Source : Webster's Revised Unabridged Dictionary (1913)

Cretaceous \Cre*ta"ceous\, a. Also Cretacic \Cre*tac"ic\ (Geol.)
   Of, pertaining to, or designating, the period of time
   following the Jurassic and preceding the Eocene.

Cretaceous \Cre*ta"ceous\ (kr[-e]*t[=a]"sh[u^]s), a. [L.
   cretaceus, fr. creta chalk. See {Crayon}.]
   Having the qualities of chalk; abounding with chalk; chalky;
   as, cretaceous rocks and formations. See {Chalk}.

   {Cretaceous acid}, an old name for carbonic acid.

   {Cretaceous formation} (Geol.), the series of strata of
      various kinds, including beds of chalk, green sand, etc.,
      formed in the Cretaceous period; -- called also the {chalk
      formation}. See the Diagram under {Geology}.

   {Cretaceous period} (Geol.), the time in the latter part of
      the Mesozoic age during which the Cretaceous formation was
      deposited.

Source : WordNet®

cretaceous
     adj 1: abounding in chalk
     2: of or relating to the Cretaceous geologic era; "cretaceous
        rocks"
     n : from 135 million to 63 million years ago; end of the age of
         reptiles; appearance of modern insects and flowering
         plants [syn: {Cretaceous period}]