Geoscience Reference
In-Depth Information
•
Fen
A peat land which receives water and nutrients from the soil, rock and ground-
water as well as rain and/or snow. Fens generally form in natural basins that have
been flooded and developed into lakes. Plants start to grow around the edges of
these lakes and eventually extend over most of the surface, often with the only
area of open water at the centre or deepest part. The fen peat forms as plants, such
as sedges, reeds and herbs, die and accumulate at the bottom of the lake.
In temperate regions, peat often occurs as fen, transition and bogs, which are lakes
or pits filled with organic material. Fen peat and bog peat are respectively sometimes
known as low-moor and high-moor. Peat lands may pass through a number of mor-
phological stages, each with its own particular plant communities, which characterize
the type of peat that develops. Figure 2.7 illustrates the various morphological stages
and the associated properties of British peat (after Hobbs, 1986).
In the above example, sediments gradually filled lakes or basins, but surface run-off
continues to bring both nutrients and sediments into the area allowing colonization of
vegetation. The vegetation contributes plant remains in greater and greater abundance
to the sedimentary deposit. The peat land eventually adopts a marsh-like landscape,
which has been referred to as fen. As can be inferred from above, fen peat is frequently
underlain by very soft organic mud.
Next follows a transition stage when, because of upward growth, the water to
the peat lands is supplied more and more by direct precipitation. The peat is generally
mixed and woody, forming what is termed as basin bog, or transition peat. Lastly,
the ombrotropic stage is reached, when the peat land grows beyond the maximum
physical limits of its groundwater supply and therefore relies entirely for its water
supply on direct precipitation. The peat itself acts as a reservoir holding water above
ground water level. The water associated with such peat land is typically acidic. The
peat deposit formed is termed
raised bog
or
bog
.
The above process of peat land development is also called a lake-filled process (also
referred to terrestrialization; see Figure 2.8). The differences between fen and bog peat
are attributable to the types of plant remains in the peat and their mode of origin.
The differences involve the degree of humification, structure, fabric and proportion
of mineral material contained in the peat, and this in turn affects their engineering
behaviour. Raised bogs or bogs are typically fibric (fibrous). The above successive
stages of peat land development are also known as wetland succession.
According to Hobbs (1986), some fen peats in Britain, because they occur in
areas of carbonate rocks such as chalk or limestone, were supplied with water which
was slightly alkaline. As such, the plant communities are more diverse, giving rise
to what is called rich fen peat. This develops a much higher degree of humification
than acid peat. Because the strength and permeability of peat declines significantly
as humification increases, rich fen peat presents more problems to engineers than
acid peat.
Valley bogs are formed along the flatter parts of valley bottoms and generally occur
as a result of water draining from relatively acidic rocks. These bogs have complex
lateral zonation due to differences in the vegetation that developed: for example, it is
richer along the border of the bog and along streams flowing in the valley.
