The Geology and Landscape around Cardigan
- By: Monika Nolte
Cardigan’s hills, valleys and coastline demonstrate a diverse range of physical features that reveal its complex geological history. The area’s rocks are mostly mudstone, deposited in a deep ocean basin about 450 million years ago. Associated locally with the mudstone are beds of hard sandstone, which can be found at Poppit Sands. These are often between one and two metres thick but elsewhere may be only a few millimetres thick. They were deposited as mud and sand on the sea bed but over time became deeply buried. Approximately 410 million years ago the deposits were squeezed and folded by massive earth movements, which eventually changed the mudstones into slate.
These folded rocks are best seen along the coast at Ceibwr and Poppit Sands. The action of folding led to the injection of fluids, and the paths those fluids took are now in-filled by a system of white quartz veins. Few fossils are found in the rocks due to the sediments having been deposited in deep water, although a few ‘graptolites’, which are free-swimming organisms that look like and are about the same size as flies’ legs have been found. In the 1800’s the local slates were worked extensively along the gorge of the River Teifi between today’s Wildlife Centre and Cilgerran. Cut into slabs, rather than roofing slates, the slate was used mainly in the production of troughs, flooring and fireplaces. Some of the slate waste found its way into the River Teifi, changing its flow pattern; this led to enhanced flooding upstream and silting downstream.
Much of Cardigan Bay is in-filled with a thick succession of Jurassic rocks similar to those in southern England. These reach near to the seabed within 3 miles north-west of Cemaes Head. During the last Ice Age the landscape of the Cardigan area became much modified, with the ice sheet crossing the area a few hundred thousand years ago. The southern edge of this ice sheet lay south of Cardigan, close to the crest of the Preseli Hills, but it flowed further south in what is now St. George’s Channel. Prior to the Ice Age the River Teifi flowed in a wide, meandering valley similar to that presently between Llechryd and Cenarth.
During a period when ice covered the region, the river cut a narrow gorge similar to that found presently between the Wildlife Centre and Llechryd. Deposits were laid down by the river flowing out of the last ice sheet as it melted, and the present location of these deposits highlights how the ice sheet must have been hundreds of metres thick and dominated the whole landscape.
At Cwm Degwel, St Dogmaels, gorge-like valleys and channels that were incised by melt water flowed, beneath the ice sheet, and when the ice sheet retreated the barren landscape was subjected to strong winds, with sand blowning high onto Towyn Warren at Gwbert, south of the Cardigan Golf Club. The landscape in the estuary continues to change. The spit at Pen yr Ergyd is expanding, causing erosion on the south-western bank of the river, and the dunes in the western part of Poppit Sands have expanded greatly in the past twenty years.
Cardigan’s hills, valleys and coastline demonstrate a diverse range of physical features that reveal its complex geological history. The area’s rocks are mostly mudstone, deposited in a deep ocean basin about 450 million years ago. Associated locally with the mudstone are beds of hard sandstone, which can be found at Poppit Sands.
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