Tryweryn River Sections

Site of Special Scientific Interest · 52.9183, -3.6041

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Also within

Gwynedd County Eryri / Snowdonia National Park

Habitats

Fast flowing river and stream

Geology & landscape

Geological Period: Ordovician · Rock Type: Limestone

Description

Description Tryweryn River Sections is of special scientific interest because it contains the type locality of the Rhiwlas Limestone, an important fossil-bearing unit of late Ordovician age which has facilitated international biostratigraphical correlation. The site is located approximately 1 km north of Bala and comprises two short sections situated on opposite banks of the Afon Tryweryn. This site is the type locality of the Ashgill Rhiwlas Limestone and provides the best exposed outcrops of this rock in the classically important Bala area. The Rhiwlas Limestone contains a rich trilobite and brachiopod fauna which has proven to be of profound significance in international correlation of Ashgill rocks, having affinities with faunas as far afield as Bohemia and Poland. The Rhiwlas Limestone yields rare cystoid material, and has long been an important “marker” horizon, representing the lowest Ashgill sediments in the Bala area. Remarks: The site lies partly within Snowdonia National Park.

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