A primary division of mire types has long
been recognised according to these
major environmental differences (Tansley, 1939). Mires that are formed in
topographic situations which allow peat formation only when effective wetness of
climate is high, are described as ombrogenous (i.e. originating through rainfall).
Other mires are restricted to topographic situations which obviously favour the
collection of water. Such situations occur either where superficial moving drainage
water becomes localised along definite tracks (often in sloping terrain) or where
local relief results in a permanently high water table as in depressions or on
extensive flat areas such as coastal plains. It has become customary to refer to the
first of these conditions as soli-genous and the second as topogenous.
These three classes intergrade and are
not always truly separable. For instance,
the frequency of occurrence and extent of soligenous mire is strongly influenced by
the supply of run-off water, which in turn depends on wetness of climate, as well as
on the nature of the ground. On some topogenous mires, too, upward growth of the
peat hasgradually raised the mire surface above the influence of ground water, so
that the living vegetation eventually becomes dependent on the atmosphere for its
water and nutrients (i.e. it becomes ombrogenous). Further examples of such
intergradation are the distinct water-tracks within ombrogenous mires which can be
regarded as small-scale soligenous components. The concepts of topogenous, soli-
genous and ombrogenous mire nevertheless express different degrees of
interaction of important factors, and the extreme types diverge considerably in
gross morphology, floristics and distribution. Topogenous mires can occur in any
region but are the major type represented in the dry lowlands of southern and
eastern England, whereas active ombrogenous mires are now largely confined to
the humid west and north of Britain.
The topographic situation, hydrology and
gross morphology of mires vary
considerably and some workers have produced a more elaborate classification of
mire types based on variations in these features. The disadvantage of such a
scheme is that it involves features which can only be identified by careful
stratigraphical surveys and water studies, and thus is of limited use during rapid
field survey. The more conservative classification adopted here is based on six
major topographic or structural mire types.
Ombrogenous mires are subdivided into
blanket mires (blanket bogs), which are
predominantly upland and restricted to the north and west, and raised mires (raised
bogs), which occur mainly in lowland situations north-west of a line from the Severn
to the Wash. Topogenous mires are of three main types: open water transition and
flood-plain mires (fens), basin mires, and valley mires (valley bogs). These are
mainly lowland but geographically widespread. Soligenous mires (flush bogs) can
occur in association with some of the other main types, but are most distinctively
developed on the uplands of the north and west.
