埋没水田遺構の地形環境分析 ("陸の古環境復元"特集号)

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The purpose of this paper is to elucidate the change in the topographical environment of the plains after the end of the prehistoric age, and also to clarify the relationship between the change and the land use. Geomorphological analysis for environmental reconstruction is employed. It consists of four different analyses: landform surface analysis, landform zone analysis, micro landform analysis I and micro landform analysis II.The purpose of landform surface analysis is to clarify the topographical change taking place in a time frame of 104 years (for example, the formation of terraces). Landform zone analysis can clarify the topographical change on a scale of 103 years order (for example, delta formation). These two analyses correspond to what was formerly called the history of topographical development. On the other hand, micro landform analysis I and micro landform analysis II are connected with the investigation of archaeological excavation. The purpose of micro landform analysis I is to clarify topographical change on a scale of 102 years. For example, a natural levee must have greatly influenced the location of a village. In micro landform analysis II, what is looked at is the topographical change which occurs every time a river floods. In geomorphological analysis for environmental reconstruction, we pay attention not to "natural topography" but to "topography of the human living environment". In other words, geomorphological analysis for environmental reconstruction is not a mere history of topographical development; it includes the history of natural disasters and of land development.As the subject of our research, we used the Mihara Plain on Awaji Island in Setonaikai. In Shichigawa Okitaminami site, part of delta I in the Mihara Plain, minute topographical changes and the appearance of the land used from the end of Yayoi era to the Kofun (old tomb) era-about 1, 700 years ago-can be clarified by geomorphological analysis for environmental reconstruction. The following is a summary:1) The area surrounding the Shichigawa Okitaminami site was a shallow sea bed at the height of the Jomon transgression (stage III, about 6, 400y.B.P.). This sea area became a lagoon 2, 600y.B. P., when the sea level changed and a sand spit was formed at the inlet (stage IV).2) The accretion of natural deposits progressed rapidly here from the end of the Yayoi era to the beginning of the Kofun era. This accretion caused the area where rice fields could be developed to spread down the river. The loam brought by the flood has a negative aspect in that it buries the rice field and a positive aspect in that it changes swampland into dry land with high productivity.3) The rice fields are divided by a footpath in accordance with the level of the ground so that the difference in the height of each division would not exceed six centimeters. Where the incline is steep the rice field was smaller, where the ground is flat, the rice field is larger.4) When floods or accumulation occur, the rice fields revert to their original state. Therefore it can be assumed that this process occurred without delay in the Mihara Plain.5) Rice fields of stage IV16 are of two different kinds: high and dry, and low and wet. The fields can also be classified as good or bad in terms of the water supply.
日本第四紀学会の論文

埋没水田遺構の地形環境分析 ("陸の古環境復元"特集号)

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