Introduction
We rarely stop to think about the ground beneath our feet and yet it is fundamental to life itself - providing scenery and habitats, soils for agriculture, and raw materials for building and industry. The Dartmoor landscape is dominated by rocks - tors, gullies, boulder fields, stone walls and buildings. This exhibition explores the fascinating story of the creation and molding of Dartmoor rock starting nearly 400 million years ago and continuing today. Humans first appeared on Dartmoor a mere 10,000 years ago and our own lifetimes seem a trifle in comparison! Nevertheless, the interaction between geology, farming, wildlife, archaeology, industry and water supply revealed here reminds us all of our dependence on the Earth’s resources for our future. Welcome to this celebration of Dartmoor’s earth heritage.
The Geological History Of Dartmoor
370 million years ago Sea sand & mud!!
The Dartmoor we see today has a long history going back to times before the existence of the granite. However, we know very little about the early geological history of the area. Rocks from this time are very deeply buried and little can be said about what they are and how they were formed. The better known history starts during the Devonian period, about 370 million years ago. The Devonian period is named after the county of Devon and lasted for around 50 million years. At this time a tropical sea, with coral reefs, fish and strange sea creatures, covered the area that would become Dartmoor. This sea deepened to the south and had a northern shoreline in North Devon or the Bristol Channel. On the bottom, mud, sand and other lime rich sediments were deposited which eventually became the shale, sandstone and limestone we see today in the region. In some parts volcanic islands appeared and lava and ash were spewed into the sea. Later on, in the Carboniferous period, when forests covered South Wales, sand and mud flowed through deltas in North Devon to be deposited in the Dartmoor and South Devon areas.
280 million years ago The birth off the Dartmoor granitite!!
By Carboniferous times the Earth’s crust was on the move in this part of the world. Pressure resulting from this movement destroyed the Devonian and Carboniferous sedimentary basin and squeezed the sediments up into mountain chains. Across what is now Devon, the shales, sandstones and limestone's were intensely folded and faulted. At the same time, the heat generated by this pressure and movement melted some of the deep crustal rocks to form molten material which slowly shouldered its way up into the deformed sedimentary rocks. It finally solidified several kilometers below the surface, as a ground mass of quartz, sodium feldspar and mica crystals, with larger potassium-rich feldspar crystals. This was the birth of the Dartmoor granite. Some molten material came up through cracks and reached the surface to formlava (rhyolite) flows. The granite also contained lots of hot liquids carrying elements such as tin, copper, iron and boron. These liquids found their way up cracks in the granite and surrounding rocks, and deposited the minerals to form metal ore veins and quartz tourmaline veins. Some of the hot liquids altered the granite itself, turning one of the minerals, the sodium feldspar, into china clay. The hot granite altered and baked the surrounding rocks over a distance of up to 5 km. This altered zone is known as the metamorphic aureole. The boundary of the National Park is very similar to the outer edge of the metamorphic aureole - a clear demonstration of links between geology and landscape value.
270 million years ago Erosion!!
No sooner had the mountain chain been pushed up than it was attacked by the elements. The climate at this time was harsh. It was hot and dry but with occasional periods of heavy rainfall and flash floods. The cover of sedimentary rocks was stripped off and then the granite itself was attacked. The resulting debris was carried by flash floods down gullies and rivers from the mountains and out into the desert below, forming cones of red gravel debris and red desert sands which were then blown into dunes. A considerable part of the country around Exeter, Crediton and East Devon is made of this debris - the source of the rich red soils visible today.
100 million years ago Dartmoor as an island!!
After the intense erosion of the Permian and Triassic periods, the sea levels rose during the Jurassic and Cretaceous periods and progressively submerged Dartmoor. By the late Cretaceous, 100 million years ago, Dartmoor was reduced to an island in the Upper greensand tropical sea. Sand, gravel and shells, deposited on beaches during this period, can be found to the east of Dartmoor near Kingsteignton. It is likely that dinosaurs such as Rhabdodon (illustrated) wandered on the island. In late Cretaceous times Dartmoor was completely covered by the sea, and a white limestone (chalk - the remains of microscopic organisms which lived in the sea) was deposited over the granite.
