OR/14/043 Site description - Hen-Allt Common
|Farr, G, Graham, J, and Stratford, C. 2014. Survey, characterisation and condition assessment of Palustriella dominated springs 'H7220 Petrifying springs with tufa formation (Cratoneurion)' in Wales. British Geological Survey Internal Report, OR/14/043.|
Hen-Allt common is situated on land managed by the BBNP at SO2354039946. The area comprises a woodland and common land that is designated as a SSSI and its special features include unimproved grassland, Flat-sedge Blysmus compressus and Meadow Saffron Colchium autumnal. There is also significant localised tufa deposition.
Geological and hydrogeological data
Hen-Allt Common is located upon the Devonian Old Red Sandstone, Raglan Mudstone Formation and the St Maughans Formation and although not mapped on the site the Bishops Frome Limestone Member occurs in the area. During the site visit an eroded section of bedrock was identified below a more competent sandstone unit visible along the footpath between Penhenallt Farm and the site. It is possible that this eroded bed is in fact a calcrete horizon within the Devonian that has not been mapped on the 1:50K Geological Map and it could be related to the nearby Bishops Frome Limestone Member which has known calcrete horizons. This calcrete unit may also be located above the main site and can be proposed as a suitable source for the site.
The area of tufa formation is on the face of a small bedrock outcrop and in resulting mounds and runnels, the site occupying the side of a small gully. The tufa morphologies displayed at this site are very fragile and great care was taken whilst undertaking the survey. For this reason no auger holes were undertaken.
Two water quality samples were obtained from the site one from the seepage and drips direct from the tufa block (HaC1.1 at SO2354039946) and the other from below the site in a small runnel (HaC1.2 at SO2351339962). pH values ranged from 8.09–8.17 and dominant ions are calcium and bicarbonate. Neither sample showed elevated nutrients with results for nitrate and phosphate close to or below the lower limit of detection.
Land use and pressures
Hen-Allt Common is grazed by sheep although there is currently no evidence of extensive poaching at the site. There is no evidence for burning or obvious signs of nutrient enrichment at any part of the site. There is a private water supply located downgradient from the site. The supply which is possibly a spring collection pit or similar takes water from the base of the site and it flows under gravity to a neighbouring property. The supply is fenced off to protect it from grazing animals and the installation is downgradient from the main area of tufa deposition. Although the abstraction should not have a negative impact on the site, or effect the formation of tufa or supply to the upper part of the site, it is possible that the location of the private water supply is preventing water that would be flowing across the surface in small runnels from reaching further down the small gully and thus limiting the potential downstream area for Palustriella.
7.1.4 Vegetation (spring) for Hen Allt Common
A small site with Palustriella commutata dominated vegetation occurring below a spring head which seeps through joints within a large natural rock face. The immediate spring head and semi- shaded rock outcrops have extensive and intricate tufa deposits that are dominated by cushions of Eucladium verticiallatum with Leiocolea badensis and occasional characteristic woodland flowering plants such as Geranium robertianum, Potentilla steralis, occasional Rubus fruticosus. Although this is a small site, locally below the rock face, species-rich Palustriella commutata dominated vegetation occurs with Campylium stellatum, Bryum psuedotriquetrum, Fissidens adianthoides, Scorpidium cosonnii and very locally Linum Catharticum, Riccardia multifida, Pinguicula vulgaris (single plant) and Carex lepidocarpa. An interesting feature of this flush is the presence of the “acid mire” species Molinia, Succisa mixed with calcicoles. The charophye Chara vulgaris was locally abundant and often becoming petrified as part of active tufa formation.
|Agrostis stolonifera||R (<1%)|
|Carex lepidocarpa (with old fruits)||R|
|Eucladium verticillatum (spor.)||F|
|filamentous algae (Chlorophyta)||F|
|Odontites vernus||R (<1%)|
|Pinguicula vulgaris||R (<1%)|
|Poa annua||R (<1%)|
|Rubus fruticosus agg.||R|
Rocks within the area of flush/spring but above level of seepage water supporting occasional calcareous bryophytes.
Vegetation (adjoining spring) for Hen Allt Common
Tall rush pasture (to 35 cm) dominated by Juncus acutiflorus with a broad number of tall wetland plants including Cirsium palustre, Equisetum fluviatile, Rumex conglomeratus and in one place a species of Marsh Orchid (Dactylorhiza sp.).
|Dactylorhiza sp. (Marsh Orchid)||R|
|Epilobium cf tetrahit||R|
|Juncus conglomeratus||R (<1%)|
|Senecio cf aquaticus||R (<1%)|
Hawthorn scrub with acid grassland (Molinia) and Bracken beneath.
|Rubus fruticosus agg.||O|
The overall assessment is that Hen Allt Common should be classified as being in favourable ecological and hydrogeological condition. However, this is a small site and the existing seepage face (rock outcrop with tufa blocks) is vulnerable to encroachment by Bramble and Hawthorn scrub. The private water supply well located further down from the site may limit the lower extent of the site, however as it appears to be gravity driven (rather than pumped) it is not thought to pose a threat to the hydrological functioning or water supply mechanisms that feed the site.