What’s so particular about South Devon?
Make a journey to the South Devon coast round Easter time and you might be certain to return throughout scholar events from universities engaged in fieldwork. What’s it about this space that makes it so common as a centre for this? The straightforward reply lies in a single phrase — selection. There’s most likely no different space within the UK the place such all kinds of rock varieties and ages is well-exposed in such a small geographical compass.
Let’s take a look at a few of the components.
The geological succession
The oldest rocks uncovered in South Devon are of Devonian age and, in contrast to many different areas of the UK, the Devonian rocks are in marine facies nearly all through. Wanting again over the historical past of geology, the age of those rocks had initially proved troublesome to determine and it was solely after Murchison had seen the marine successions in The Rhineland and Russia that he realised that these marine rocks have been the equal of the Previous Pink Sandstone farther to the north. The Devonian rocks current quite a lot of marine facies, with the Center Devonian limestones being of explicit observe. The limestones are an area growth whose presence, in an in any other case deeper water succession, is due solely to native shallowing of the water brought on by thicknesses of volcanic rocks extruded alongside extensional fault traces because the native basins developed. This shallowing allowed reef-building organisms to flourish and the principal ones of these are the stromatoporoid sponges. These helped lure sediment and constructed up reef buildings, whose presence enabled each rugose and tabulate corals to thrive.
The succeeding Carboniferous rocks, collectively often called the Culm, usually are not nicely uncovered in South Devon and, to see higher exposures of them it’s essential to journey northwards. Once more, the Carboniferous succession differs markedly from, its equivalents to the north. The extra easterly outcrops of the Decrease Carboniferous are limestone turbidites and their supply is the Carboniferous Limestone sediments on the shallow-water cabinets to their northeast. Additional west in Devon, the Decrease Carboniferous is essentially represented by cherts or siliceous shales. The Higher Carboniferous is represented by turbidites after which sandstones with skinny sooty coals (the ‘culm’ of earlier writers).
Succeeding the deposition of the Carboniferous was the Variscan orogenic episode, which migrated northwards throughout southwest England through the later phases of the Carboniferous, producing a thrust-dominated predominantly northward-facing sequence of buildings. The orogeny continued into the early Permian, when the main pluton of the Dartmoor Granite was intruded. The Carboniferous rocks have been stripped from most areas of South Devon shortly afterwards.
The current day floor of the Dartmoor Granite was most likely little-removed from the unique floor of the intrusion, since, in some place, xenoliths of the nation rock are frequent. It appears doubtless that the intrusion broke floor and rhyolites extruded over the Dartmoor space, as such rocks are seen above coeval granite in east Cornwall and, though no rhyolite is understood at outcrop within the Dartmoor space, it’s a frequent constituent of clasts in breccias of the decrease elements of the Permian succession within the Crediton trough to the north.
On the coast to the south, the Permian and Triassic rocks are uncovered in a largely steady sequence of fantastically uncovered cliff-sections, most of that are readily accessible. The facies is principally non-marine red-beds, which collected within the Pangaean desert throughout a time-interval approaching 100 Ma. The rocks relaxation with marked unconformity on the Devonian: the basal facies are coarse breccias, however the succession consists of finer breccias, conglomerates, sandstones (together with spectacular aeolian dune-bedded sands), mudstones and evaporites. The Sherwood Sandstone Group, within the higher a part of this succession, gives the principal reservoirs for hydrocarbons within the Wytch Farm oilfield of Purbeck, the place it lies some 1,600m under the floor, and the coastal exposures of this and the overlying Mercia Mudstone group (that gives the seal to the Wytch Farm reservoir) have change into traditional websites for trainee oil geologists.
Relationship of this thick Permo-Triassic succession is simply approximate partly, because the absence of any marine fossils precludes correct correlation with the worldwide normal that’s based mostly on marine successions. Some courting is offered by spores and pollen which might be recognized from intercalations of gray or inexperienced horizons, and reptilian stays also can present some dates. Newest work from magnetostratigraphy implies that there could also be appreciable non-sequences inside this succession.
