This type of mire occurs in small, shallow
valleys or channels which are not
enclosed, so that movement of water along the long axis is possible even though
ground slope in that direction may be very slight. Such topography is characteristic
of the soft and relatively young rocks in many parts of southern England and East
Anglia. Valley mires are especially associated with wet, elongated depressions in
the acidic heathlands of these regions, and were first described in the New Forest,
where they are numerous. Although widespread in Britain and not confined to the
lowlands, valley mires are most distinct in low rainfall districts. In the uplands, their
distinctness is often lost through the general occurrence of blanket mire over more
gently undulating terrain, or by transitions to various types of soligenous mire. Valley
mires could, in fact, be regarded as lowland soligenous mires but, from their
dependence on a particular kind of topography it seems best to regard them as a
separate class. Valley mires are typically larger than basin mires, but seldom
exceed 300 ha. The valley sides vary from 1-150 m or more in vertical height but
are usually under 10 m, and they range from gentle to steep in angle of slope. The
peat is often very shallow, but in the case of certain northern glacial overflow
channels is extremely deep and fluid.
As in basin mires, the vegetation of valley
mires consists largely of communities
described under the mire classes on pp. 252-60, and ranges from oligotrophic to
eutrophic according to the base content of substrata within the catchment. Valley
mires often show marked differences in water chemistry and vegetation within a
small space, indicating hydrological complexities within a single mire system. A
characteristic feature of oligotrophic examples is a relatively base-rich central
watercourse or soakway, with lateral zonation of communities associated with
decreasing base-status towards the valley sides. At the edges there is often a
transition to dry heath through zones of wet heath and humid heath varying in width
according to steepness of the valley sides. Some oligotrophic valley mires contain
communities virtually identical with those of ombrogenous bog, but more typically
there are species which appear to need lateral water movement, and the
vegetation is of the poor-fen type.
Mesotrophic to eutrophic valley mires
are restricted to areas where base-rich
substrata occur within the catchment, and are best developed in East Anglia.
Nearly all the major types of rich-fen occur in valley mires, and 'brown moss' carpets
are often especially well represented in the calcareous types. Most of the
communities of lowland mires in Britain, from the poorest to the richest, are in fact
represented in valley mires. Some examples contain spectacular contrasts
between acidophilous Sphagnum carpets and rich herb fen, and many show great
vegetational diversity and richness of flora. Many valley mires also show degrees of
hydroseral development to carr and woodland, with birch, pine, willow and alder
according to varying conditions. As many acidic valley mires are situated within
extensive areas of uncultivated heathland, they are less threatened by
eutrophication than some basin mires with similar vegetation. This is, nevertheless,
a class of habitats rather vulnerable to disturbance in various forms.
The separation between some of the larger
and richer valley mires (e.g. Redgrave-
South Lopham Fen in Norfolk and Suffolk) and typical flood-plain mires is perhaps
rather tenuous, but the smaller acidic valley mires are very distinct. The distinction
between valley and basin mires is also rather fine at times, and both types may be
fed either by springs or a more extensive inflow of water from the catchment; but the
first type tends to show a more particular direction of water movement than the
second. Vegeta-tionally, valley and basin mires show a similar range of variation,
taking the country as a whole; but few basin mires individually approach the degree
of diversity shown by many valley mires, and there is a preponderance of
acidophilous vegetation in basin mires generally. Where raised mires have well-
developed laggs, these have certain affinities with valley mire in both form and
vegetation. In the uplands, extensive complexes of blanket mire may contain
soakways or other areas of obvious lateral water seepage (often, but not always,
associated with depressions or channels) where poor-fen communities suggest a
clear analogy with lowland valley mire. Unlike typical valley mires, however, these
seepage areas have no clear limits defined by transitions to dry mineral soils, for
they merge laterally into deep ombrogenous mire. Valley mire in these situations
tends to be a vegetational rather than a topographic concept, for many shallow
valleys are filled by the general mantle of ombrogenous mire, and the only
distinguishing feature may be a higher water table (sometimes with a patterned
surface) compared with the surrounding mire.
