Devon (including Plymouth and Torbay)
Devon is one of the most geologically diverse counties in England.
The County spans approximately 400 million years and includes rocks that belong to the Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary and Quaternary. Devon has seen changing environments from dominantly marine in the Devonian and Carboniferous, arid deserts of the Permian and Triassic, the return of marine conditions in the Jurassic and Cretaceous and the retreat of seas through the Tertiary to the present.
Mountain building towards the end of the Carboniferous has further complicated the succession. This caused uplifting of the Devonian and Carboniferous rocks leading to extensive folding and faulting together with the deep intrusion of extensive granite of Dartmoor which is one of the largest and most southerly upland areas in England.
This diverse geological history has produced a varied landscape including the granite upland of Dartmoor and Exmoor, red Triassic sandstone and mudstone cliffs around Budleigh Salterton and the Cretaceous chalk cliffs of Beer.
Perhaps most notably Devon has lent its name to the Devonian Period of geological time – a term which has been recognised globally since the 19th Century.
Devonian
During the Devonian (417-354 million years ago) Britain was part of a large continental landmass (the Old Red Sandstone Continent also know as Pangaea) that included Europe and North America, which lay approximately 10o south of the Equator. What is now Devon, was at this time located on the margin of a subsiding marine basin with the Old Red Sandstone Continent coastline stretching approximately from the Bristol Channel to the Thames estuary (or at least where they are now). There is a contrast between the Devonian rocks of North and South Devon. In North Devon and Exmoor (which were nearer to land in the Devonian) the rocks comprise shallow water muds, shales, sandstones and occasional limestones.
In contrast in South and East Devon, though shallow water conditions dominated the Lower Devonian water deepened in the Middle and Upper Devonian. Deposition is dominated by deeper water siltstones and limestones the latter containing fossil corals and brachiopods which lived on contemporary reefs which were potentially associated with local volcanic islands.
Carboniferous
Marine conditions continued through into the Carboniferous Period (354-290 million years ago), resulting in the deposition of a thick succession of sandstones, shales and mudstones within the marine basin. Lower Carboniferous shales and thin limestones outcrop in two narrow bands to the north of and either side of the Dartmoor granite as well as between Bideford and Bampton. Volcanic activity was widespread during this time and extensive sheets of lava and volcanic ash occur in the Lower Carboniferous rock succession.
Upper Carboniferous shales and sandstones form the bulk of central Devon, north of Dartmoor. This thick sequence of rocks, which forms part of the so-called Culm Measures, were originally muds that were deposited on the floor of the sea basin, and which were occasionally disturbed by currents of muddy water carrying sands and even pebbles that swept down the flanks of the basin. Where the Culm Measures outcrop, they give rise to heavy acidic soils, making farming difficult. Reflecting this and the oceanic climate, the predominant land use is grass production for livestock, but where this is not intensive then areas of species-rich habitat known locally as Culm Grassland still occurs.
A period of mountain building towards the end of the Carboniferous (know as the Variscan Orogeny) resulted in the spectacular folding and faulting of the Carboniferous rocks as seen in places like Hartland Point, as well as the low heating of the shales to form slates. Higher temperatures affected the Devonian sediments of Start Point and Bolt Tail and these were changed (metamorphosed) to rocks known as greenschist and mica-schist. The culmination or late stages of the Variscan Orogeny was marked by the intrusion at depth of a large mass of granite, the Cornubian Batholith. The surface of this body intrudes through the older rocks as the granite masses that now stretch intermittently between the Isles of Scilly and Dartmoor. This body is often interpreted as the product of the melting of part of the Earth’s crust caused by continent-continent collision.
Fluids escaping along fractures from the cooling granite bodies were responsible for depositing a wide range of minerals including metal ores, including those rich in tin, copper, lead and zinc. The granite, upland mass of Dartmoor today provides a strong geological contrast with the surrounding Devonian and Carboniferous shales and mudstones. This contrast is reflected by the heathland vegetation, granite landforms and upland acid moorland as opposed to the more fertile grasslands found on the surrounding slates.
Permian and Triassic
The last event of the Variscan Orogeny was a general uplift of the area that is now Devon to form mountains, possibly up to 3,000m high. Under a subtropical, arid climate, large amounts of sediment from the mountain chain were swept down by temporary rivers to the surrounding desert valleys and plains. This climate continued throughout the Permian (290 to 248 million years ago) and much of the Triassic (248-205 million years ago). Today thick sequences of conglomerates, sandstones and breccias, deposited by these rivers outcrop between Paignton and Uffculme and are particularly well exposed on the coast between Exmouth and Sidmouth. The arid conditions were responsible for oxidation of iron compounds within the rocks, leading to the characteristic red colouration and the descriptive name for the sequence as the New Red Sandstone.