On the japanese facet of the Exe estuary, the higher elements of the Permian and the entire of the Triassic succession are discovered on the western finish of the ‘Jurassic Coast’ and the western restrict of this heritage website is marked by the ‘Geoneedle’ above the cliffs on the japanese facet of Exmouth. The Heritage Coast extends from there to Previous Harry Rocks to the east of Swanage in Dorset. In Devon, the best Triassic rocks (the Penarth Group) exhibits a return to marine circumstances. which then existed by way of nearly the entire of the Jurassic Interval. Solely the basal a part of the Lias Group, on the base of the Jurassic, crops out alongside the Devon coast.
Resting with unconformity, however with little angular discordance on the underlying rocks, the most recent Early Cretaceous Gault and Higher Greensand regularly overstep older formations westwards. On the Dorset-Devon boundary, the Gault rests on the Lias Group, round Axmouth on the most recent Triassic. West of there at Seaton, the Gault has been overlapped and the Higher Greensand oversteps older members of the Triassic westwards, till on the Haldon Hills, west of Exeter, the Higher Greensand rests on Permian rocks. Farther west nonetheless, round Newton Abbot, the Higher Greensand rests instantly on Carboniferous or Devonian rocks — within the latter case, the time hole represented by the unconformity exceeds 250myrs. Succeeding the Higher Greensand is the Chalk, which is current on the cliff-tops from the Dorset boundary westwards nearly so far as Sidmouth. Palaeogeographical concerns counsel strongly that the Chalk as soon as prolonged farther to the west and it’s doubtless that the Chalk even coated the best floor in Devon over Dartmoor. Remnants of the previous Chalk cowl are to be present in Palaeogene flint gravels that cap such areas because the Haldon Hills, west of Exeter, and are recognized in varied areas throughout the county.
Along with the Tertiary flint gravels, the Palaeogene can also be represented by thick deposits of clays in a sequence of pull-apart basins developed alongside the road of the Sticklepath-Lustleigh fault that runs throughout Devon in a line from Torbay to Barnstaple Bay. These basins yield commercially vital deposits of ball clays which might be exploited for sanitary ware. Pupil guests are capable of see not solely how the clays are labored, however how they’re graded and packed, and far of the manufacturing is exported. One of many surprises to many is that the darkest clays hearth to provide the whitest sanitary ware, because the darkish color is because of finely divided carbon, derived from plant particles within the lakes during which the clays collected. On firing, the carbon is misplaced as carbon dioxide, leaving a white residue behind.
Igneous rocks
There are numerous igneous rocks uncovered in Devon, of which the obvious is the Dartmoor granite that was intruded within the early a part of the Permian Interval. Pure exposures happen as the numerous tors of Dartmoor and many aged granite quarries additionally exist. Dartmoor has an in depth metamorphic aureole (that’s, an space of rock altered in composition, construction or texture by contact with an igneous intrusion) and the rocks inside it have been thermally altered, with an accompanying change in mineralogy. There are coastal exposures of primary lavas and of tuffs, and inland exposures of pillow lavas and of dykes could also be seen in previous quarries.
Buildings
The Variscan orogenic episode had a profound impact in South Devon. The rocks have been deformed in a thrust-dominated sequence of buildings that face predominantly northwards. The rocks have responded in differing methods to the deformation. Within the mudrocks, cleavage is well-developed and, in lots of instances, turns into the dominant rock-fabric, so the bedding is, in some instances, not instantly apparent. Within the huge limestones, there was appreciable recrystallisation, which has altered and even destroyed a few of the fossils – bedding too could also be troublesome to discern. Within the thinner-bedded limestones, notably these with shaley interbeds, the limestones themselves might develop a crude cleavage. Within the case of shaley sandstones, particularly these with interbedded sandstones and shales, deformation has brought about the formation of mullions or boudins. Lots of the folds show traditional examples of axial planar cleavage, with the cleavage planes fanning outwards from the fold axes.
