How does a raised bog form?
This is the story of how and why a raised bog
forms and how it manages to keep itself growing using
just rainwater.
Schematic
Hydrosere
Many raised
bogs, like Greenhead Moss, began life as a
lake. A layer of very fine minerals - clay and
silt - settled at the bottom of the lake during the
cold climatic conditions of the last Ice Age, which
ended around 11000 years ago. As the climate
warmed, reedswamp and then fen plants grew
around the lake edge on the damp ground, with aquatic
plants growing in the water (a).
Meadowsweet is a common fen plant
When these
plants died, they settled on the bottom of the
lake. Because they were underwater, there was
little oxygen for bacteria that normally break down
or decompose organic material, so the partly decayed
dead plants formed a layer of fen peat over
the silt, clay and mud in the bottom of the lake
(b). The clay lining the lake basin probably
helped form a waterproof lining that stopped water
from draining away. Over time, as more plants
grew and died, this fen peat layer built up, and the
water slowly became shallower. Eventually the
lake was overgrown by fen plants which gradually
moved in from the edge of the lake as the water got
shallower (c). Often birch trees and willows
grew on the fen peat if it was solid enough and not
too wet. This is called a fen carr.
Fen
Carr © SNH
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