沖ちゅう積せき平へい野や

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Meanings

1

noun

alluvial plain

A flat landform created by the deposition of sediment over a long period by a river. Used in geography and earth science contexts.

open sea, offing, rise high into sky

積volume, product (x*y), acreage

平even, flat, peace

野plains, field, rustic

平へい野や
平野 is a general term for a plain or flat area, while 沖積平野 specifically refers to an alluvial plain formed by river sediment.
沖ちゅう積せき層そう
沖積層 refers to the alluvial deposit or layer itself, whereas 沖積平野 is the resulting landform.

Compound of 沖積 (alluvial, from 沖 'offshore' and 積 'accumulation') and 平野 (plain). The term is a direct calque from Western geographical terminology.