Fossil Coast Drinks Co in collaboration with Gary Holpin one of the South West’s leading professional photographers explores Devon's geodiversity during the Devonian Period.
The county of Devon is located in the South West of England and has some of the most varied geology in the British Isles and enigmatic landscapes. Devon’s geological and fossil record spans 415 million years starting from the Devonian Period. It is this geodiversity through both the geological and fossil record that our blog shall explore.
Importance of Geodiversity
Across Devon, there are over 40 walks and cycle paths published by Devon County Council to observe the County’s geodiversity. Geodiversity refers to the natural portion of the Earth that is not alive, both at the surface and in the Earth's interior.
Geodiversity considers understanding the Earth's minerals, rocks, fossils, soils, sediments, landforms, topography, and hydrological features such as rivers and lakes. The term ‘geodiversity’ also spans the processes that create and modify these features. Exceptional examples of geodiversity are preserved in UNESCO Global Geoparks and natural World Heritage Sites around the world of which Devon has both.
"Geodiversity underpins food production, water management and energy production. It is also central to the ‘green’ transition: when used wisely, mineral resources can create wealth and jobs while decarbonizing development, all prerequisites for a sustainable future..." Ms Audrey Azoulay, Director-General of UNESCO October 2022
The South West county of Devon has 13 different rock types and is the home to several internationally important natural UNESCO sites such as the Jurassic Coast, the English Riviera Geopark and the North Devon Biosphere Reserve. The county has a number of iconic and accessible geological landscapes including the Valley of the Rocks and Granite Way on Dartmoor.
The county’s name was the inspiration for the 1839 designation by the English geologist Adam Sedgwick and the Scottish geologist Roderick Murchison of the Devonian Period, part of the Palaeozoic era, and sometimes known as the “Age of Fishes” because 85% of the Earth during this period was covered by ocean.
More recently the Pleistocene epoch, translated as “most recent”, a geological time period that began about 2.6 million years ago and ended 11,700 years ago saw the Earth covered in a patchwork of glaciers during the last ice age.
Devon at this time was on the ice margin of the Devensian British-Irish Ice Sheet (BIIS) that covered approximately two-thirds of Britain and Ireland and reached its “last glacial maximum” approximately 27 – 30,000 years ago.
Devon remained unglaciated and subsequently, its permafrost conditions were not levelled by ice sheet erosion or covered in glacial deposits as the ice melted and retreated. Instead, Devon and the South West provided an ecosystem that supported some hugely significant animals including the woolly mammoth, woolly rhinoceros, wolf, hyena, hippopotamus, and early humans.
Devon's Own San Andreas Fault
Devon has a naturally occurring dissecting line called the Sticklepath - Lustleigh Fault Zone referred to in some text as Devon’s own San Andreas fault line. Running from North-West to South-East from near Clovelly in North Devon, it passes through the Dartmoor village of Sticklepath near Okehampton before ending in Torquay in South Devon.
It is believed that this fault was formed during the early Tertiary Period (65 – 50 million years ago), and is a wrench or tear fault created by the tectonic pressures during the formation of the Atlantic Ocean and sea-flooring spreading.
The Devonian Period (419.2 - 358.9 million years) in Devon spans 60.3 million years from the end of the Silurian to the beginning of the Carboniferous. At this time Devon would have been located near the equator.
The Devonian period was a time of great tectonic activity, as Laurasia and Gondwanaland drew closer, and by the Late Devonian, the region was experiencing the impact of mountain building. This major mountain-building event saw the gradual closure of the Rheic Ocean and the emergence of the supercontinent of Pangaea with the collision of plate tectonics of North America and Europe giving rise to the Appalachian Mountains, the Caledonides in Britain and Scandinavia and the creation of many subduction zones and volcanic arcs.
In both North and South Devon, a complex combination of sandstone, shales, limestone, schists and fossils was formed and continually influenced by geological faults, volcanic intrusion and folding.
Among the oldest Devonian rocks in Devon are the Lower Devonian schists. These schists are metamorphic rocks that have been put under considerable pressure, heated, squashed or stretched from tectonic movements. These extreme movements converted the clay minerals of the sedimentary rocks into fragile layers of visible plate-shaped mineral grains.
In South Devon, the Lower Devonian Schists are also known as the “Start Point Complex” and include the green hornblende schists and grey mica schists.
The Start Point Complex has a fault that runs in a line from Start Point to Perranporth on the north Cornish coast known as the Start – Perranporth Fault Line and marks the transitions from Lower Devonian Schists to the Middle Devonian slate. The fault line is described as a mylonite (meaning “mill”) fault zone where intensive grinding and crushing influenced by the pressure and stresses have created mylonite a fine-grained, partially recrystallised metamorphic rock.
The grey mica schist is distinguishable by its quartz that commonly forms bands or veinlets between the mica. Schists are generally named according to their most prominent mineral rather than their origination. The dark hornblende schists are formed from lavas, sills and tuffs, and the mica schists are the result of the metamorphism of slates or shales, siltstones, and sandstones.
Schists are best seen at exposures in the South Hams near Salcombe at North Sands Bay and the surrounding area of Prawle Point, Start Point, Bolberry Down the headland of Bolt Tail. The fault line can be seen at The Shippen at Outer Hope Village.
When the sea level began to rise and marine conditions swept across the South West region the deposition of the mineral calcium carbonate began to accumulate and form the sedimentary rock of limestone between 416 to 345 million years ago. The warm shallow Devonian seas were rich in lifeforms that produced these carbonate sediments and so they have an abundance of fossils.
