THE FORMATION OF COAL
300 million years ago, long
before humans or even
dinosaurs existed on earth,
Scotland’s coal seams were
being formed. The land was
covered in forests and
looked very different.
Hundreds of thousands of years later when the water had
receded, new forests grew on top of the old ones. Then once
again the waters rose and covered the area with silt creating yet
another layer of coal. This process was repeated many times
until several layers, or seams, of coal were formed.
The shape of the earth was
continually changing. Volcanic
eruptions and earthquakes altered
the earth’s surface. Central
Scotland, and some small areas to
the north and south, were covered
with low-lying swamps. Rivers from the Highlands flowed
down the valleys and into these swamps depositing layers of
silt. The silt then covered the
swamp vegetation. Over time the
buried layers of rotted plants and
trees were compacted and put
under enormous pressure and
heat. This slowly changed the
peat of the swamps into coal.
Types of coal
There are various types of coal. Coal
seams formed at different times were
exposed to different temperatures and
amounts of pressure. This altered the
type of coal produced. The most
important factor, however, is time.
Coal that has been buried in the earth
for the longest time is the hardest and
gives out the most energy when
burned. Coal that has been buried for a
shorter period of time is softer and gives
out less energy.
Cross-section showing
different coal seams
THE FORMATION OF COAL
Peat
Peat is the first stage in the process of coal
formation. Peat forms in low-lying swamps
that have been buried by surface litter and mud
in peat bogs. The partially decayed twigs,
branches and leaves are compressed and turned into
peat. Peat bogs can be found in moorlands in Britain.
plant fragments. Bituminous coals are easy to light, but
burn with a great deal of smoke. They are also easily
identified by their sooty surface which leaves fingers dirty
when handled.
Peat
Lignite
Further burial of peat will transform it into
lignite. Not a true coal because of its high
moisture content, woody composition and
crumbly nature, lignite is rarely found in
Scotland. The leaves and bark that make up lignite
Lignite
are what give it its brownish colour. Lignite is
rarely used as fuel because of the bad smell it gives when
burned; intead oil is extracted from its core.
Bituminous
Bituminous coal is a soft coal and is the
most common type of coal found in Britain.
It is used most often in homes and factories
for energy. When lignite has been buried
Bituminous coal
deeper in the earth it is further compacted and
transformed into bituminous coal. This coal is black and
usually made up of various bright (clairan) and dull
(durain) layers. The bright layers consist of many large
plant fragments. The dull layers are made up of dense, thin
Anthracite
Anthracite is formed after bituminous coal has
been changed by exposure to high
temperatures and pressure. Anthracite is the
oldest coal. It is black, or sometimes
brownish black in colour, and has a brilliant
Anthracite
sheen. Although it is hard to light, once lit it
burns with almost no smoke and gives a good steady heat.
Anthracite can be found in the Scottish coalfields.
Cannel
Cannel is a dull, hard type of coal. It is similar
to bituminous coal in that it has thin layers of
plant fragments. But it also contains the
remains of fish and other small plant
Cannel
fragments. The frequent discovery of fish fossils
in cannel coal led geologists to conclude that it formed in
still lakes and ponds. Cannel coal is characterised by the
hollow sound it emits when struck. Other names include
stone, parrot, lantern and cornish. Commonly found in the
coalfields of Scotland, cannel coal was given its name
because it burns with a long smoky flame like a candle.
THE FORMATION OF COAL
Types of fossils
Lepidodendron
This was one of the plants that lived at the
time of the formation of coal. Lepidodendron
was a tree, which sometimes grew to over 35
metres in height. The bark of the tree was
characterised by the diamond-shaped pits
where the leaves were once attached.
Calamite
Calamite is a fossil commonly found in the
Edinburgh area. It is an ancient relative of
the horsetail fern. The fossilised stem of this
plant has long vertical striations and a few
thin bands that circle it.
Surface of a
Lepidodendron Corals
Stigmaria
Stigmaria is the fossilised root of the large
Lepidodendron tree. The
irregular pattern of
indentations marked
where tiny rootlets were
attached to the
underground branches.
Stigmaria
Mariopteris
Mariopteris is a fossil, which has
leaves that are often lobed, and
sometimes resemble oak leaves in
shape.
Mariopteri
Calamite
Corals are tiny sea animals that formed a shell to protect
themselves. Different shapes of shell
can help identify the different types of
animals. Corals may be horn shaped
with a large single animal living
inside, or colonial where many small
animals live together and often form a
Coral
dome-like shell. Fossilised corals can be found
Surface of on the beaches of East Lothian.
Stigmaria
Brachiopods
Brachiopods are soft-bodied animals living within two shells,
unequal in size. Brachiopods usually live on the sea floor by
attaching a fleshy stalk, on the shell surface, to the seabed.