The above buildings are these developed throughout compressional phases of the Variscan orogeny. There are additionally vital extensional buildings developed. These embody early post-orogenic stretching that produced fissuring within the Devonian limestones, particularly within the coastal areas that have been stuffed with the earliest red-bed sediments of the Permian (or probably newest Carboniferous). Later buildings embody important regular faulting that’s demonstrably post-Cretaceous in age and might be associated to the Alpine earth actions. The Sticklepath-Lustleigh Fault is probably the most important of a sequence of strike-slip faults that lower throughout southwest England. Its origin was actually Variscan, however there was main dextral motion alongside it through the Palaeogene, producing a sequence of traditional pull-apart basins during which collected the lacustrine Tertiary ball-clays.
Topography
The topography of South Devon may be very diversified and most of the current landforms might be seen as being derived from a Chalk cowl, which just about actually enveloped even the best floor. Early Palaeogene uplift, most likely centred to the west of Devon within the Lundy space, resulted in speedy removing of the Chalk within the west, however resulted in a ramification of Chalk-derived gravels farther to the east. The Chalk cowl over Dartmoor was eliminated early, however in that area, erosion then slowed because the resistant granitic rocks have been uncovered.
Coastal options
South Devonshire shows a variety of various coastal surroundings. There are broad, sandy seashores, pebble seashores, boulder-strewn seashores and areas the place vertical cliffs plunge straight into the ocean. The cliffs embody caves, pure arches, sea stacks and, in locations, there are raised seashores of Pleistocene age.
Quarries
The principal supplies labored at current embody the Ball Clays of the Bovey and Petrockstowe basins, and aggregates, that are largely obtained from the flint-dominated Palaeogene gravels. There’s a plethora of disused quarries, predominantly within the Center Devonian limestones that have been previously quarried throughout Torquay and Brixham, some till the current previous. Many of those quarries should be visited. The few brick-pits within the space are actually overgrown or infilled. There are additionally previous quarries scattered about on the Dartmoor granite and inside its metamorphic aureole. Youthful rocks that have been labored embody the Higher Cretaceous Beer Stone, and it’s doable to go to the previous workings to see the pillar and stall technique of working the stone that was extensively used up to now for ecclesiastical buildings.
Climate
The climate in Devon might be wonderful, however, as in different elements of the UK, is unpredictable. Dartmoor has its personal mini-climate, with very excessive rainfall and a propensity to be buried in low cloud, which can descend abruptly. It’s helpful to have a number of different methods for dealing with unhealthy climate. The Beer Stone ‘caves’ described above are one chance for a brief interval of safety from inclement climate. The identical could also be mentioned of Kent’s Cavern in Torquay, which is a long-known cave within the Devonian limestones that has yielded a wealthy vertebrate fauna – the oldest fossils are about 340,000 years previous – however the cave has additionally yielded the earliest stays of Homo sapiens recognized in Northwest Europe, that are dated at 41,500 to 44,200 years in the past. Torquay has an attention-grabbing museum that features a lot beside geological reveals.
Lodging
South Devon is a well-liked vacation space and there’s numerous resorts various from primary to luxurious. So far as scholar events are involved, as a result of their lodging wants are largely exterior of the principal vacation season, good worth packages can typically be obtained.
Taking all these components into consideration, it’s not exhausting to see why South Devon is such a well-liked space for scholar geologists.
For these inquisitive about furthering their data of the geology of Devon, I’m main a Geologists’ Affiliation (GA) area journey there in October 2017, at the side of the publication of my information to the world, which is reviewed on the web page reverse. Extra details about becoming a member of the GA and happening the sector journey might be discovered at: https://geologistsassociation.org.uk/index.html.
Concerning the writer
Professor John Cope retired in 2003 after lecturing at Swansea and Cardiff universities. Since then, he has been an Honorary Analysis Fellow on the Nationwide Museum Wales in Cardiff. He has a large area expertise within the UK and Europe, and his publications cowl many fossil teams over a large stratigraphical vary. He has additionally written guides for the Geologists’ Affiliation on Dorset and Devon.
Prof John C W Cope (Nationwide Museum of Wales, Cardiff UK)
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