50 million years ago Sub--tropical Dartmoor!
By this time Britain was beginning to take on something like its modern shape. The sea level fell again and South West England appeared out of the chalk sea. Again the Dartmoor granite was stripped of its cover by erosion. The climate was now sub tropical to warm temperate. Giant Redwood (Sequoia) forests grew on the cooler higher ground of Dartmoor and there were swampy rivers and lakes on the lower ground. Here there were also plants we would recognize today such as ferns, magnolias and swamp cypress and living amongst these were the early mammals, both herbivores and carnivores. The debris from the eroded granite and other rocks was brought down by river to be deposited in lake basins such as the Bovey Basin. Coarse sand was mostly deposited but also clays rich in a mineral called kaolinite. These clays, the ball clays, now provide a valuable source of ceramic clays used in sanitary ware, crockery and tiles all over the world.
Mineralisation
How did the minerals form ?
Cooling of the granite
As the top of the granite cooled, contraction caused steeply inclined cracks - called joints - to form. These were enlarged as the overlying sedimentary rocks were worn away reducing the pressure. Forces in the Earth’s crust also caused movements along fractures, extending and enlarging them. At the same time large quantities of hot, salty water were released by the cooling of the granite. This liquid, at temperatures well above the boiling point of water, moved upwards into the cooler top of the granite carrying dissolved metals with it. The hot fluids penetrated into the fractures and as the temperature and pressure fell, the dissolved minerals crystallised as veins which miners call lodes. Traces of this salt water can still be found in minute cavities in quartz and other minerals associated with the lodes. Some of these fluid inclusions even contain minute crystals of salt. They can be analysed to give an idea of the temperature at which the mineral lodes formed. The water was at temperatures of up to 400°centigrade and still in liquid form due to the dissolved salt and the great pressure of the overlying rock.
Crystallisation
The type of metal ore deposited partly depended on the temperature of the fluids. The lodes containing tin (in the form of its oxide cassiterite) formed at high temperatures and are mostly confined to the granite itself. There are indications that the tin lodes die out with depth, suggesting that they formed near the top of the granite. Cassiterite is commonly associated with tourmaline which also crystallised at relatively high temperatures. Another mineral found within the granite, and sometimes associated with tin, is hematite. A series of lodes in the north east part of the Dartmoor granite contain hematite in the glistening crystalline form called specular hematite. In parts of the lodes the crystals were so fine that they formed a glistening powder called micaceous hematite. It was this powdery form that was mined. The first use was as pounce, the powder sprinkled on hand written letters to dry the wet ink. Later, the hematite was mined to make a corrosion resisting paint for iron and steel. Micaceous hematite was the last mineral to be ‘mined’ on the moor. Some other minerals The copper minerals crystallised at lower temperatures than tin. As a result copper was found as lodes in the sedimentary rocks surrounding the granite. In some mines copper was found in the higher parts of the workings with tin appearing in the deeper parts where the temperatures were higher. Arsenic was worked in association with copper in some mines. Arsenic oxide obtained by roasting the sulphide ore was, at one time, exported to the United States for use as an insecticide in the cotton fields. Some of the last minerals to form, and at lower temperature, occur in north-south aligned fractures or faults that cut across and displace earlier mineral lodes. Near Mary Tavy, on west Dartmoor, Wheal Betsy worked the lead ore galena. Some silver was also separated from the lead. An engine house, which is preserved by the National Trust, marks the site of this mine beside the Mary Tavy to Tavistock road. To the east of the moor, in the Teign valley, another north-south fracture was also mineralised with galena and with the barium-containing mineral barytes. Barytes was worked at Bridford mine as late as 1958. Today, barytes is used in the oil industry as an addition to drilling mud. The high density of barytes helps to control the pressure in oil wells. The discovery of North Sea oil caused a flurry of interest in the barytes of the Teign valley but no further mining took place. Barytes is also added to the concrete used to shield nuclear reactors as it stops radiation.
