Underclay n. : A stratum of clay lying beneath a coal bed, often containing the roots of coal plants, especially the Stigmaria.

Example:


Taxonomy
underclay, clay, flint, paleosols, fireclay, Seatearth, seat, soil, coal, Ganister, fireclays, free encyclopedia, layer, weathering, pedogenic, alteration, plants, accumulated.

Defination from the web:
underclay, these soil features can include some combination of pedogenic slickensides, pedogenic ped structures, ... Underclay, which consists of siliceous refractory clay rich in hydrous aluminum silicates, is also called "fireclay" (fire clay) ...