Jurassic
The end of the Triassic Period 205 million years ago was marked by a marine incursion which flooded small areas of the south eastern part of the County with a shallow sea in which a succession of limestones and shales were deposited (the Lias). Most of the rest of the County remained as land area until the start of the Cretaceous Period, 142 million years ago, and therefore rocks of Jurassic age were either not deposited or have been largely eroded from the area. Only the lowermost Jurassic Blue Lias is exposed to the immediate west of Lyme Regis. The Blue Lias comprises dark marine limestones and shales which are a rich and important source of early Jurassic marine reptiles such as the ichthyosaur.
Cretaceous
The Cretaceous (142 to 65 million years ago) is represented in south-east Devon by three broad divisions of rock, all of which were deposited in a shallow, sub-tropical sea which covered the entire area, apart from perhaps Dartmoor which may have existed as an island for most of the Cretaceous.
At the base of the succession is the Gault Clay, a dark bluish grey clay which outcrops near Lyme Regis. The Gault is overlain by the Upper Greensand, a series of dark green to grey sands with calcareous concretions which give way to sands with chert and shell beds. The Upper Greensand outcrops over the Blackdown Hills area, forming the capping to the series of hills, ridges and escarpments. In the western part of this area the Upper Greensand is devoid of calcareous material but the sands yield superb fossils of marine bivalves and gastropods (snails) preserved in silica. An isolated outcrop of the Upper Greensand caps the Haldon Hills south of Exeter.
Overlying the Upper Greensand is the Upper Cretaceous chalk. At its base this comprises calcareous sands and sandy limestones rich in marine fossils such as sea-urchins, ammonites and bivalves. Over time, the Chalk sea deepened and the input of sand decreased, resulting in the deposition of the purer calcium carbonate-rich white Chalk. In its upper part the Chalk is coursed through with bands of flint and marl (calcareous clay-rich layers) and yields well preserved fossil sea-urchins. Exposures of the Chalk are limited to the coastal section between Sidmouth and Lyme Regis, as epitomised by the well known cliffs at Beer.
Tertiary (comprising Neogene and Palaeogene)
The start of the Tertiary Era (65-2 million years ago) was marked by a period of Alpine mountain building known as the Alpine Orogeny, which affected the Devon area by causing land uplift and retreat of the Cretaceous sea. Aerial weathering and erosion removed much of the Chalk that would have covered the new landmass, leaving behind large expanses of residual flint gravel in the east of the County and on Haldon Hill.
In the Bovey Tracey area there are a series of clays, sands, gravels and brown coals (or lignites) which were deposited within a large lake basin. This basin lies on the line of a great fault (the Sticklepath Fault) which runs through the rocks of Devon from Torquay to Bideford Bay and which through subsidence during the Tertiary accumulated a great thickness of sediment. The lake, which was at least 10 miles long, was fed by mountain torrents coming off Dartmoor and which transported sand and mud together with large amounts of plant debris into the basin. Dating of the plant fossils indicates that the sediments are of Oligocene (about 30 million years ago) age. The clays at Bovey are worked through opencast extraction for the production of high grade earthenware, drainpipes, tiles and bricks. The lignite has been used as a fuel in the potteries. Similar lake deposits are also found in a smaller basin on the line of the Sticklepath Fault at Petrockstow in North Devon.
Quaternary
Over the last two million years ago the climate of Britain has varied tremendously with periods of temperate climate interrupter by repeated advances and retreats of glaciers and ice sheets. Collectively these have been known as the Ice Age and the actions of the ice sheets have been instrumental in forming the landscape we see today.
However Devon did not experience the full effects of the repeated advances of ice sheets, as their southward movement did not extend into the County. However, during these glacial periods it is likely that much of the County resembled the tundra of today’s arctic provinces. Sea-level rises and falls associated with the repeated growth and decay of ice sheets are recorded in the various raised beaches seen around the Devon coastline, such as those exposed at Croyde in North Devon and at Hope’s Nose, Torquay. Clays deposited by ice (known as till or boulder clay) are present at Fremington near Barnstaple. Today, these are a prime source of pottery clay, but they also provide the only indisputable evidence that glacier ice reached the south-west peninsula during the Quaternary, probably during the Wolstonian Glacial.
During the past 10,000 years the rise in sea level following melting of the ice sheet caused many of the rivers and estuaries within Devon to become drowned by the sea, with the accumulation in the lower reaches of the valleys of alluvial silt, clay and peat. Fossil remains of forests which were drowned by the advancing sea are commonly exposed by storm and tides. Examples include those found in Tor Bay and at Westward Ho! Deposits of more recent times include river alluvium, blown sand and modern beaches. Spits have formed at Dawlish Warren and Den Point, together with shingle barriers along the coast, such as the one at Slapton.
