Source : Webster's Revised Unabridged Dictionary (1913)

Hardpan \Hard"pan`\, n.
   The hard substratum. Same as {Hard pan}, under {Hard}, a.

Source : WordNet®

hardpan
     n : crust or layer of hard subsoil encrusted with
         calcium-carbonate occurring in arid or semiarid regions
         [syn: {caliche}]