What’s so special about South Devon?

Professor John CW Cope (National Museum of Wales, Cardiff UK)

Take a trip to the South Devon coast around Easter time and you are bound to come across student parties from universities engaged in fieldwork. What is it about this area that makes it so popular as a centre for this? The simple answer lies in a single word — variety. There is probably no other area in the UK where such a wide variety of rock types and ages is well-exposed in such a small geographical compass. Let’s have a look at some of the factors.

The geological succession

The oldest rocks exposed in South Devon are of Devonian age and, unlike many other areas of the UK, the Devonian rocks are in marine facies virtually throughout. Looking back over the history of geology, the age of these rocks had initially proved difficult to identify and it was only after Murchison had seen the marine successions in The Rhineland and Russia that he realised that these marine rocks were the equivalent of the Old Red Sandstone farther to the north.

The Devonian rocks present a variety of marine facies, with the Middle Devonian limestones being of particular note. The limestones are a local development whose presence, in an otherwise deeper water succession, is due entirely to local shallowing of the water caused by thicknesses of volcanic rocks extruded along extensional fault lines as the local basins developed. This shallowing allowed reef-building organisms to flourish and the principal ones of those are the stromatoporoid sponges. These helped trap sediment and built up reef structures, whose presence enabled both rugose and tabulate corals to thrive.

The succeeding Carboniferous rocks, collectively known as the Culm, are not well exposed in South Devon and, to see better exposures of them it is necessary to travel northwards. Again, the Carboniferous succession differs markedly from, its equivalents to the north. The more easterly outcrops of the Lower Carboniferous are limestone turbidites and their source is the Carboniferous Limestone sediments on the shallow-water shelves to their northeast. Further west in Devon, the Lower Carboniferous is largely represented by cherts or siliceous shales. The Upper Carboniferous is represented by turbidites and then sandstones with thin sooty coals (the ‘culm’ of earlier writers).

Succeeding the deposition of the Carboniferous was the Variscan orogenic episode, which migrated northwards across southwest England during the later phases of the Carboniferous, producing a thrust-dominated predominantly northward-facing series of structures. The orogeny continued into the early Permian, when the major pluton of the Dartmoor Granite was intruded. The Carboniferous rocks were stripped from most areas of South Devon shortly afterwards.

The present day surface of the Dartmoor Granite was probably little-removed from the original surface of the intrusion, since, in some place, xenoliths of the country rock are common. It seems likely that the intrusion broke surface and rhyolites extruded over the Dartmoor area, as such rocks are seen above coeval granite in east Cornwall and, although no rhyolite is known at outcrop in the Dartmoor area, it is a common constituent of clasts in breccias of the lower parts of the Permian succession in the Crediton trough to the north.

On the coast to the south, the Permian and Triassic rocks are exposed in a largely continuous series of beautifully exposed cliff-sections, most of which are readily accessible. The facies is principally non-marine red-beds, which accumulated in the Pangaean desert during a time-interval approaching 100 Ma. The rocks rest with marked unconformity on the Devonian: the basal facies are coarse breccias, but the succession includes finer breccias, conglomerates, sandstones (including spectacular aeolian dune-bedded sands), mudstones and evaporites.

The Sherwood Sandstone Group, in the upper part of this succession, provides the principal reservoirs for hydrocarbons in the Wytch Farm oilfield of Purbeck, where it lies some 1,600m below the surface, and the coastal exposures of this and the overlying Mercia Mudstone group (that provides the seal to the Wytch Farm reservoir) have become classic sites for trainee oil geologists.

Dating of this thick Permo-Triassic succession is only approximate in part, as the absence of any marine fossils precludes accurate correlation with the international standard that is based on marine successions. Some dating is provided by spores and pollen that are known from intercalations of grey or green horizons, and reptilian remains can also provide some dates. Latest work from magnetostratigraphy implies that there may be considerable non-sequences within this succession.

On the eastern side of the Exe estuary, the upper parts of the Permian and the whole of the Triassic succession are found at the western end of the ‘Jurassic Coast’ and the western limit of this heritage site is marked by the ‘Geoneedle’ above the cliffs on the eastern side of Exmouth. The Heritage Coast extends from there to Old Harry Rocks to the east of Swanage in Dorset. In Devon, the highest Triassic rocks (the Penarth Group) shows a return to marine conditions. which then existed through virtually the whole of the Jurassic Period. Only the basal part of the Lias Group, at the base of the Jurassic, crops out along the Devon coast.

Resting with unconformity, but with little angular discordance on the underlying rocks, the latest Early Cretaceous Gault and Upper Greensand gradually overstep older formations westwards. At the Dorset-Devon boundary, the Gault rests on the Lias Group, around Axmouth on the latest Triassic. West of there at Seaton, the Gault has been overlapped and the Upper Greensand oversteps older members of the Triassic westwards, until on the Haldon Hills, west of Exeter, the Upper Greensand rests on Permian rocks.

Further west still, around Newton Abbot, the Upper Greensand rests directly on Carboniferous or Devonian rocks — in the latter case, the time gap represented by the unconformity exceeds 250Ma. Succeeding the Upper Greensand is the Chalk, which is present on the cliff-tops from the Dorset boundary westwards almost as far as Sidmouth. Palaeogeographical considerations suggest strongly that the Chalk once extended farther to the west and it is likely that the Chalk even covered the highest ground in Devon over Dartmoor. Remnants of the former Chalk cover are to be found in Palaeogene flint gravels that cap such areas as the Haldon Hills, west of Exeter, and are known in various areas across the county.

