The Cardigan Geological Landscape
The hills, valleys and coast of the Cardigan area display a wide range
of physical features that reveal its complex geological history.
The
rocks of the area are mostly mudstone that were deposited in a deep ocean
basin some 450 million years ago. Associated locally with the mudstone
are beds of hard sandstone (turbidites); at Poppit Sands (Grid Ref:150489)
these beds are up to a metre or two thick but elsewhere may be only a few
millimetres thick. These rocks were deposited as muds and sands on the
ocean floor but became deeply buried and about 410 million years ago were
squeezed and folded by major earth movements. This event changed the mudstones
into slates
These folded rocks are best seen along the coast at Ceibwr (109461) and
Poppit Sands (147490).
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The folding led to the injection of fluids through the rocks and the paths which these fluids followed are now infilled locally by a network of white quartz veins.
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Very few fossils are found in the rocks because the sediments were deposited in deep water. The few graptolites (free swimming organisms that look like and are about the same size as flies legs) that have been found indicate that the rocks belong to the Ordovician System. In the 19th century the local slates were worked extensively in the gorge of the River Teifi between the Wildlife Centre (191449) and Cilgerran (206430). The slate was cut into slabs, rather than roofing slates, and used for making troughs, flooring and fireplaces. Some of the slate waste created by the workings was dumped into the river changing its flow pattern and leading to enhanced flooding upstream and silting downstream.
No evidence of rocks younger than the Ordovician period is found in the Cardigan area, though much of Cardigan Bay is infilled with a thick succession of Jurassic rocks similar to those in southern England. These rocks reach near to the seabed within 5km northwest of Cemaes Head (131502).
The Ice Age modified the landscape of the Cardigan area. The first ice sheet crossed the area a few hundred thousand years ago. Most of the evidence of this event was destroyed by the last ice sheet to cross the area which reached its maximum extent some 18 000 years ago. On land the southern limit of this ice sheet lay south of Cardigan, close to the crest of the Preseli Hills, but it flowed further south in St George’s Channel. Processes related to the ice sheet created a number of features:
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•Prior to the Ice Age the River Teifi flowed in a wide, meandering valley
similar to that presently between Llechryd and Cenarth.
At some time, when
ice covered the region, the river cut across some of the meanders and incised
a narrow gorge-like valley such as presently found between Llechryd and the
Wildlife Centre.
The old cut off meanders are now infilled but can be located
(south of the Wildlife Centre) by careful examination of maps and the landscape.
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•
A raised beach a few metres above the high tide line west of Poppit Sands
(147489). This beach was cut by wave action about 125 000 years ago and
is covered by rounded wave-eroded boulders which in turn is covered by
sticky boulder-rich clay laid down during the last ice age.
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This clay, deposited some 18 000 to 15000 years ago contains many boulders and pebbles from North Wales and southwest Scotland (glacial erratics) that were transported south in the ice.
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• The extensive sand bodies forming Banc-y-Warren (204475) and the
quarries to the north.These
were laid down by a river flowing out of the last ice sheet as it melted.
The present location of these deposits highlights how the ice sheet must
have been hundreds of metres thick and dominated the whole landscape. Similar
deposits are found at Trefigin(132434)
• Local gorge-like valleys (Cwm Degwel, St Dogmaels-162453) and channels that were incised by melt water flowing beneath the ice sheet.
• Once the ice sheet retreated the barren landscape was subject to strong winds and sand was blown high onto Towyn Warren (168 490) at Gwbert, the south of the Cardigan Golf Club.
The
landscape in the estuary continues to change. The spit at Pen yr Ergyd
(160486) is expanding causing erosion on the southwestern bank of the river,
and the dunes in the western part of Poppit Sands have expanded greatly
in the past two decades.
More specific information may be obtained from the British Geological Survey 1:50 000 Sheet 193 Cardigan and Dinas Island: Solid and Drift Geology Map, obtainable from Map Sales, BGS, Keyworth Nottingham , www.bgs.ac.uk.
Click here for information on bird watching around Cardigan
The Teifi River Trail is a great way to discover the wildlife of Cardigan
Read about the Special Area of Conservation that is Cardigan Bay
©Menter Aberteifi