Geological Highlights:
- The County of Devon gives its name to one of the periods of geological time, namely the Devonian (417 to 354 million years ago). The defining of this period of time, which is applicable throughout the World, was subject to a significant amount of controversy at the time. There were heated debates and arguments among the members of the British "Geological Society," based in London, in roughly the last half of the 1830's, over the mapping and interpretation of the geological strata of the county of Devonshire. The leading geologists of the day, Roderick Murchison and Adam Sedgwick had mapped the strata in Wales and established the classic Silurian and Cambrian systems, and did not believe the results of mapping undertaken of what were considered similar rocks by their colleague Henry Thomas De la Beche in Devonshire. Following the presentation of various views, counterviews and arguments, Murchison in 1839 was forced to give up his original ideas, largely through his own scientific re-interpretation, and the Devonian was coined by him as a period of time to cover the rock sequence described by De la Beche.
- During the Lower Carboniferous there was widespread volcanic activity and extensive sheets of lava and volcanic ash occur in Cornwall and south Devon. The knoll of Brent Tor on the western fringe of Dartmoor represents a well exposed example of a large amount of lava that erupted onto the seafloor during this time. The exposures at the site demonstrate the effect of water current reworking, carrying volcanic debris down a submarine slope for a considerable distance.
- The brown coal (lignite) beds of the Bovey Tracey Basin have yielded an abundant and diverse fossil flora – 33 named species belonging to 31 families of trees and other vegetation have been identified. Tertiary deposition in the Bovey Basin is believed to have taken place on a river flood plan and, to a lesser extent, in freshwater lakes. The flora, which probably grew on the mountainous slopes of Dartmoor and in the valley flood plain is dominated by redwood conifers, swamp-palm, cinnamon, other species of palms, various climbers, trees and shrubs. This vegetation indicates that the climate at the time was sub-tropical and the area must have resembled a lush jungle similar to those present in parts of Indonesia.
- Several caves in the Devonian limestones of south Devon are famed for the remains of extinct animals and evidence of man’s early occupation of the British Isles that they provide. Perhaps the most famous is Kent’s Cavern, near Torquay, which has yielded large numbers of bones of hyaenas, woolly rhinoceros, cave lion, cave bear, reindeer, sabre-tooth tiger and bison. The large number of hyaena bones and gnawed bones of other animals suggest that the cave was used as a den by hyaneas during the Quaternary. Man-crafted implements, notably hand axes, flint scrapers and bone tools have been found at the site indicating that man occupied the cave at times over the past 200,000 years.
- The tors of Dartmoor represent the products of millions of years of weathering and erosion. Between 30-60 million years ago the Dartmoor granite stood above sea-level and this upland area would have been covered by lush tropical vegetation as Britain lay closer to the equator. Weathering under the hot and humid conditions resulted in the decay of much of the granite, mainly along the joints and fractures in the rock. During the ice ages of the last two million years, the granite was subject to repeated freezing and thawing. These processes resulted in the removal of weathered material from around the main areas of unaltered granite revealing the final form of the granite tors and producing the large amount of boulders and blocks around the tors.
- Gravels deposited by the River Axe and worked for aggregate have yielded large numbers of beautifully worked Palaeolithic hand axes. These were produced by man approximately 10,000 years ago from the large deposits of chert and flint derived from erosion of the Upper Greensand and the Chalk.
- The quarrying of Beer Stone from the Chalk, has, led to the production of an extensive series of caves at Beer. The disused caves now act as a very important site for hibernating bats. In recent years there has been c. 70--80 individuals of up to eight species of bat present in the caves, including the very rare Bechstein's bat and greater and lesser horseshoe bats.
- In 2001 the stretch of cost between Exmouth and Lyme Regis was inscribed by UNESCO as part of the Dorset and East Devon Coast World Heritage Site. This is the first natural World Heritage Site on the UK mainland and was inscribed on the basis of its spectacular and diverse geology and the contribution this coastline has and continues to make to geological study. The World Heritage Site, a 95 mile stretch of coast, provides a walk through geological time from the oldest Triassic rocks in Devon, up through the Jurassic and Cretaceous as you travel from Devon to Dorset and onto the end of the World Heritage Site at Studland Bay.
- The area around Torbay in Devon was inscribed as a European Geopark in 2007. Known as the English Riviera Geopark, the geology comprises marine Devonian rocks, Permian desert sandstones and caves. European Geopark status recognises the importance of the geology of the area and the opportunity that it presents for people to learn more about their surrounding environment. The English Riviera is part of a growing network of European Geoparks and membership of this network will lead to more events activities, trails and leaflets focusing on the geology of the area.
Local sites
The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible.
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