Fossils are the remains or traces of an animal or plant
that are preserved in rock. They come in many forms, from footprints and faint
impressions of leaves to shells and bones of animals. Fossils tell us about the
life of the past, and when different species evolved and died out. They are
also a guide to the age of the rocks in which they are found. Animals and
plants are FOSSILIZED when they are buried in sedimentary deposits
that are hardened and compressed into sedimentary rocks.
It is rare for the actual body parts of an animal or plant to remain
intact when it is fossilized. Sometimes the parts are mineralized (replaced by
minerals), or they may rot away, leaving a cavity called a mould. Sometimes the
mould fills with rock to form a cast. A trace fossil is a fossil of a mark made
by an animal, such as a footprint.
BIOGRAPHY: WILLIAM SMITH British, 1769-1839
William Smith is known as the father of British geology. While
working as a surveyor, Smith noticed that layers of sedimentary rocks (known as
strata) were laid down in a particular order and that the layers could be
matched by the fossils they contained. This enabled him to draw the first
proper geological map of England.
The process that turns the remains of an animal or plant
into rock is called fossilization. It takes many thousands or millions of
years. The remains become fossils only if they are buried by sediments before
they rot away. This can happen at the bottom of lakes and seas or when dead
animals beside a muddy river are quickly covered by floodwater.
When a fish dies, its body falls to the bottom of the sea or a
lake. Its soft parts are eaten by other animals, leaving just a skeleton.
A layer of sediment settles over the skeleton, which becomes
buried. More sediment settles on top, burying the skeleton deeper.
The buried sediment slowly turns to sedimentary rock. The fish
skeleton is partly or wholly replaced by minerals that preserve its shape.
Millions of years later, earth movements thrust the rock with the
fossil upwards. The fossil is revealed when the rocks above are eroded
away.
