noun
alluvium; alluvial stratum
Technical term in geology and geography for sediment deposited by rivers or floods.
この地域の沖積層は主に砂と粘土からなる。
The alluvial stratum in this area consists mainly of sand and clay.
沖積層 is the sediment layer itself, while 沖積平野 is the flat landform created by that sediment.
Compound of 沖積 (alluvial, from 沖 'offshore' + 積 'accumulate') and 層 (stratum).