China Clay (Kaolin)
China clay is formed by decomposition of feldspar minerals in the granite to form the white clay mineral kaolinite. This process is known as kaolinisation. There has been much research and some controversy as to how this took place. Two main processes have been suggested. The most important was probably the circulation of hot water through the granite, during the final stages of cooling. More recent work has shown that much of the alteration may have been caused by water derived from rainfall. It is now known that the interior of the granite is still heated by its natural low levels of radioactivity. A current view is that rain water falling on the granite surface penetrated through fissures to depths where it was heated and rose upwards by convection. This heated water broke down the feldspar into kaolin. The kaolinisation process was probably accentuated by weathering during periods of warmer climate 50-60 million years ago. The working of china clay in South West England developed in the 18th century. A Devonshire man, William Cookworthy, discovered a method of using kaolin to make hard porcelain similar to that produced in China. The term kaolin derives from Gaoling, a locality in China where kaolin is found.
Formation of Todays Landsacpe
Sub-tropical Dartmoor
Between 65 and 2 million years ago, during the Tertiary period, Dartmoor was nearer the Equator and although the climatevaried it was predominantly hot and wet. As can be seen on Panel 5 the landscape was covered by dense sub-tropical forest which included the Giant Redwood tree (Sequoia) which today grows in northern California. Other plants included palms, ferns and heathers. The result of the hot wet climate and abundant vegetation was that acidic waters penetrated deep into the granite enlarging the existing joints. Minerals in the granite, particularly the biotite and feldspar, take up the excess hydrogen ions in the water and break apart - a process known as hydrolysis or chemical weathering. This is one of the causes of tor formation.
Arctic Dartmoor
Around 2 million years ago, when the British Isles had already reached their present latitude, the climate cooled and a series of semi-regular oscillations of climate occurred - known as the Ice Ages. It is now believed there were at least 20 oscillations with the coldest periods occurring over the last half million years. The remains of plants and pollen from the end of the last cold period (Devensian) show that Dartmoor experienced an arctic climate (average January temperatures as low as -20° C) with a tundra vegetation of small flowering plants (arctic and alpine herbs) and small shrubs with the only trees being dwarf willows. Beneath the rocky soil was a permanently frozen layer (permafrost), the upper part of which melted in the summer causing rock and soil to flow downslope - a process called solifluction. Although chemical weathering did not occur during this time, when the water froze and expanded in cracks and joints it exerted a physical pressure breaking up the exposed rock faces and shattering boulders. These processes, along with solifluction, physical weathering, and with the action of gravity, caused the formation of the extensive slopes of jagged boulders (or clitter fields), long lines of large boulders (boulder runs), and terraces of finer gravelly material (growan) which surround many tors. Recent studies have suggested that the boulder accumulations that surround many of the tors (such as Great Mis Tor) were formed by a very slow process called ‘creep’ which operated under permafrost conditions creating what is also called a ‘rock glacier’.
Were there glaciers on Dartmoor?
At its maximum extent, during the Pleistocene, the edge of the ice sheet was somewhere in the Bristol Channel some 40 kilometres to the north. However, because of the height of Dartmoor (over 300 m), ice would probably have persisted throughout the year during the coldest periods (glacial maxima about 18,000 years ago) as snow or ‘nival’ patches on predominantly northerly facing slopes. This would have protected the face from solifluction and degradation leaving it steep and forming typical blind or blunt-ended valleys. In other areas these patches would have melted and the unfrozen ground (called the active layer) moved downslope through solifluction. These processes moulded the landscape we see today with many blind or steep bowl-shaped slopes, especially in north and west Dartmoor, and long low slopes in other areas. So, although there were probably no glaciers on the moor, the landscape has been shaped by ice and snow.
Tor formation
Tors are an excellent example of how the landscape has been fashioned over thousands and millions of years by many different geological processes. Their ultimate origin is the intrusion of the granite some 280 million years ago and the mix of minerals and sub-terranean dome structure that was the result (see earlier sections). On the cooling of the granite, contraction caused the formation of vertical cracks or joints. Hot water moving through these joints lined or infilled them with quartz, tourmaline and in some areas metals such as tin. These joints were not uniformly distributed across the moor but concentrated in certain areas by pressures in the Earth’s crust and faults, many of which trend northwest-southeast.
