Peat varies in nature according to the
factors which produce differences in the
vegetation of the mire surface, i.e. acidity/alkalinity and height of water table, both
of which are affected by degree of human disturbance. Peat formed by
acidophilous vegetation tends to have certain common features regardless of the
type of mire. With a high water table, Sphagnum spp. are typically the important
peat-forming plants, producing a fast-growing, loose-textured deposit known to
peat-cutters as moss-litter. Where the mire surface is drier but still wet enough to
prevent high humification, Sphagnum cover is reduced, and mono-cotyledonous
plants such as Eriophorum vaginatum, E. angustifolium, Trichophorum cespitosum
and Molinia caerulea are important peat-formers, producing a dense fibrous
material. In former ages, the now very rare Scheuchseria palustris locally
contributed a good deal to the formation of this fibrous peat, under conditions of
high water table which were usually transitory. On still drier mire surfaces, dwarf
shrubs such as Calluna vulgaris, Erica tetralix and Empetrum nigrum, along with
Eriophorum vaginatum and Rhacomitrium lanuginosum contribute significantly to
peat formation, but the greater rate of oxidation produces a more highly humified,
amorphous deposit in which it is frequently more difficult to identify the remains of
particular species.
In general, on ombrogenous mires, the
shallower the depth of peat, the greater the
degree of humification (and vice versa), and there is a gradual transition to the mor
surface horizons of wet heaths as drainage improves. The peat most valued for fuel
is that of relatively shallow, well-humified deposits. The peat of raised mires is often
deep (up to 10 m), but the acidic material sometimes passes below into a fen peat
laid down under mesotrophic or eutrophic conditions, and this in turn may overlie
lake, fluvio-glacial or marine sediments. The remains of trees and shrubs, varying in
size from twigs to large trunks, may occur at various levels, indicating periods when
forest or scrub cover developed on the site. Though there is a general tendency for
humification to increase towards the bottom of a deposit the deeper profiles of
acidic peat are often stratified into layers of markedly differing humification,
indicating changes in water table which may or may not be related to drier or wetter
climatic phases. Often, too, deep peat-cuttings reveal the past small-scale surface
structure of the mire, with systems of hummocks and hollows, either corresponding
in position to similar features of the present day, or differing somewhat in their
spatial relationships. The complete profile of a raised mire thus provides a unique
record of the developmental history of the site, dating back in many instances to the
period immediately following the end of the last Ice Age.
The peat of blanket mires differs from
that of raised mires mainly in being shallower
(not usually more than 5 m and most often from 2-4 m), and thus often more
humified and generally in lacking underlying deposits formed under fen or open
water conditions. Blanket mire is, however, variable in age; in many districts it
evidently began to form widely at the onset of the Atlantic Period (around 5000
B.C.) as ground which carried forest or heath became waterlogged and soils
impoverished under the wetter climate following the dry Boreal Period. Yet in many
places, blanket mire did not begin to develop until much later; and topography,
through its influence on drainage, appears to have been a crucial factor in the
initiation of the process. There is also some evidence that blanket mire formation
was accelerated in Neolithic times by human activities of forest clearance and
stock grazing. Many blanket mires have peat with a basal layer containing remains
of trees and tall shrubs which grew on the underlying mineral soil, typically of glacial
drift. Some deposits contain tree remains at higher levels in the profile and, as in
raised mires, the peat is usually stratified, with layers of differing humification
reflecting vege-tational change according to variations in wetness of bog surfaces
over long periods. These changes may sometimes be climatic, but it is again
possible that some apparent changes in wetness may be the result of early human
disturbance. Patterns of surface structure are again shown by the undulations of the
horizons exposed in cut profiles. In many districts upland blanket peat is now
subject to severe and widespread erosion, resulting in breakdown of the mire as a
continuous system. This may be due to changing climatic conditions, or to inherent
instability in the growth of blanket mire, but there is abundant stratigraphical
evidence that heavy grazing and attendant land-use practices, especially burning,
have played a part in initiating peat erosion.
Basin mires have often developed over
kettle-holes and tend to have underlying
deposits which are very deep, especially in relation to surface area. Some have
basal lake sediments, and quite often the mire surface is a floating raft of
vegetation (schwingmoor) overlying open water or semifluid peat. Where there is a
deeper layer of more solid peat it tends to be like that of raised mires, except in the
rather rare mesotrophic or eutrophic basin mires. The oligotrophic valley mires and
soligenous mires tend to have rather shallow peats overlying the fluvial, fluvio-
glacial or drift deposits of the channels, hollows and depressions where they occur.
The peat is nevertheless often the unhumified moss-litter type, except at the shallow
edges of a mire where there is a transition to the podsolic soils of dry ground. In
some examples, especially of soligenous mires, there are good examples of peaty
gley soils, and where periodic flooding of the surface occurs there may be varying
mixtures of mineral sediment.
The open water transition and flood-plain
mires, basin mires, valley mires and
soligenous mires with poor-fen vegetation have peat formed from sedges or
Sphagnum spp., or mixtures of these, varying again in humification between
different places, and within the same profile. The mesotrophic and eutrophic
examples with rich-fen vegetation generally have a distinctive peat formed from
sedges and their allies, Phragmites communis and other tall grasses, forbs and '
brown mosses', in varying proportion. This peat tends to be moderately to highly
humified, probably because higher pH allows some decomposition even under
anaerobic conditions, but in open water transition and flood-plain mires the
deposits are often quite deep (up to 8 m). When drained these rich-fen peats give
agricultural soils of high fertility, as in the East Anglian Fenlands, but drying is
accompanied by rapid oxidation and a tendency to wind-blow, so that over much of
this district the land surface has shrunk by a depth of several metres, and in places
the underlying clay is exposed. Conversely, the undrained remnants of the old
Fenlands appear now as isolated blocks of peat forming more elevated islands,
which can only be kept moist by artificial methods of maintaining water tables. Fen
peat often contains the remains of alder and willow, indicating phases of carr
development. In some localities, nuclei of oligotrophic vegetation amongst areas of
rich-fen on the present surface may indicate incipient acidification of the kind which
occurred on a large scale where flood-plain mire changed into raised mire.
