Travel Reference
In-Depth Information
ous rocks the Cambrian System and used, in his definition of this group of
rocks, the characteristic fossils enclosed and found within these strata. The
Cambrian
Period
was denned, then, as the block of
time
during which these
strata—the Cambrian System—were deposited. We now know that the
Cambrian Period started about 530 million years ago and ended about 500
million years ago. Although Sedgewick's strata are found only in a small part
of Wales, we refer to all rocks on earth as belonging to the Cambrian System
if it can be demonstrated, through fossil content or some other technical
means, that they were formed between 530 and 500 million years ago. (The
assignment of ages in years to various geological units of time is a twentieth-
century advance, as we will see later in this chapter.)
Even larger-scale divisions were soon recognized—defined by mass ex-
tinction events, which are sudden global catastrophes causing major biotic
turnovers and extinction. Two of these were especially dramatic. At the top
of strata named the Permian System, and again at the top of a much
younger group of strata known as the Cretaceous System, the vast majotity
of animal and plant fossils was replaced by radically different assemblages of
fossil. Nowhere else in the stratigraphic record are such abrupt and all-
encompassing changes in the faunas and floras found. These two wholesale
turnovers in the makeup of the fossil record were of such magnitude that the
Englishman John Phillips used them to subdivide the geological time scale
into thtee large-scale blocks of time. The Paleozoic Era, or "time of old life,"
extended from the first appearance of skeletonized life 530 million years
ago until it was ended by the gigantic extinction of 250 million years ago.
The Mesozoic Era, or "time of middle life," began immediately after the great
Paleozoic extinction and ended 65 million yeats ago with The Cenozoic Era,
or "time of new life," extending from the last great mass extinction (the "K/T
event") to the present day.
A hierarchical assemblage of units was thus established, each based on
actual rocks and the fossils they contained. The largest-scale units were the
eras defined by the mass extinctions. These eras were in turn made up of
the systems and their accompanying periods, such as the Cambrian and the
Jurassic. Yet these units were also quite large in scale and were surely of long
