Osaka Plain

A plain that stretches from Osaka Prefecture to the eastern part of Hyogo Prefecture. It covers an area of ​​about 1,600 square kilometers, stretching from the foothills of the Hokusetsu Mountains in the north, the Ikoma and Kongo Mountains in the east, and the Izumi Mountains in the south, to Osaka Bay in the west. It is also called the Setsu-Kaisen Heiya after the old name of the province, and is the largest plain in the Kinki region.

Topographically, it is made up of hills and plateaus formed in the Pliocene to Pleistocene epochs of the Tertiary Period (Pleistocene Epoch), and alluvial plains mainly consisting of deltas created by modern rivers in the Holocene Epoch (Alluvial Epoch). The main hills and plateaus include the Senri Hills, Toyonaka Plateau, and Itami Plateau in the north, the Hirakata Hills and Katano Plateau in the northeast, the Kasen Hills, Shinodayama Plateau, and Mikunigaoka Plateau in the south, and the Uemachi Plateau that extends into Osaka city. The alluvial plains include the Muko Plain created by the Muko River and Ina River to the north, the Mishimano Plain created by the Ibaraki River and Akutagawa River to the north, the Kawachi Plain created by the Yodo River and Yamato River to the east, and the Izumi Coast Plain created by the Ishizu River, Otsu River, and Onosato River to the southwest.

Development dates back a long time, with Jomon ruins on the edges of the hills, Yayoi ruins and ancient tombs distributed from the plateau to the high delta, as well as reservoirs for rice paddy development and the remains of the jori system. The northern lowlands of the Kawachi Plain are new rice fields developed by reclaiming ponds and marshes from the early modern period. It was developed as an advanced agricultural area early on, and suburban agriculture is still thriving today, with famous garden cultivation in the Itami Plateau and Ikeda, and onion cultivation in the Izumi Kaigan Plain.

Cities also developed quickly, with castle towns such as Naniwa in ancient times, Sakai in the Middle Ages, and Amagasaki, Takatsuki, and Kishiwada thriving in the early modern period. After the Meiji period, the development of commerce and industry in Osaka brought about modern urbanization in the surrounding areas, and industrial cities such as Amagasaki, Suita, Ibaraki, Moriguchi, Higashiosaka, Sakai, and Kishiwada formed the Hanshin Industrial Zone from the coast to the alluvial plains of the Yodo River. Meanwhile, residential cities such as Nishinomiya, Takarazuka, Toyonaka, Hirakata, and Fujiidera were developed on the hills and plateaus. With a total population of about 9.87 million, these satellite cities form the foundation of the Osaka metropolitan area, and together with the Kanto Plain that forms the capital region, they play a central role in Japan.

[Juichi Inoki]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

大阪府から兵庫県東部に広がる平野。北は北摂(ほくせつ)山地、東は生駒(いこま)、金剛(こんごう)山地、南は和泉(いずみ)山脈の各山麓(さんろく)から西の大阪湾に及ぶ面積約1600平方キロメートル。旧国名から摂河泉平野(せっかせんへいや)ともいい、近畿地方第一の平野。

　地形上、第三紀鮮新世から更新世（洪積世）に形成された丘陵、台地と、完新世（沖積世）に現河川のつくった三角州を主とする沖積平野とからなる。おもな丘陵、台地には、北に千里(せんり)丘陵と豊中(とよなか)台地や伊丹(いたみ)台地、北東に枚方(ひらかた)丘陵と交野(かたの)台地、南に広がる河泉(かせん)丘陵と信太山(しのだやま)台地、三国丘(みくにがおか)台地、さらに大阪市内に延びる上町(うえまち)台地などがある。沖積平野には、北に武庫(むこ)川・猪名(いな)川のつくる武庫平野、茨木(いばらき)川・芥(あくた)川などの三島野(みしまの)平野、東に淀(よど)川・大和(やまと)川の河内(かわち)平野、南西に石津(いしづ)川・大津川・男乃里(おのさと)川などの和泉海岸平野が連なる。

　開発は古く、丘陵縁辺には縄文遺跡、台地から高位三角州には弥生(やよい)遺跡や古墳の分布、さらに水田開発のための溜池(ためいけ)や条里制遺構をとどめる。河内平野の北部低地は、近世池沼を干拓した新田開発地である。早くから先進農業地として開け、現在も近郊農業が盛んで、伊丹台地や池田の植木栽培、和泉海岸平野のタマネギ栽培は有名である。

　都市の発達も早く、古代の難波(なにわ)、中世の堺(さかい)、近世にはこのほか尼崎(あまがさき)、高槻(たかつき)、岸和田などの城下町が栄えた。明治以降、大阪の商工業の発展に伴い、周辺の近代都市化をもたらし、工業都市として尼崎、吹田(すいた)、茨木、守口(もりぐち)、東大阪、堺、岸和田など海岸から淀川の沖積地に阪神工業地帯を形成。一方、住宅都市として西宮、宝塚、豊中、枚方、藤井寺などが丘陵、台地に開発をみた。これらの衛星都市を擁して総人口約987万人、大阪都市圏の基盤を構成し、首都圏を形成する関東平野とともに、日本の中枢的役割を果たしている。

［位野木壽一］

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