Geoscience Reference
In-Depth Information
F
IGURE 2 The sweep of geologic time and evolution. Geologic time is
divided into eras, periods, and epochs. The earth formed 4.5 billion years
ago; life began sometime in the Archean, possibly as early as 3.8 billion years
ago, and exploded in the Cambrian. Dinosaurs arose in the Permian and
disappeared about 160 million years (abbreviated m.y.) later at the boundary
between the Cretaceous period of the Mesozoic era and the Tertiary period
of the Cenozoic era. [After
Geologic Time,
U.S. Geological Survey Publication.]
The Cretaceous period began 145 million years ago and lasted
until 65 million years ago, when the Tertiary began. Not surpris-
ingly, geologists break the periods down more finely, first into
epochs and then into stages. We are particularly concerned with the
Maastrichtian, the last stage of the Cretaceous, which began about
75 million years ago and ended at 65 million years ago; and with
