Cregennen a Pared y Cefn Hir
Also within
Climate exposure
This is about exposure to a nationwide pressure this habitat type carries — not a verdict on this site's own management.
Coastal habitats are exposed to sea-level rise and storm erosion. Where hard sea defences sit behind them, there's nowhere landward to retreat to — a squeeze that keeps compressing the habitat no matter how well it's managed.
Geology & landscape
Geological Feature: Igneous Intrusion · Geological Period: Ordovician
Landscape processes: Coastal, Volcanic / Igneous
Description
Description Cregennen a Pared y Cefn Hir is of special scientific interest because it provides the best exposed sections through the lower part of the Aran Volcanic Group. Sedimentary and volcaniclastic units within the succession have yielded fossil material which has facilitated precise biostratigraphical dating of the sedimentation and volcanism. The site is located in a mountainous tract of land lying on the north-western flank of the Cadair Idris massif, overlooking the Mawddach Estuary, approximately 7km west-south-west of Dolgellau. Cregennen a Pared y Cefn Hir is a nationally important geological site which includes the clearest and most complete sections through the lower part of the Aran Volcanic Group, a sequence of volcanic and marine sedimentary rocks which, at this site, spans the Arenig and Llanvirn epochs of the Ordovician Period. Four formations are present at the site, which in ascending sequence include the Allt Lëyd Formation, the Offrwm Volcanic Formation, the Cregennen Formation with its two volcanic and volcaniclastic units, the Bryn Brith and Cefn Hir members, and the Llyn y Gafr Volcanic Formation. A diverse acritarch flora from the Allt Lëyd Formation indicates an Arenig age for these marine sandstones and siltstones, and a graptolite fauna from the overlying Offrwm Volcanic and Cregennen formations indicates an early Llanvirn Didymograptus artus Biozone age. The site also displays a variety of extrusive and intrusive volcanic rock types such as subaqueously emplaced basic and acid tuffs, pillow lavas and dolerite sills, together with the Cregennen microgranite sill. These rocks exhibit evidence for a wide range of igneous processes and textures which include welding fabrics in the tuff units, ophitic textures and chilled margins in the dolerites, and a basic margin and xenolithic zone in the microgranite. This is an important site for observing the relationships between rocks of mixed origin and for dating the volcanic episodes which interrupted normal marine sedimentation. In addition, the site is a key teaching, educational and research locality which is complementary to the classic volcanic terrain of nearby Cadair Idris. Remarks: The site lies within Snowdonia National Park and is partly owned by the National Trust.