The Geologic Time Scale
In the early 19th century, William "Strata"
Smith noticed that specific fossils were found in specific
types of rocks, not randomly distributed like people first thought.
Throughout his life, he created a stratigraphic
map of the entire world.
A stratigraphic map is a diagram of the
different types of strata, or layer of bedrock, that occur in
an area.
Between 1820 and 1880 the Geologic Time Scale
was developed with help from Smith's work. It did not have ages
attached, but was based on relative dating.
The early Geologic Time Scale included
the development of plants and animals by identifying fossils
that were in the rocks.
Scientists realized that when a fossil dissappeared
in the rock record, it did not return (which implies the organism
went extinct).
The age of fossils and rocks were later determined
by using radiometric
dating, a process that measures age by the amount of
decay in a radioactive material.
The geologic time periods to the left are distinguished
by major events that occured between their boundaries. For example,
at 65 million years a meteor hit the earth which caused the
extinction of many animals, including the dinosaurs.
Note: This scale is only of earth's recent
history. It has been more easily documented because organisms
began to grow "hard-parts" like shells, skeletons
and armor. The Earth was formed over 3 billion years before
the Cambrian.
