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Why is the Cretaceous Period named after chalk?

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Cretaceous paleogeography — Cretaceous Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Cretaceous Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The Cretaceous Period is named after the Latin word creta, which means "chalk". The name was first proposed in 1822 by the geologist J.B.J. Omalius d’Halloy, who was tasked with creating a geologic map of France.123

In his work, d’Halloy identified a specific geologic unit, the Terrain Crétacé, which included significant deposits of chalk and underlying sands. While not all rocks from this period are chalk, the Late Cretaceous was a time of high marine productivity, leading to the widespread deposition of thick chalk beds across regions such as Western Europe, Russia, and North America. These deposits are famously visible today in the white cliffs bordering the Strait of Dover.32

Chalk itself is a soft, fine-grained limestone composed primarily of the microscopic, armorlike plates of coccolithophores—tiny marine algae that flourished during this era.234