The Collaborative International Dictionary of English v.0.48:

Cretaceous \Cre*ta"ceous\ (kr[-e]*t[=a]"sh[u^]s), a. [L.
   cretaceus, fr. creta chalk. See Crayon.]
   1. Having the qualities of chalk; abounding with chalk;
      chalky; as, cretaceous rocks and formations. See Chalk.
      [1913 Webster]

   2. (Geol.) Of, pertaining to, or designating, the period of
      time following the Jurassic and preceding the Tertiary,
      generally given as from 144 million years b. p. to 65
      million years b. p.. Also called

WordNet (r) 3.0 (2006):

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