through drifting and b) Accumulation through stirring and the latter into a) Accumulation through burying and b) Accumulation through sand-flying. The way of accumulation on the coast of the Inland Sea of Seto is different from that on the coast of the Japan Sea.(b) From the relative height of level of the full tide and the cultural fossil bed in a sand deposit it is clarified that some local earth movements have occurred since the last stage of Jomon Ages. This accounts for the fact that the coast-line from the river Saba to the river Koto is somewhat elevated, while the north-eastern coast of Suo-nada is depressed.(c) Dunes generally develop on the coast of a bay or baylike part of the sea. They form rows of beach ridge dunes on a shoaling beach which abounds in stuff carried by rivers, but where mountains rise from waters one row of beach ridge dune develops, in which deposit beds belonging to several periods lie one upon another. On the northern coast of Suo-nada the coastal sand dune is a beach ridge or row of beach ridges mainly built by wind and waves. On the other hand, on the eastern coast of Hibiki-nada the deposit piled by the end of the Third period (I) forms a row of beach ridge and that of the Fourth period (IV) the dune accumulated through drifting by wind.
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