Fabulous folds: Variscan tectonics in southwest England

Jens Lehmann (Germany)

Plate tectonics drove the continent-continent collision of Euramerica and Gondwana, roughly 280 to 380mya. This mountain-building phase of the late Palaeozoic era is referred to as the Variscan Orogeny and eventually formed the supercontinent Pangaea. This was largely complete by the end of the Carboniferous and many of today’s secondary mountains in Europe are ascribed to the Variscan phase. In the UK, this event formed a couple of spectacular places for geotourism at the boundary between Devon and Cornwall. When visiting southwest England, you should not miss these spots – they are surely among the most impressive places in the world showing the effects of tectonics.

A tiny settlement but tremendous in geology

A tiny settlement on Cornwall’s coast gives the name for surely the most famous spot for folded sedimentary rocks in the UK – Millook Haven. A narrow, winding road leads down the hillside, with a gradient of 30° (Fig. 1).

But, after surviving the journey downhill, you are rewarded by a cliff with spectacularly folded Carboniferous sediments (Fig. 2).

On a global scale, coal swamps are typical of the late Carboniferous period. However, in Cornwall and Devon, no economically viable coal was deposited. In southwest England, alternating sand- and claystones, which had been transported by submarine currents with material mainly in suspension, were formed in an environment far away from the coastline (Fig. 3).

These deposits are referred to as the Crackington Formation. Marine organisms, like early ammonoids (the goniatites) indicate the early phase of the Crackington Formation. They represent a marine environment in contrast to the largest timespan of the Crackington Formation, during which deposits were laid down in a large freshwater lake.

The Crackington Formation is Namurian and earliest Westphalian in age (stage subdivision for NW Europe; note that the international stage subdivision is more detailed, but difficult to apply in SW England; see Gradstein et al, 2012). The largest portion of the Westphalian stage belongs to the Bude Formation, which is characterised by a generally similar lithofacies to the underlying Crackington Formation, but was deposited at a shallower water depth, with occasional with marine intercalations.

The Culm Basin and the greater structure

The Crackington and Bude Formations are part of the late Carboniferous basin, north of an orogenic belt of the Variscan orogeny, called the Rheno-Hercynian. The Rheno-Hercynian fold belt extends as a large curved structure from the UK to the Harz Mountain in Germany and then turns east into Moravia and Silesia (Franke, 2006).

The structure of the Rheno-Hercynian appears simple on the first sight: it roughly looks like an ‘S’, but the whole Variscan orogeny is not (Franke, 2006). A complicated assemblage of a number of microplates is included in this process, involving numerous collisions and therefore the detailed reconstruction of plate motions is difficult. Furthermore, its evolution is challenging, since the general convergence that is responsible for the Variscan orogeny in the Devonian and Carboniferous continuously arose from the Caledonian orogeny during the earlier Silurian.

Back in Cornwall and Devon, the tectonic movements were gradually incorporating the basin sediments of the late Carboniferous Basin (referred to as the Culm Basin) and gave rise to the beautiful folds seen today (Fig. 4). As mentioned above, the Crackington Formation at scenic Millook Haven consists of an alternation of thin beds of sandstones and claystones. Often, these thin-bedded claystones are called, since they are finely bedded and show a good ability to split along horizontal planes of weakness. However, ‘slate’ is a term that should be restricted to metamorphic rocks.

Chevron folds

The characteristic of these folds is that both limbs are uniform and straight, with sharp fold hinges. In other words, the limbs meet in a very acute angle, immediately before the hinge – as a result a set of ‘V’-shaped beds is arranged in a zigzag pattern (Fig. 5).

The inter-limb angles are very tight, being between 5° and 70°, which is also called chevron folding. At the Millook Haven locality, these folds are several meters high and are even more complex, with a horizontal fold axis. Folds are formed in response to compressive stress, and chevron folds, in the first instance, develop when the bedding consists of alternating layers of weak (claystones) and hard (sandstone) rock – in tectonics, the contrasting rigidity of rocks is called competence.

