In a manner relating to or resembling chalk, or in a way characteristic of the Cretaceous period of geological time.
From cretaceous (from Latin cretaceus meaning 'of chalk') plus the adverbial suffix -ly. Cretaceous as a geological term was named because rocks from that period often contain chalk deposits.
The Cretaceous period is famous for being when dinosaurs ruled Earth, but it's literally named 'the chalk period' because European geologists noticed all the white chalk in rocks from that era.
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