In addition to the Tertiary flint gravels, the Palaeogene is also represented by thick deposits of clays in a series of pull-apart basins developed along the line of the Sticklepath-Lustleigh fault that runs across Devon in a line from Torbay to Barnstaple Bay. These basins yield commercially important deposits of ball clays that are exploited for sanitary ware. Student visitors are able to see not only how the clays are worked, but how they are graded and packed, and much of the production is exported. One of the surprises to many is that the darkest clays fire to produce the whitest sanitary ware, as the dark colour is due to finely divided carbon, derived from plant debris in the lakes in which the clays accumulated. On firing, the carbon is lost as carbon dioxide, leaving a white residue behind.

Igneous rocks

There are various igneous rocks exposed in Devon, of which the most obvious is the Dartmoor granite that was intruded in the early part of the Permian Period. Natural exposures occur as the many tors of Dartmoor and many old granite quarries also exist. Dartmoor has an extensive metamorphic aureole (that is, an area of rock altered in composition, structure or texture by contact with an igneous intrusion) and the rocks within it have been thermally altered, with an accompanying change in mineralogy. There are coastal exposures of basic lavas and of tuffs, and inland exposures of pillow lavas and of dykes may be seen in old quarries.

Structures

The Variscan orogenic episode had a profound effect in South Devon. The rocks were deformed in a thrust-dominated series of structures that face predominantly northwards. The rocks have responded in differing ways to the deformation. In the mudrocks, cleavage is well-developed and, in many cases, becomes the dominant rock-fabric, so the bedding is, in some cases, not immediately obvious.

In the massive limestones, there has been considerable recrystallisation, which has altered or even destroyed some of the fossils – bedding too may be difficult to discern. In the thinner-bedded limestones, particularly those with shaley interbeds, the limestones themselves may develop a crude cleavage. In the case of shaley sandstones, especially those with interbedded sandstones and shales, deformation has caused the formation of mullions or boudins. Many of the folds display classic examples of axial planar cleavage, with the cleavage planes fanning outwards from the fold axes.

The above structures are those developed during compressional phases of the Variscan orogeny. There are also important extensional structures developed. These include early post-orogenic stretching that produced fissuring in the Devonian limestones, especially in the coastal areas that were filled with the earliest red-bed sediments of the Permian (or possibly latest Carboniferous). Later structures include significant normal faulting that is demonstrably post-Cretaceous in age and can be related to the Alpine earth movements. The Sticklepath-Lustleigh Fault is the most significant of a series of strike-slip faults that cut across southwest England. Its origin was certainly Variscan, but there was major dextral movement along it during the Palaeogene, producing a series of classic pull-apart basins in which accumulated the lacustrine Tertiary ball-clays.

Topography

The topography of South Devon is very varied and many of the present landforms can be seen as being derived from a Chalk cover, which almost certainly enveloped even the highest ground. Early Palaeogene uplift, probably centred to the west of Devon in the Lundy area, resulted in rapid removal of the Chalk in the west, but resulted in a spread of Chalk-derived gravels farther to the east. The Chalk cover over Dartmoor was removed early, but in that region, erosion then slowed as the resistant granitic rocks were exposed.

Coastal features

South Devonshire displays a wide range of different coastal scenery. There are wide, sandy beaches, pebble beaches, boulder-strewn beaches and areas where vertical cliffs plunge straight into the sea. The cliffs include caves, natural arches, sea stacks and, in places, there are raised beaches of Pleistocene age.

Quarries

The principal materials worked at present include the Ball Clays of the Bovey and Petrockstowe basins, and aggregates, which are largely obtained from the flint-dominated Palaeogene gravels. There is a plethora of disused quarries, predominantly in the Middle Devonian limestones that were formerly quarried all around Torquay and Brixham, some until the recent past. Many of these quarries may still be visited. The few brick-pits in the area are now overgrown or infilled. There are also old quarries scattered about on the Dartmoor granite and within its metamorphic aureole. Younger rocks that were worked include the Upper Cretaceous Beer Stone, and it is possible to visit the old workings to see the pillar and stall method of working the stone that was widely used in the past for ecclesiastical buildings.

Weather

The weather in Devon can be glorious, but, as in other parts of the UK, is unpredictable. Dartmoor has its own mini-climate, with very high rainfall and a propensity to be buried in low cloud, which may descend suddenly. It is useful to have a few alternative strategies for coping with bad weather. The Beer Stone ‘caves’ described above are one possibility for a short period of protection from inclement weather. The same may be said of Kent’s Cavern in Torquay, which is a long-known cave in the Devonian limestones that has yielded a rich vertebrate fauna – the oldest fossils are about 340,000 years old – but the cave has also yielded the earliest remains of Homo sapiens identified in Northwest Europe, which are dated at 41,500 to 44,200 years ago. Torquay has an interesting museum that includes much beside geological exhibits.

Accommodation

South Devon is a popular holiday area and there is a large number of hotels varying from basic to luxurious. As far as student parties are concerned, because their accommodation needs are largely outside of the principal holiday season, good value packages can often be obtained.

Taking all these factors into account, it is not hard to see why South Devon is such a popular area for student geologists.

For those interested in furthering their knowledge of the geology of Devon, I led a Geologists’ Association (GA) field trip there in October 2017, in conjunction with the publication of my guide to the area (Geology of the South Devon Coast from the Dorset County boundary to the Brixham area: Geologists’ Association Guide No 73). This field trip is covered by Mervyn Jones in his article: Excursion to the South Devon coast led by Prof John CW Cope (National Museum Wales). More information about joining the GA and going on the field trip can be found at: https://geologistsassociation.org.uk/index.html.

About the author

Professor John Cope retired in 2003 after lecturing at Swansea and Cardiff universities. Since then, he has been an Honorary Research Fellow at the National Museum Wales in Cardiff. He has a wide field experience in the UK and Europe, and his publications cover many fossil groups over a wide stratigraphical range. He has also written guides for the Geologists’ Association on Dorset and Devon.