Geo-environmental Changes and Land Development History after the Middle of the Yayoi Period in the "Asuka-Fujiwara Region", Nara Basin, West Japan

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  • 「飛鳥・藤原地域」における弥生時代中期以降の地形環境変遷と土地開発
  • アスカ フジワラ チイキ ニ オケル ヤヨイ ジダイ チュウキ イコウ ノ チケイ カンキョウ ヘンセン ト トチ カイハツ

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The Asuka-Fujiwara region contained the capital of Japan during A.D. 694-710. Many land developments have been carried out in this area over the past two thousand years. It is one of the most important areas in which to consider an analysis of the land development history of Japan. The purpose of this paper is to examine the relationship between geo-environmental changes and land development process of the Asuka-Fujiwara region after the middle of the Yayoi Period. The study identifies five stages in the geo-environmental history of this region. These stages are closely related to land development and land-use change.<br>The following results were obtained:<br>(1) The middle of the Yayoi Period (B.C.1-A.D.2th c.); A natural levee was formed on the Asuka river basin during this period, followed by soil formation. This soil formation indicates the existence of a stable geo-environment at this stage. The Shibu archaeological site was developed on this dry geo-environment natural levee.<br>(2) From the late Yayoi Period to the Kofun Period (A.D.2-6th c.); During this period, the back swamp was buried around the natural levee by sediments from the Asuka river floods. On account of this sedimentation, the natural levee located in the Shibu archaeological site became wetter than it was during the middle of the Yayoi Period. Paddy fields were also developed in this back swamp area during this period.<br>(3) From the Asuka Period to the middle of the Heian Period (A.D.7-about A.D.10th c.); By the end of the Kofun Period, sediments from the Asuka river floods resulted in a completely flat landscape. Following this, The soil formation was occured on the floodplain, indicating that the area had become a stable environment during this stage. The development of the Fujiwara Capital and the Jori Grid Plan were constructed on the Asuka river floodplain during this period.<br>(4) From the late Heian Period (A.D. 11th c.) to A.D.14th c.; The channel entrenchment that occurred along the Asuka river led to the formation of a Holocene terrace by A.D.12th c. This entrenchment made the floodplain small, and transformed the terrace geo-environment from wet to dry. Paddy fields on the Holocene terrace were abandoned because of this environmental change. As a result, these cultivated lands, including Hida-sho, were later redeveloped by Sho-en developers.<br>(5) After A.D. 15th c.; Active sedimentation after A.D. 15th c. produced a natural levee along the Asuka river. This active sedimentation and the building of embankments led to the formation of a raised bed river on the natural levee zone of the Asuka river basin.

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