Geological term for a dense, impervious layer of soil or subsoil, often cemented by minerals.
この地域の硬盤層は水はけが悪い。
The hardpan in this area has poor drainage.
Compound of 硬 (hard), 盤 (plate/pan), and 層 (layer). The exact historical derivation is uncertain; the spelling is conventionally associated with the geological meaning.