Nara Basin

A tectonic basin in the northwest of Nara Prefecture. It covers an area of ​​about 300 square kilometers. It is a long rectangle from north to south, covered with fertile alluvial deposits, and the bottom of the basin is 40 to 80 meters above sea level. To the east is the Yamato Plateau, which is separated from the basin by the Kasuga Fault Cliff, and to the west is the Ikoma-Kongo Mountains, which run north to south and show the characteristic fault topography between the basin and the Osaka Plain. To the north is the Narayama Hills, which form the border with the Kyoto Basin, and to the south is the low-altitude Ryumon Mountains, which are surrounded by green mountains. Although it accounts for only 8% of the prefecture's total area, the flat and fertile alluvial deposits are suitable for rice paddy cultivation, and in ancient times, Heijo-kyo and other cities were built here, making it the center of politics and culture. Even today, it is well-connected to the Kyoto-Osaka-Kobe metropolitan area, and with major cities such as Nara City, it is the most densely populated area in the prefecture.

At the end of the Pliocene epoch of the Tertiary geological period, the peneplain-like ground collapsed in the shape of a graben, while the surrounding mountain ranges separated by faults rose up. The sinking basin was filled with soil and sand flowing from the uplifting mountains, forming an alluvial fan at the foot of the mountains. The Yamato River, the main river flowing out from the southeastern corner of the Yamato Plateau, flows westward, joining various tributaries within the basin, passing through Kamenose Gorge and emptying into Osaka Bay from the Osaka Plain. The climate is warm and with little rain, typical of an inland area with extreme heat and cold.

[Ichiro Kikuchi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

奈良県北西部にある構造盆地。面積約300平方キロメートル。南北に長い長方形で、肥沃(ひよく)な沖積層に覆われ、盆地底の標高は40～80メートルである。東方は春日(かすが)断層崖(がい)によって盆地と画される大和(やまと)高原、西方は大阪平野との間に断層地形の特徴を示す生駒(いこま)・金剛(こんごう)山地が南北に走り、北方は京都盆地と境をなす奈良山丘陵、南方は低山性の竜門山地など四周を青垣の山々に囲まれる。面積は県全体の8％にすぎないが、平坦(へいたん)で肥沃な沖積層は水田耕作に適し、古代は平城京などが造営され政治・文化の中心であった。現在も京阪神大都市圏との交通に恵まれ、奈良市のほか主要都市が立地し県内でもっとも集落・人口密度の高い地である。

　地質時代の第三紀鮮新世末に準平原状の地盤が地溝状に陥没し、他方、断層によって画された周囲の山地は隆起していった。沈降する盆地部は、隆起する山地から流出する土砂で埋積され、山麓(さんろく)に扇状地が形成された。大和高原南東隅から流れ出る主流の大和川は、盆地内の諸支流をあわせて西流し、亀ノ瀬峡谷を経て大阪平野から大阪湾に注ぐ。気候は温暖少雨で、寒暑の大きい内陸性を呈する。

［菊地一郎］

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