Folding at Broadhaven
Pembrokeshire, Wales
The outer parts of mountain chains are commonly marked by folds and thrust faults that climb up to the Earth’s surface and involve strata that had just been deposited from the detritus eroded from the fledgling chain. All too often these outermost regions are eroded away in ancient settings but the outcrops of SW Wales are an exception.
Here the Variscan orogenic front is preserved with structures developed in Upper Carboniferous fluvio-deltaic sandstones and shales. The sea cliffs between Little Haven and Broadhaven display spectacular fold structures, made more complex by faulting.
But to understand ideal relationships between thrusting and faulting, the best place lies just north of Broadhaven beach. Asymmetric (so-called North-facing) folds are cut by northward-directed thrusts. The relationship between these different types of structures was established by Williams and Chapman (1983) who showed that folds form in response to displacement gradients on thrusts, most probably developed as thrust surface nucleated and grew.
With SW Wales being an important training ground for UK geology students, the Broadhaven outcrops are commonly the first place many future Earth Scientists experience the delights of structural geology.
Text: Professor Rob Butler