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. Farther 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 250myrs. 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.
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Prof John C W Cope (National Museum of Wales, Cardiff UK)