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Era

Period

Geology

Biology

Cenozoic

(65 million years ago)

Quaternary

Holocene epoch

Pleistocene epoch (Ice Age)

Cascades

Humans

Dinosaurs extinct

Tertiary

Mesozoic

(250 millions years ago)

Cretaceous

Rockies

Sierra Nevada

Flowering plants

Jurassic

Birds

Triassic

Dinosaurs, primitive mammals

Paleozoic

(570 million years ago)

Permian

Wichitas

Appalachians

Coal-forming swamps

Insects

Pennsylvanian

Mississippian

Reptiles

Devonian

Amphibians

Silurian

Plants (land)

Ordovician

Fish

Cambrian

Mollusks

Precambrian

(1,800 million years ago)

This table is meant to acquaint the average reader--in a general way--with the geologic time scale. An "era" is the largest block of geologic time. "Eras" are further divided into "periods" and "epochs." (Only the Holocene, or modern epoch, and the Pleistocene, or "Ice Age," are shown here). Under "Geology," several mountain-building episodes are listed. There are many others, but the mountain ranges given are familiar to most people. "Biology" reflects the fossil record: mollusks first appeared during the Cambrian, while modern man developed much later (closer to the present) during the Quaternary. Finally, "Mesozoic (250 millions years ago)," for example, indicates the Mesozoic began about 250 million years ago.

Last reviewed: 2004/08/07