There are principally three depositional Devonian of limestones sediments. These are the Plymouth, Brixham and Torbay Highs and depending on the location they show the three types of Devonian limestone known as organic, chemical, and detrital or clastic.
Firstly, organic limestones are simply sediments created from the carbonate remains of sea creatures; secondly, detrital or clastic limestones are made from the deposition of pre-existing limestone or carbonate rocks and finally, and much rarer, chemical limestones are deposits derived from carbonate minerals already dissolved in the water.
Amongst the fossil record of the Devonian Limestones in Devon are the prolific corals that created huge reefs.
In the oceans, brachiopods, echinoderms including stemmed “blastoid” echinoderms, tabulate and rugose corals, ammonites, graptolites, and trilobites were widespread alongside an ancient fossil group that first appeared in the seas of the mid-Cambrian Period called crinoids.
The most notable and diverse group of animals to evolve during the Devonian was the primitive fish. The most formidable of them were the armoured Placoderms that first appeared in the Silurian with powerful jaws lined with bladelike plates that acted as teeth that fed on molluscs and predated on other fish.
The Devonian ancestors of fishes living today belonged to two main nonarmored groups. The cartilaginous fish later gave rise to sharks and rays. The second group were the bony fish covered in scales and had manoeuvrable fins and gas-filled swim bladders for controlling their buoyancy. Most modern fishes are bony fish following the Permian-Triassic Extinction Event.
Kents Cavern rock is Devonian Limestone - Image by Fossil Coast Drinks Co
In North Devon, the characteristic red-brown colour sandstone is known as the "Old Red Sandstone" and was the first major group of nonmarine rock to be preserved in the geological record of the Devonian Period.
These sandstones were deposited in a delta that drained south from the neighbouring Welsh mountains across what would become the Bristol Channel into a region of the Rheic Ocean that covered Devon.
The sandstones of North Devon date from Mid Devonian Epoch to the Late Devonian Epoch spanning over 34 million years including the oldest Hangman Sandstone of the Eifelian Age and Givetian Age to the Famennian Age where the Pickwell Down and Baggy Sandstones found near Morte Bay and around Barnstaple were formed.
The sandstones have a complex fossil record because they were deposited at the same time but under two different environmental conditions combining both the river delta and marine conditions. Ordinarily, the sandstone does not contain marine fossils but fossil fish, plant assemblages and animal trace fossils are found in delta-originated sandstone.
Devonian slate is widespread across South Devon. Slate is a fine-grained, foliated metamorphic rock created by the alteration of shale or mudstone from the pressure and heat from mountain building activities. A common characteristic is not only its colour but also under pressure the clay minerals line up parallel to each in layers or foliations that can be split and traditionally used as a building material.
In North Devon, the shoreline of Lynmouth and Combe Martin exposes the Lynton and Ilfracombe slates. The Ilfracombe Slates Formation comprises a combination of shallow marine-originated slates and limestones as well as sandstones of deltaic origin. The Lynton Formation sits beneath the Hangman Sandstone Group and has some shells and trace fossils of Chondrites, the remnants of a highly branched burrowing system of organisms that lived in the sediments of the seafloor.
Slates are far more prevalent in South Devon. The Dartmouth Group of slate was formed during the Early Devonian Epoch during the Lochkovian Age and Pragian Age between 419.2 - 407.6 million years ago. This group of slates is found around Dartmouth and exposed between Adurn Point in the west and Scabbacombe Head in the South Hams. Other locations include Castle Cove and Sugary Cove south of Dartmouth Castle and the cliffs at Strete Gate the start of the long coastal bar called Slapton Sands that stretches to Torcross.
The Meadfoot Group of slate dates to between 410.8 – 393.3 million years spanning the Pragian Age and Emsian Age. The Meadfoot slates can be seen at low tide along Meadfoot Beach in Torquay continuing to the east to Thatcher Point forming a rocky coastline. This group of slates include the Staddon and Bovisand formations. These formations within the slate show subsidiary or minor faults of onshore limestone, mudstone, sandstone, and siltstone.
The Norden Group slates are found around Totnes including Bridgetown and Dartington and contain limestone and volcanic complexes. These grey slates weather to an orange-brown colour. The Gurrington Slate Formation dates 382.7 – 346.7 million ago spanning the Frasnian Age of the Late Devonian into Tournaisian Stage the first of six stages of the Carboniferous Period. These green and purple colour slates occur in the Ashburton and Buckfastleigh areas.
Devon has a rich geodiversity and the Devonian Period alone forms part of our overall natural and cultural heritage that deserves to be better known and conserved for the benefit of future generations.
About Gary Holpin
Gary Holpin is a professional photographer based in East Devon, not far from the Jurassic Coast. Gary fell in love with the coastline of the southwest whilst walking the South West Coast Path, and this led him to become a photographer with a particular love for seascapes and capturing the beauty of our coastline. These days he does it professionally, and as well as landscapes he also specialises in providing photography services to tourism businesses, including property interiors & exteriors (hotels, B&B), leisure & tourism locations (from glamping sites to tourist attractions to golf courses), aerial (drone) photography, and people and events (from corporate headshots to informal shots of customers enjoying tourist attractions). You can explore more about Gary and his work on his website at www.garyholpin.co.uk as well as on Twitter, Instagram, and LinkedIn.