THE SEDIMENTATION AND PALAEOGEOGRAPHY OF THE OLD RED SANDSTONE OF ANGLESEY, NORTH WALES

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Abstract
Summary: The Old Red Sandstone is not less than 1,645 feet thick and comprises four lithological formations. Each formation is recognized as a major sedimentary facies and is interpreted from a knowledge of modern sediments and sedimentary processes. The lowest facies, of conglomerates and pebbly sandstones, lies diachronously above the gently sloping base of the Old Red Sandstone. The next up comprises siltstones and caliche-like carbonate mineral beds. Overlapping earlier beds, the third facies which dominates the succession comprises numerous cycles of conglomerate, sandstone and siltstone. Rapidly alternating sandstones and siltstones make up the highest facies. These facies are considered to represent piedmont, playa, stream and lacustrine deposition respectively. The Old Red Sandstone probably accumulated in a valley whose axis sloped south-eastward. Palaeontological evidence is lacking, but indirect evidence is consistent with an early Lower Old Red Sandstone age. The beds probably belong to the southern British development of the Old Red Sandstone, rather than to the Midland Valley (Scotland) province as was formerly believed.