The Clwydian Hills are formed from an upstanding block of deep seasediments formed during the Silurian period as debris slurries originating on the nearby continental shelf. The older mudstones and siltstones of the Nantglyn Flags Formation form parts of the west-facing scarp slope and the overlying Elwy Formation, which consists of mudstones and siltstones deposited in deep marine conditions with numerous sandstone beds, forms most of the higher ground.[2] Both formations are of Ludlovian age. The range's rocks are intensely faulted; the major Vale of Clwyd Fault is responsible for the impressive west-facing scarp of the Clwydian Range. It downthrows the rocks to the west and separates the younger Carboniferous and Permo-Triassic rocks of the Vale of Clwyd from those of the hills.[3]
Ice from the Welsh ice-sheet moved eastwards over the Clwydian Hills during the last ice age, impinging on the Irish Sea Ice to the east of the range. Numerous glacial meltwater channels occur around the range whilst the valley of the River Wheeler which cuts the range in two was a significant drainage channel.[4]
A wide range of wildlife thrives in the range, including red kites and red foxes, which both prey on rabbits and voles. There is also one of the few Welsh populations of black grouse and there is a project to conserve the European water vole, which is suffering a large decline in numbers across the United Kingdom. Another project is trying to get rid of the non-native Himalayan balsam which has invaded the Alyn Valley area.[9]
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