Geoscience Reference
In-Depth Information
5. The main divisions of geological time (not to scale). Ages (on the right, in millions
of years before present) are those agreed by the International Commission on Strati-
graphy in 2000.
It was an English civil engineer, William Smith, who, in the early 19th century, began to
make sense of it all. He was surveying for Britain's new canal network and started to real-
ize that rocks in different parts of the country sometimes contained similar fossils. In some
cases the rock types too were the same, sometimes only the fossils were similar. This en-
abled him to correlate the rocks in different places and work out an overall sequence. As a
result, he published the first geological map. Once the dates were added in the 20th century,
and the rocks correlated between different continents, it was possible to publish a single
sequence of layers representing periods of geological time for the whole world. The geolo-
gical column we know today is the product of many techniques, refined over the years and
agreed by international collaboration.
Extinctions, unconformities, and catastrophes
It became clear that some of the changes in the geological column were bigger than others,
and these provided convenient places to divide the geological past into separate eras, peri-
ods, and epochs. Sometimes there was a sudden and significant change in the nature of the
rocks across such a boundary, indicating a major environmental change. Sometimes there
was what is known as an unconformity, a break in deposition, caused, for example, by a
change in sea level so that either deposition stopped or the layers were eroded away before
the column continued. They are often also marked by major changes in fauna, represented
by fossils, with many species becoming extinct and new ones beginning to arise.
A few intervals in the geological record stand out for the severity of the extinctions across
them. The end of the Cambrian period and the end of the Permian period were both marked
by the extinctions of around 50% of families and up to 95% of individual species of marine
invertebrates. The extinctions that marked the late Triassic and late Devonian saw the loss
of about 30% of families and, slightly smaller at 26%, but the most recent and the most
famous, is the mass extinction at the end of the Cretaceous period 65 million years ago.
That so-called K/T boundary is famous not only because it saw the extinction of the last of
the dinosaurs but also because there is good evidence for the cause.
Threat from space
The first suggestion, by Walter and Louis Alvarez, that the extinction might be due to an as-
tronomical impact at first received little scientific support. However, they soon discovered
that sediments in a narrow band at that point in the geological column were enriched in