The rivers of Dartmoor
The arctic climate also affected the rivers. In fact, the river pattern of Dartmoor is world famous, appearing in numerous textbooks as the classic example of a radial drainage pattern (radiating outwards). Dartmoor rivers are unusual for north west Europe as they have their headwaters in sand, gravel and granite, then go through steep bedrock gorges and finally meander across the lowlands to the sea. Although the approximate location of the rivers and classic radial drainage pattern was established after the original unroofing of the granite, what we see today is the result of their activity over the last few hundred thousand years - during the later Pleistocene. During the cold periods (Ice Ages or Glacials) snow melted and floods, earth flows, rock flows and ice jams eroded the valley floor. During the warm periods (Interglacials), a relatively short period of time but warmer than today, finer sand, growan and gravel were deposited, sometimes even burying trees. These formed the river terraces and are important in the history of Dartmoor as the terraces are one of the principal sources of tin ore (cassiterite) which underpinned the Dartmoor tin industry. Virtually all the humps and bumps so characteristic of the Dartmoor rivers above the gorges are the result of the hydraulic mining or streaming of this material over the last thousand years. The classic gorges, cataracts and waterfalls which occur at the edges of the moor are due to the increase in slope of the valleys at the edge of the granite. Large fans of gravel and boulders known as alluvial fans, accumulated downstream of these gorges, the best example of which underlies the town of Ivybridge. Away from Dartmoor the rivers meandered across the lowlands. The result of this geological history is that Dartmoor rivers are wonderfully diverse, varying over short distances from small gravelbed streams, to cascades, cataracts and waterfalls and more typical meandering rivers.
Peat and humans
Using pollen analysis, the peat on Dartmoor allows us to recreate the vegetation and climate history of the moor. Some of the pollen produced by plants is transported by wind and insects and then lands on wet boggy areas of the moor. Over time it is entombed in the growing peats and preserved for thousands of years. In the laboratory the pollen can be extracted and used to plot the changes in vegetation that have occurred since the last Ice Age (see diagram below). About 11,500 years ago, at the end of the last Ice Age, the climate suddenly warmed and the Dartmoor landscape began to change into the landscape we see today. Climatic change was rapid but the vegetation took longer to change as plants had to migrate from refuges to the south of Britain where they had survived through the severe glacial episodes. This migration was helped for almost 5,000 years by the fact that southern England was still joined to the continent until it was eventually cut off by rising sea levels. Dartmoor was therefore a scene of constant change with at first birch and hazel, pine and eventually oak woodland spreading over the developing soils. It seems likely that for some time all of Dartmoor was covered by trees, with perhaps only the highest and most blocky tor summits retaining open heath vegetation. This woodland cover was first affected over 7,500 years ago by communities, reliant on hunting and gathering, using fire, probably to hunt or encourage game. By opening up the highest woodland to the high rainfall then characteristic of Dartmoor, peat began to form and over time spread from the highest surfaces preventing woodland recovery. This process gradually accelerated and woodland regeneration was further prevented by the introduction of domestic grazing animals and the establishment of permanent pasture around 4,000 years ago. Grazing by animals and the widespread expansion of peat has probably meant that the moorland landscape has looked very much like that which we see today for at least 2-3,000 years, with tree cover largely confined to steep valley sides. Isolated pockets of early woodland, such as Wistman's Wood, can still be seen today. They are remnants of the woodland that must have covered Dartmoor before the impact of human communities produced the landscape we see today.
