cretaceous (adj.)

1670s, "chalky," from Latin cretaceus "chalk-like," from creta "chalk." As a geological period (with a capital C-) between the Jurassic and the Tertiary, it is attested from 1851, a reference to the extensive chalk beds of southeastern England that were laid down during the Cretaceous and studied in the early years of geology. In reference to the chalk beds, cretaceous is attested in geology writing by 1832.

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Definitions of cretaceous from WordNet
cretaceous (adj.)
abounding in chalk;
Cretaceous (n.)
from 135 million to 63 million years ago; end of the age of reptiles; appearance of modern insects and flowering plants;
Synonyms: Cretaceous period
Cretaceous (adj.)
of or relating to or denoting the third period of the Mesozoic era;