Geology of the Cardigan and Dinas Island district: a brief explanation of the geological map

This Sheet Explanation describes the geology of the area between Dinas Island in north Pembrokshire and the town of Cardigan in south Ceredigion, and summarises the results of the first detailed geological survey of the district. The spectacular coast between Dinas Head and Cemaes Head, where the cliffs stand up to 160 m above the sea, is part of the Pembrokeshire Coast National Park. Inland, the rolling landscape is incised by deep valleys and precipitous rock gorges. The exposed bedrock of the district is composed exclusively of deformed Ordovician sedimentary rocks deposited between about 450 and 440 million years ago in a deep-water environment. The strata were deformed during the Acadian orogeny, around 400 million years ago, and show complex folding, cleavage and metamorphism. The Fishguard–Cardigan Fault Belt crosses the district; this has a long history of movement and played a major role during the accumulation of the Ordovician sediment and subsequently parts of it underwent major reversals of movement. The solid rocks are mantled by a range of Quaternary glacial and postglacial sediments. Two episodes of glaciation are evident: an early pre-Late Devensian glaciation by Irish Sea ice was followed by the main Late Devensian glaciation, around 20 000 years ago. It was during the latter advance and recession of the ice that most of the glacial and periglacial deposits of the district were deposited. After the ice melted and since the postglacial sea level rose to its present-day level, around 5 000 years ago, a suite of alluvial, beach and blown sand deposits (Flandrian) has accumulated. Extensive sand and gravel deposits are currently worked as a source of local aggregate. Abandoned slate quarries of the Cilgerran gorge and pits previously worked for brick clay in Cardigan bear testimony to an earlier extractive industry. The geological survey has also provided information on a range of applied earth science issues including mineral and water resources, conservation, and geohazards including land instability, gas emissions and flooding.