The detailed explanation of the formation of chevron folds is highly complex, because they are a result of the combination of several kinematical mechanisms, “whose magnitude and order of application vary within certain limits” (Bastida et al, 2007). Folding in chevrons is preferred (from an energetic point of view) to sinusoidal folds, because, in chevrons, the ductile flow is minimal as the bending is very local. In other words, it costs less to bend a chunk of rock at a few hinges strongly, than to deform it a little but continuously.

The alternating occurrence of claystone and sandstone can be formed by turbidity currents, a gravity flow transporting clayey and sandy sediments into deep basins in the ocean. The ideal turbiditic sequence would be – from base to top – a grading from coarse to fine grains, within the single beds. However, this ideal sequence is rarely seen. The bedding planes of the claystones contain abundant plant debris, which is fractured and partly rounded during transport down the slope.

Conspicuous features

Some limbs at Millook Haven show an additional fine internal folding, which is also called ‘parasitic’. These can be interpreted as synsedimentary folds, such that they are not formed by immediate tectonic compression, but by the sliding of stacks of soft sediments into the basin during the sedimentation process.

The southwardly inclined fold limbs are thickened by a factor of up to 1.5 compared to the northwardly inclined limbs – which is a matter of shear. Sometimes tension gashes are visible on slickensides at the Millook Haven locality. Additionally, the folding sediments are also affected by rock fracturing. Many boulders at the beach show ‘healed’ fractures in the rock that are filled by quartz – evidence for tectonic stress (Fig. 6).

Hartland Quay

The Culm Basin was folded from south to north (for example, Franke, 2006). Therefore, folding is most intensive at the southernmost localities. The intensively folded cliffs at Millook Haven belongs to the latter, but a short drive north reveals the less strongly influenced folds near Hartland in Devon. At Hartland Quay, the sequence contains ammonoids (among other fossils), confirming a Late Carboniferous age. The chevron folds near Hartland show vertical fold axes (Fig. 7) in contrast to that of the Millook Haven cliffs.

Additionally, their enormous height of up to almost 100m is different to the Cornish locality (Fig. 8), which is related to the slightly greater distance from the head of the Variscan mountain building belt.

A coastal cliff gets famous

The folds described in this article have not only been the focus of scientific literature (for example, Hecht, 1992 and Bastida et al, 2007), but are considered ‘text-book examples’ in the field of public understanding of science – even in the literature of continental Europe in recent years (for example, Reuther, 2012 and Lehmann, 2014). In 2014, the Geological Society of London voted the Millook Haven site as the top “folding and faulting” site in the UK and placed it among the top ten geology spots the UK. On the Internet, in particular, the folding at Millook Haven is well illustrated.

Both sites are also very scenic from a tourist point of view. At Millook Haven, you cannot miss the best place to visit, as there is no other access than to walk along the beach. At Hartland Quay, it is nice to walk along the road above the hotel and enjoy an oblique view into the bay and onto the folds (Fig. 9).

However, the best way to imagine how the deformation process took place is to view the folds from the beach (Fig. 10). This is reached by leaving the car park and taking the path between the restaurant buildings down to the sea. At high tide, there is no way to get down the beach, but the spot at the end of the paved path is also excellent for a view of the giant folds.

References

Bastida, F., J. Aller, N. C. Toimil, R. J. Lisle & N. C. Bobillo-Ares 2007. Some considerations on the kinematics of chevron folds. Journal of Structural Geology 29: 1185-1200.

Franke, W. 2006. The Variscan orogen in Central Europe: construction and collapse. Geological Society, London, Memoirs 32: 333-343.

Gradstein, F. M., J. G. Ogg, M. D. Schmitz & G. M. Ogg 2012. The Geologic Time Scale 2012. 1176, Elsevier, Boston.

Hecht, C. A. 1992. The Variscan evolution of the Culm Basin, south-west England. Proceedings of the Ussher Society 8: 33-38.

Lehmann, J. 2014. Fantastische Falten – Millook Haven und Hartland Quay. Fossilien 31(6): 24-29.

Reuther, C.-D. 2012. Grundlagen der Tektonik. Kräften und Spannungen der Erde auf der Spur. 274, Spektrum Akademischer Verlag in SBM, Heidelberg.