Cwm Dewi
Climate exposure
This is about exposure to a nationwide pressure this habitat type carries — not a verdict on this site's own management.
Peat habitats store centuries of carbon and depend on a stable, waterlogged water table. Warmer, drier summers dry out peat, making it far more vulnerable to wildfire and to carbon release — a pressure that can undo years of restoration work regardless of how well the site is otherwise managed.
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
Landscape processes: Coastal, Glacial, Peatland / Mire
Description
Description Cwm Dewi is of special interest for its geomorphology. It is a spectacular example of a glacial meltwater channel, owing its origins to erosion by meltwater from the Irish Sea ice sheet. The Cwm Dewi channel is some 1300m long, about 100m wide along its length and opens onto the sea at both ends, although sea defences at Cwm yr Eglwys and Pwllgwaelod mean that the sea has not flowed into the valley for some considerable time. The floor of the channel lies at c. 5m OD, and virtually isolates Dinas Island to the north from ‘mainland’ to the south. The waterlogged valley floor contains thick peat deposits with important pollen records. Geology: Together with Esgyrn Bottom, Cwm Dewi is a representative part of the classic Gwaun- Jordanston meltwater channel system. The channels were originally interpreted as overflow features from glacially impounded lakes, but are now believed to have been cut by meltwater flowing subglacially. They are particularly noted for their large size and extent and may have been occupied by meltwaters on more than one occasion. In addition to being an outstanding geomorphological feature, the channel at Cwm Dewi also contains thick peat sequences with important pollen records. Remarks: The site lies within the Pembrokeshire Coast National Park. Most of Dinas Island is owned by the National Trust. Part of the valley floor (western end) is registered as common land. Cwm Dewi SSSI forms part of a GCR site known as Dinas and Esgyrn Bottom. This section has been selected as a result of the former Nature Conservancy Council's Geological Conservation Review, a national survey and evaluation of sites of geological and physiographical interest.