The Collaborative International Dictionary of English v.0.48:

Cretacic \Cre*tac"ic\
   [Webster 1913 Suppl. +PJC]

   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, and at the end of which the dinosaurs died out.
      See Cretaceous.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

Cretaceous period
    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]