Cwm Mill Section, Mardy

Site of Special Scientific Interest · 51.8345, -3.0019

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Also within

Gwent County

Habitats

Fast flowing river and stream

Geology & landscape

Landscape processes: Glacial

Description

Description Cwm Mill Section is situated on gently undulating land on the edge of the village of Mardy to the North of Abergavenny. The stream flows from the east where it emerges from under the A465, the main road between Abergavenny and Hereford, and the adjacent railway line to join the River Gavenny in the west. The rock outcrops form part of the bed and banks of the stream. An important death assemblage of Dittonian ostracoderms occurs in a thin horizon in this section. Complete, but compressed, cephalaspids were found in a lens; all but three specimens belong to a single species, Cephalaspis cradlevensis, the other three species described are unique to this site, these are Cephalaspis cwmmillensis, Cephalaspis abergavenniensis and Cephalaspis (Cwmaspis) billcrofti. Cephalaspis cradlevensis shows apparent incomplete ossification of the exoskeleton suggesting that cephalaspids became ossified evenly throughout the animal once full size was achieved. The associated sediments contain disarticulated but well preserved pieces of Cephalaspis and a mass of specimens of Rhinopteraspis crouchi, made up of various sized dorsal and ventral discs of young animals plus the only known completely articulated specimens of body armour of this species. This assemblage allows the study of the growth stages of the individual armour plates, including examples of the youngest stages of development, and shows the probable existence of sexual dimorphism in R crouchi. R crouchi is used biostratigraphically as a zone fossil through northern Europe, and it occurs in Britain, France, Belgium, Germany and the Ukraine.

Plant communities

A9 — Fast flowing river and stream, Marginal and inundation, Pond ditch and canal, Sluggish river and stream

Potamogeton natans community

Schoenoplectus lacustris, Juncus bulbosus, Sparganium angustifolium, Sparganium erectum, Sparganium minimum, Alisma plantago-aquatica, Lobelia dortmanna, Equisetum fluviatile, Myriophyllum alterniflorum, Elodea canadensis, Lemna minor, Lemna trisulca, Nymphaea alba, Callitriche platycarpa, Callitriche stagnalis, Littorella uniflora, Potamogeton natans, Ranunculus trichophyllus, Glyceria fluitans, Chara subspinosa

A14 — Fast flowing river and stream

Myriophyllum alterniflorum community

Isolepis fluitans, Juncus bulbosus, Fontinalis antipyretica, Lobelia dortmanna, Equisetum fluviatile, Myriophyllum alterniflorum, Lemna minor, Callitriche stagnalis, Littorella uniflora, Potamogeton filiformis, Potamogeton gramineus, Potamogeton natans, Ranunculus peltatus, Chara spp., Nitella hyalina

A17 — Chalk river and stream, Fast flowing river and stream

Ranunculus penicillatus ssp. pseudofluitans community

Berula erecta, Nasturtium officinale, Elodea canadensis, Lemna minor, Callitriche stagnalis, Veronica beccabunga, Polygonum amphibium, Polygonum hydropiper, Potamogeton crispus, Potamogeton pectinatus, Ranunculus penicillatus ssp. pseudofluitans

A18 — Fast flowing river and stream

Ranunculus fluitans community

Fontinalis antipyretica, Fontinalis squamosa var. curnowii, Myriophyllum alterniflorum, Myriophyllum spicatum, Elodea canadensis, Mentha aquatica, Lemna gibba, Lemna minor, Potamogeton perfoliatus, Ranunculus fluitans, Glyceria fluitans, Phalaris arundinacea