Cretaceous Period, a division of geologic time that began about 140,000,000 years ago and lasted about 65,000,000 years. It was the last and longest of the three Mesozoic Era periods. The Cretaceous Period takes its name from the Latin word for chalk, creta, after the chalk deposits of southern England and northern France formed during this period.
During Cretaceous times, shallow seas covered much of what is now land, and thick sedimentary deposits were laid down, including some now containing oil and ores of such metals as copper and aluminum. Near the end of the period the seas began to withdraw as widespread uplifting and mountain building began. Extensive folding and faulting in the Western Hemisphere marked the birth of the Rocky Mountains and the Andes. In the Middle East and Far East, vulcanism and folding reshaped the land.
The climate during Cretaceous times was relatively warm in most regions of the world. Deciduous trees such as elm, oak, and maple developed late in the period. In the animal kingdom, dinosaurs and other reptiles continued the dominance they had achieved earlier in the Mesozoic Era. By the close of the Cretaceous Period, however, mammals and birds were evolving rapidly and dinosaurs had become extinct. In the sea, ammonites and other mollusks were abundant. The shells of tiny Cretaceous sea organisms called foraminifera became the source of most of the world's chalk.