Habitat account - Raised bogs and mires and fens
7220 Petrifying springs with tufa formation (Cratoneurion) * Priority feature
Background to selection
|Distribution of SACs/SCIs/cSACs with habitat 7220 Petrifying springs with tufa formation (Cratoneurion). Click image for enlarged map.|
Description and ecological characteristics
Tufa formation is associated with hard-water springs, where groundwater rich in calcium bicarbonate comes to the surface. On contact with the air, carbon dioxide is lost from the water and a hard deposit of calcium carbonate (tufa) is formed. These conditions occur most often in areas underlain by limestone or other calcareous rocks, and particularly in the uplands of northern England and the Scottish Highlands.
Tufa-forming spring-heads are characterised by the swelling yellow-orange mats of the mosses Cratoneuron commutatum and C. filicinum. Many rare, lime-loving (calcicole) species live in the moss carpet, particularly arctic-alpine species, such as bird’s-eye primrose Primula farinosa, Scottish asphodel Tofieldia pusilla, alpine bartsia Bartsia alpina and false sedge Kobresia simpliciuscula.
There are two main NVC types associated with tufa formation:
- M37 Cratoneuron commutatum – Festuca rubra spring
- M38 Cratoneuron commutatum – Carex nigra spring
The former community is widely distributed, while the latter is found only at moderate to high altitudes and has a flora especially rich in rare arctic-alpine species, which is best-developed in upper Teesdale and the Scottish Highlands.
Tufa-forming springs are often associated with 7230 Alkaline fens, where they may form prominent upwelling masses of short open vegetation around the spring-heads that feed the fen system. There may also be transitions to a wide range of other habitats, particularly calcareous grassland, acid grassland, heath, 8240 Limestone pavements, and calcareous cliff and scree.
Tufa also forms in some highly-calcareous lowland alkaline fens in southern Britain, but these fens are not considered to fall within the Cratoneurion in terms of their detailed floristic composition; they generally conform to Annex I type 7230 Alkaline fens.
European status and distribution
Petrifying springs are found in a number of EU Member States, in diverse environments from forest to open countryside.
UK status and distribution Click to view UK distribution of this habitat
Cratoneurion tufa formation is a relatively rare phenomenon in the UK. Petrifying springs occur as small, scattered flushes, and the total area of this habitat type is small.
Site selection rationale
The SAC series reflects the distribution of petrifying springs in the UK, with a concentration of sites on the Carboniferous limestones of northern England and on base-rich rocks in Scotland, where a high proportion of the habitat is found. Site selection has taken account of the habitat’s priority status, and the site series encompasses a high proportion of the total UK resource.
Sites have been selected to encompass the geographical distribution and ecological variation of the habitat type (including the range of ecological contexts in which it is found). Many of the sites selected contain a rich flora, and include a wide range of rare and local calciphile species.
|Asby is one of three sites selected on Carboniferous limestone in northern England. Sunbiggin Tarn and Moors is considered to be the most important site in Britain for petrifying springs with tufa formation, owing to the extent of the habitat type and the degree of conservation of spring-head structures. Nearby, Crosby Gill has areas of tufa with transitions to 7230 Alkaline fens and holds good populations of alpine bartsia Bartsia alpina. Large tufa mounds formed around spring-heads are frequent. There are transitions to a range of 7230 Alkaline fens, calcareous grasslands and acid heath.|
|Beinn a`Ghlo||Perth and Kinross|
|Beinn a’Ghlo represents upland petrifying springs with tufa formations in Perthshire. Springs are associated with extensive exposures of metamorphosed limestone of the Dalradian series. Tufa-forming mounds are frequent and are found with other qualifying habitats (e.g. 7240 Alpine pioneer formations of Caricion bicoloris-atrofuscae, 6230 species-rich Nardus grasslands, and 8240 Limestone pavements). At Fealar Gorge tufa formations are found with mountain avens Dryas octopetala within the highest upland birch woodland known in Britain.|
|Cairngorms||Aberdeenshire; Highland; Moray|
|The Cairngorms is one of three sites representing upland petrifying springs with tufa formation in north-east Scotland. The springs occur particularly at Inchrory, where there is an extensive series of springs associated with metamorphosed limestones and calc-schists. There are transitions to 7230 Alkaline fens, 6230 Species-rich Nardus grasslands and more acidic grassland and heath communities.|
|Craven Limestone Complex||North Yorkshire|
|Craven is one of three Carboniferous limestone sites in northern England selected for petrifying springs with tufa formation. The site contains extensive complexes of tufa-forming springs associated with a wide range of other habitats, including 7230 Alkaline fens, calcareous grasslands, 8240 Limestone pavements, cliffs and screes. Locally calcareous springs emerge within areas of acid drift supporting heath and acid grassland. The flora of these habitat mosaics is outstandingly species-rich and includes many rare northern species, such as alpine bartsia Bartsia alpina and bird’s-eye primrose Primula farinosa.|
|Creag nan Gamhainn||Moray|
|Creag nan Gamhainn, a limestone crag and small, wooded gorge along the River Avon, is one of three sites selected in north-east Scotland for this habitat. An unusual feature of the site is the occurrence of Petrifying springs with tufa formation (Cratoneurion) within woodland. Although unusual in the UK, the habitat type is found in similar woodland situations in Fennoscandia. The mixed broad-leaved woodland has a ground flora rich in ferns. Other associated habitats include dry calcareous grassland and fen communities.|
|The petrifying springs with tufa formation on Dalradian Durness limestone at Inchnadamph are considered to be the best examples in the north-west Highlands on account of the large number of springs developed and their overall extent. Inaccessible springs on rocky slopes show a more continuous moss development while others are accessed by deer and are more open in structure as a result of trampling. They are formed on the slopes below outcrops of Durness dolomite. Petrifying springs occur at relatively low altitude at this site.|
|Moor House – Upper Teesdale||Cumbria; Durham|
|This is one of three sites in northern England that have extensive series of petrifying springs with tufa formation. At this site Carboniferous limestones are thinly-bedded amidst shales, sandstones and slates. Tufa springs often occur at the junction between limestone and these other, less permeable, rocks at a range of altitudes. Tufa springs are associated with calcareous glacial drift and can be found in calcareous grasslands, in fen systems of grazed pastures, associated with limestone scar cliffs and screes and amidst acid heathland and grassland. The flora is exceptionally rich and includes rare northern species such as bird’s-eye primrose Primula farinosa and Scottish asphodel Tofieldia pusilla.|
|North Pennine Moors||Cumbria; Durham; Northumberland; North Yorkshire|
|The petrifying springs habitat is very localised in occurrence within the North Pennine Moors, but where it does occur it is species-rich with abundant bryophytes, sedges and herbs including bird’s-eye primrose Primula farinosa and marsh valerian Valeriana dioica.|
SACs/SCIs/cSACs where this Annex I habitat is a qualifying feature, but not a primary reason for site selection
|Border Mires, Kielder – Butterburn||Cumbria; Northumberland|
|Eller`s Wood and Sand Dale||North Yorkshire|
|Eryri/ Snowdonia||Conwy; Gwynedd|
|Ingleborough Complex||North Yorkshire|
|West Fermanagh Scarplands||Fermanagh|
Many designated sites are on private land: the listing of a site in these pages does not imply any right of public access.