The Geological Time Scale

A sequence of divisions of geological

time comprising in order from oldest to youngest: Precambrian, Cambrian, Ordovician, Silurian, Devonian,

Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary and Quaternary.

Each of the geological periods is characterised by groups, or suites, of fossils. The picture below shows a typical fossil embedded in a rock.  The geological periods are grouped into three major divisions of Phanerozoic time. The
block of "ancient life" is dated from some 540 million years before present (the Cambrian) to about 245 million years before present (the Permian). Fossils such as
trilobites, graptolites, early fish and ancestral plants belong to this "Era", known as the Paleozoic. The Paleozoic Era is replaced by the time of "middle life" (the Mesozoic Era),
charcterised by dinosaurs and marine organisms such as the great marine reptiles and the ammonites. The Mesozoic Era commenced with the Triassic Period (starting about 245

million years ago) and concluded with the Cretaceous Period (66.4 million years ago).
The last block of geological time is the Cenozoic Era with two geological periods, the
Tertiary and the Quaternary. This era is characterised by widespead evolution of the
mammals, and concludes with the appearance of modern Homo sapiens (our own
species), in late Quaternary time. We are living in the Quaternary Period.