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Arthur’s Stone, Cefn Bryn (Walks 8, 10 and 28)

The Devonian period ended approximately 360 million years ago when changing sea levels caused the sea to advance northwards. Initially mainly muddy marine sediments were laid down over the continental conglomerates, becoming dark, fine-grained shales, but gradually the amount of river-borne detritus diminished to leave clearer waters.

In these equatorial warm, clear waters calcium carbonate precipitated in the form of shells and skeletons from the abundant corals, shellfish, brachiopods and crinoids (sea-lilies). This became the Carboniferous limestone series that comprises grey calcareous shales and massive limestones. The rocks are divisible into three groups: Lower Limestone Shales, Main Limestone and Upper Limestone Shales; however, there are many different rock types within these groups, each with varying textures, thicknesses and fossils as a result of subtle environmental changes. Overall it is about 800 metres thick, but becomes progressively thinner to the north, where the sea was shallower and more susceptible to interruptions of sedimentation as sea levels fluctuated, leading to the absence of some layers.