The Uses of Dartmoor Rock
Dartmoor’s rocks and people Buildings and stuctures
Some 10,000 years ago groups of hunters and gatherers would have roamed through Dartmoor’s largely wooded landscape. Stone tools used by these (and later Prehistoric) hunters were made of flint and chert imported from areas to the east and south of the moor. Gradually, people cleared the moor of its tree cover and, by the Bronze Age, were living in stone built houses and pasturing their animals in stonewalled fields. Dartmoor granite has travelled throughout Britain and beyond, to build houses, museums, prisons, bridges, dams, dockyards, seadefences, lighthouses, forts, and many other structures. Their permanence and grandeur is a testament to its ancient origins, forged deep in the Earth’s crust.
Dartmoor’s rocks and people Tools and technology
As an abundant and free material, Dartmoor granite has inevitably been fashioned into a range of practical forms for people living and working on the moor. The earliest uses, for instance as benches in Bronze Age houses, would have been based simply on the selection of suitably shaped pieces of natural stone. However, it was not until iron tools were available, that the hard quartz-rich granite rock could be carved more easily and shaped into every day tools and utensils. With the establishment of medieval settlements around the moor, and the rise of tin mining, the use of granite by farmers and miners increased dramatically. The use of granite in manufacture and in farming continued until at least the mid 20th century - next time you pass through a Dartmoor village, try and spot examples still in use today! Throughout its history, mining on Dartmoor used granite, most conspicuously for buildings but also as part of various processes. ‘Stamping mills’, in which the ore was first crushed using water power, were closely associated with ‘blowing houses’, where the ground rock was smelted and the tin extracted. This type of processing is typical of late medieval mining activity, when the ore was mainly obtained from surface stream and river deposits and subsequently by the excavation of substantial gullies. Later mining, especially in the 19th century, pursued the ore deposits underground and Dartmoor granite was mainly used for buildings and foundations for machinery, rather than as part of the process itself. Many of the simpler artefacts were made on the farm itself. The techniques for splitting and shaping large stones, using wedges and chisels, continue to be used today although on a much smaller scale.
Dartmoor’s rocks and people Mining of minerals on Dartmoor
When valuable minerals were found on the moor they were worked and mined in many places. Now nature has reclaimed most of the mined areas. The mine buildings, water wheels, steam engines and ore crushing stamps have gone and the refuse dumps lie beneath the heather. Nevertheless, mining remains an important part of the heritage of Dartmoor. The oldest mineral working on the moor was for tin ore weathered from solid rock and concentrated in the gravel beds of the streams and rivers. The earliest documentary reference to tin working comes from the 12th century. The methods used were simple, essentially using the abundant supplies of water to wash away the lighter, sandy parts of the deposits from the heavy grains of tin ore. There are hardly any stream or river valleys on the moor which have not been worked over. Deepened stream gullies and valleys lined with hummocks or banks of waste material are common and bear witness to the extent of this activity. Working of minerals in the solid rock started later. Fifteenth century to 17th century miners created openwork's, but from the 18th century tunnels and shafts followed the mineral deposits underground. Some of the larger and more successful ventures continued into the early part of the 20th century. Great Rock Mine, the last of the mines in the Dartmoor National Park area, worked iron ore as micaceous hematite until 1969.
Dartmoor’s rocks and people Cultural and inspirational
The earliest granite structures surviving on Dartmoor are not the remains of houses, but monuments associated with ritual and ceremony. These included stone rows, stone circles and burials. Neolithic inhabitants first erected standing stones and rows and constructed elaborate chambered tombs and stone-capped burials. There is a very long tradition of burial monuments fashioned from Dartmoor granite, with a history of over 4,000 years. Changes in style over time reflect different religious beliefs and cultural fashions. Of all monuments, however, the simple monolithic column is a style which links ancient and modern cultures from Neolithic standing stones to 19th and 20th century memorials for fallen heroes. The inspirational potential of Dartmoor’s rocks draws countless thousands of visitors each year to marvel at the bizarre forms of its tors, with their boulder strewn slopes flanking stony streams and babbling rivers. But how many people really realise how the landscape has evolved over hundreds of thousands of years, carved by water and moulded by ice from the tough 300 million year old granite bed-rock? The story of Dartmoor’s evolution as a landscape, an interaction between natural and human processes, is as much an inspiration as the multitude of monuments and art forms fashioned from the granite itself.
