Gradually, the fen peat became thicker
and finally plant roots were cut off from the soil
below and they had to rely on water and minerals from
rain water, especially in the middle of the old lake
where the old ground surface was buried more
deeply. The outer fringes of the fen soaked up
the nutrients flowing in from the surrounding ground
and rain washed out or leached the nutrients
and minerals away from the peat surface. Nutrients
are like vitamins and minerals for plants.
So the centre of the swamp became
waterlogged with few nutrients. These
conditions are just right for a bog to begin forming
because bog plants are adapted to nutrient-poor
conditions. They gradually took over from fen
plants as conditions changed (d).
One of the main bog plants is bog moss
or Sphagnum. It is very good at keeping
in water, in fact the plant is made up of cells which
are specially designed to hold water, like little
barrels, so Sphagnum acts like a sponge.
Sphagnum and bog plants form peat which is
almost ‘waterproof’ - it doesn’t let
water drain down very easily, making the surface even
wetter and forming a soft, multicoloured, spongy
carpet.
Sphagnum carpet picture
© SNH
Sphagnum also makes the peat
acidic, so it resists decay and makes it even harder
for most bacteria and fungi to break down any organic
materials. This is why bogs are so good at preserving
a whole range of things.
Preserving the past>
The permanently wet and acidic
conditions stopped trees from growing on the bog and
it became more open.
The bog holds its own water so well that
as plants on the bog died, they stacked up on the bog
surface and the top of the peat slowly began to rise
above the water table in the surrounding soils
(e). This is known as bog peat and, for
obvious reasons, the mound which began to grow higher
than the dry land around it is called a raised
moss (f). These can be anything up to 10
metres high and all because the plants and peat hold
water like a giant sponge.
It is easy to tell the story of how a
bog forms but in reality we would only ever be able
to see a tiny part of this over our lifetime.
It takes thousands of years for these changes to
happen and for peat to build up. For example,
at Greenhead Moss it has taken over 10000 years for
around 4.5 metres of peat to build up – it takes
around 29 years for just one centimetre of
peat to form, or around 0.4 mm of peat for
each year.
A moving image
of bog formation
www.burnsbog.org/what.html
More
Back
