静岡県浮島ケ原・雌鹿塚遺跡における自然環境と人間活動の変遷 Environmental Change and Human Activity around Megazuka in the Ukishimagahara Lowland, Shizuoka Prefecture.
静岡県浮島ヶ原は駿河湾奥部に面し, 3列の海岸砂礫州が海側へ発達することに伴って, その背後に形成された低湿地である. 内陸側の2列の砂礫州は, 本地域の沈降運動のために, 低地の地下に埋没している. 最も内陸側の埋没砂礫州は, 一部微高地として認定することができ, 雌鹿塚遺跡はその上に立地している.<br>雌鹿塚は, 縄文時代中期から古墳時代中期まで営まれた遺跡である. 今回, 沼津市教育委員会によって, 雌鹿塚遺跡の本格的な発掘調査が行われた. その際, 埋没砂礫州の微地形, ならびに周辺の地質層序, 年代測定値, テフラ等の資料を得ることができた. さらに, 雌鹿塚遺跡周辺における自然環境の変遷と人間活動との関係について考察した結果, 浮島ヶ原への人間の進出が砂礫州の発達過程と対応すること, 火山活動や地殻変動が雌鹿塚を放棄させる原因となった可能性が大きいことが推定された.
The Ukishimagahara Lowland is situated along the innermost margin of Suruga Bay. A barrier-backswamp system is recognized along the present coastline. In addition, two barriers buried beneath the present backswamp are developed behind the present coastal barrier in the most part of the lowland. In this paper, these three coastal barriers are called barriers I to III, in order from landward to seaward.<br>Megazuka is an archaeological site located in the middle of the Ukishimagahara Lowland. It is situated on the oldest coastal barrier (barrier I). Excavated materials indicate that Megazuka was occupied by pre-historical people from the middle of the Jomon period (ca. 4, 000yrs BP) until the middle of the Kofun period (ca. 1, 500yrs BP).<br>The purpose of this paper is to discuss the influence of environmental changes on human activity in the Ukishimagahara Lowland.<br>The development of coastal barriers in the Ukishimagahara Lowland and the environmental changes around Megazuka are summarized as follows (Matsubara, 1988, 1989). A bay had been formed in the Ukishimagahara Lowland at least 9, 000yrs BP. Coastal berrier I began to develop around 8, 000yrs BP and began to enclose the bay about 7, 000yrs BP. Coastal barrier II was constructed between 5, 000 and 4, 000yrs BP, and the area around Megazuka changed into a lagoon. The completion of coastal barrier II about 4, 000yrs BP caused the lagoon to change into a backswamp. The construction of coastal barrier III was finished around 2, 000yrs BP.<br>A series of excavations at Megazuka were carried out from 1988 to 1989. Microlandforms and the stratigraphic sequence at Megazuka were clarified along with the periods of human activity.<br>The relationships between environmental changes and human activity at Megazuka are inferred as follows.<br>The initial stage of human activity coincided with the period when the area around Megazuka changed from a lagoon to a backswamp with the construction of the coastal barrier II (ca. 4, 000yrs BP). In addition, the initial stage of human settlement was coincident with the period when coastal barrier III was completed and came to enclose the backswamp (ca. 2, 000yrs BP).<br>The human settlement at Megazuka was abandoned at the time of the fall of the Obuchi Scoria (ObS) from Mt. Fuji (ca. 1, 500yrs BP).<br>Furthermore, tectonic movement in the Ukishimagahara Lowland is characterized by subsidence (westward and landward downtilting), which buried the former coastal barriers beneath the backswamp. Continued burial of barriers made the environment around Megazuka unsuitable for human settlement.<br>These findings indicate that volcanic activity and tectonic movement influenced the development of human settlement in the Ukishimagahara Lowland.
第四紀研究 31(4), 221-227, 1992