Amroth-Saundersfoot-Tenby Transect, Wales

The Variscan (sometimes called “Hercynian”) orogeny was the last great period of mountain building to affect the British Isles. The heart of this mountain belt lay above much of France (e.g. the Britanny peninsula) and it is here that once deeply-buried rocks are now exposed at the Earth’s surface. The outer parts of the Variscan belt forms a mountain front that runs through northern Germany, Belgium, beneath Kent and southern England, into the Mendips before running through southern Wales and on into Munster province in Ireland. This site in SW Wales captures part of the Variscan mountain front, completely exposed in a few km of coast section.

	Variscan mountain belt during the Carboniferous Period: © Woudlouper

It is a feature of active mountain belts such as the Himalayas that they shed material into adjacent sedimentary basins as they develop. A lot of material collects into a moat (so-called foreland basin or foredeep) ahead of the developing orogen. In South Wales the sediments shed from the orogen were deposited in deltas –these became progressively buried and eventually hosted the famous South Wales coalfields. The fate of this sedimentary material is to become incorporated back into the mountain belt as it advances into its moat. This is what can be seen in the Tenby area. The rocks here are chiefly of Westphalian (upper Carboniferous – 323 to 290 million years ago) age.

Starting at the village of Amroth you can take a walk south to Tenby that passes through Upper Carboniferous strata that becomes increasing folded and disrupted. This includes the famous Lady’s Cave anticline (Saundersfoot) – a classic example of a tight chevron fold. Further south the folds become disrupted by thrusts so that bed continuity around structures is lost. The best example of this disruption is to be found at Monkstone Point. Continuing towards Tenby the structure becomes increasingly complex and disrupted with older Namurian (326 to 313 million years ago) rocks brought up and significant tracts of over-turned rock layers. All this deformation culminates in the Ritec Fault – a significant thrust that brings up Lower Carboniferous limestones (at Tenby’s Castle Hill).

Note – it is possible to follow the walk along the foreshore but this requires careful judgment re-tides.  

Further reading: 

An exceptionally detailed guide book to this and other itineraries in South Wales is:
George, G.T. 2008.The Geology of South Wales: A field guide. Pp. 247. ISBN 978-0-9559371-0-1.

Smallwood, S. 1985. A thin-skinned thrust model for Variscan Pembrokeshire, Wales. Journal of Structural Geology, 7 (6), pp. 683-687

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