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1. Of, relating to, or being the period of geologic time from about 146 to 66 million years ago, the third and most recent period of the Mesozoic Era. The Cretaceous Period is characterized by the formation of modern continents from the supercontinent Pangaea and by the development of flowering plants. The Period ended with the sudden extinction of the dinosaurs and many other forms of life. See Table at geologic time.
2. cretaceous Of, containing, or resembling chalk.
The Cretaceous Period.

[From Latin crētāceus, chalky, from crēta, chalk, from Crēta (terra), Cretan (earth).]

cre·ta′ceous·ly adv.