海岸沖積平野の環境史は陸上生態系と海洋生態系の間の遷移的過程のなかにある.裾礁島の海岸平野は完新世後期,サンゴ礁の上方成長が海面に到達し,浜堤が形成されると急速に発達する.数年にわたるクック諸島ラロトンガ島の発掘調査によって,人間居住と海岸平野の地形発達との関係が明らかになった. (1)約6000年前の海岸線は,海岸平野の最奥,現海岸線から内陸500mの地点(平均海面上1.70m)に侵入しており,海岸平野の発達はその後の海退現象と関連している. (2)その後,間もなくオオハマボウ,パンダヌスが優占する海浜植生が成立した.この時期にさかのぼって,この島にはマングローブ植生が欠けており,その生態地位をオオハマボウの群落が占めていた. (3)4000年前から3000年前にかけて,サンゴの破砕礫からなる浜堤と州島が形成され,後背湿地や潟湖をもつ海岸平野が発達した.(4)初期的な人間居住の痕跡は,それら浜堤と州島の上で発見された.考古学的調査によって得られた最も早い居住は, 1030年前にさかのぼる. (5)内陸丘陵斜面における焼畑農耕によって森林破壊と土壌のラテライト化が急速に進んだ.豪雨などによって渓谷に滑落した土壌が,焼畑の炭化物とともに土石流となって海岸低地に運ばれ,堆積した.こうして渓谷の出口に小さな扇状地が複合的につくられ,現在の海岸平野が完成した. (6)おそくとも,300年前頃になると扇状地堆積が安定し,扇頂部にマラエ(祭祀神殿)が構築されて,谷筋を領有する社会単位(タペレ)が形成されたと考えられる.
Coastal alluvial plains are transitional environments caught between terrestrial and aquatic ecosystems in the formation process. Our concern is to examine the environmental history of Rarotonga, the principal island in the Cook Islands of central Polynesia. In terms of human settlement, probably the most important change occurred after the Holocene high sea level when the slight drop in sea-level would have exposed shallow sea floor and increased near-shore sedimentation, creating coastal plains. Our excavation at the site located at the foot of hill terraces, inland side of coastal plains, revealed tidal marine deposits at 1.7 metres above mean sea level. On top of the layer we have found peat soil deposits containing waterlogged plant remains. This suggests that coastal woodland was formed and the land forming process was started around 6000BP. The beach-ridge, less than 2500 years old, provided enough protection from wave action and allowed for the rapid creation of coastal plains. These deposits laid down around 600 to 500 years BP were interpreted as having been derived from hillslope soils exposed by agricultural forest clearance. The occurrence of slips and transportation of soils in the valley system is closely related to periods of heavy rain such as cyclone season. After the sedimentation activity had slowed down until about 300 years ago, an apex of alluvial fan at valley debouch was selected as the site for building Marae or ceremonial court which is most important social focus by people.
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