hard·pan /ˈhɑrdˌpæn/
  硬質地層,粘土層,砂礫層
  Hard·pan n. The hard substratum. Same as Hard pan, under Hard, a.
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  hardpan
       n : crust or layer of hard subsoil encrusted with
           calcium-carbonate occurring in arid or semiarid regions
           [syn: caliche]