The Cretaceous Period covers about 80 million years, making it the longest period of the Phanerozoic. It began with the end of the Jurassic and a minor extinction event about 145 million years ago, and lasted until 65 million years ago, when it ended with a big bang, the asteroid impact that produced one of the larger extinctions in earth history.
Because it’s so long, there have been attempts to subdivide the Cretaceous into two periods, but those proposals have not been accepted. Unlike some of the shorter periods that often are broken into early, middle, and late epochs, the basic subdivisions of the Cretaceous are only two – early and late. Beyond that, there are 12 stage names for the rocks. The corresponding Ages, the time divisions, were defined mostly in Europe and have European-derived names, but are used worldwide. Most of those ages range from 3 to 6 million years in their timespan, but the Aptian, Albian, and Campanian ages are 11 to 13 million years long.
Generally, in marine rocks, the various ages of the Cretaceous are defined by international agreements that peg the boundaries to the first appearance of particular fossils, usually ammonites, but for some of them, the boundaries are defined by appearance or disappearance of other fossils, including certain species of crinoids, bivalves, and coccolithophores, the algae whose shells make up much of the Cretaceous chalk.
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Today’s geologic birthday is Marie Morisawa, born November 2, 1919, in Toledo, Ohio. Her specialty was geomorphology, the study of landforms and their origins. She wrote eight technical books and founded the journal Geomorphology.
—Richard I. Gibson