underclay

 : 
Noun
 : 
un=der=clay

CIDE DICTIONARY

underclayn. 
     A stratum of clay lying beneath a coal bed, often containing the roots of coal plants, especially the Stigmaria.  [1913 Webster]

OXFORD DICTIONARY

underclay, n. a clay bed under a coal seam.


For further exploring for "underclay" in Webster Dictionary Online


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