Widecombe History Group Talk on Mercer's Dartmoor

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Mercer’s Dartmoor by Prof. Ian Mercer.

The meeting then enjoyed a interesting illustrated talk given by Prof. Ian Mercer entitled "Mercer’s Dartmoor" which explained so much about the geology of Dartmoor and its present management.

Prof. Ian Mercer started work in Devon as a Geologist based at Slapton Ley in the South Hams, he then became County Conservation Officer based at Exeter and ultimately became Dartmoor National Park Officer.

As a geologist his interests relate to the formation of Dartmoor and its development over what must be 10 billion years. This is a length of time that is virtually impossible to grasp, but this is the time scale that geologists have to work with. After considering this ‘before history period’ as he explained it, we will look at the last few decades as a glimpse of Dartmoor today.

The Galaxy as we know it today has been calculated as 10 billion years old, the Sun and the Planets are 4.5 Billion years old, the oldest rocks in the U. K. are 3.3 Billion Years old and Dartmoor Granite is a mere 280 Million years old. Looking at fossils the earliest are fishes 300 million years ago and man is but a recent arrival on the scene by comparison.

Those of us that are fortunate enough to live on or near Dartmoor can appreciate the granite formations, and by looking at it realise how it was formed and weathered to its present state. The joints in the tors visible in many of the photographic images shown are a result of the forces both exterior and interior that acted on the stone as it was forming and cooling, and the effects of the weathering since. How it varies, due partly to the coarseness or fineness of the crystals within the rock. The rock was subject to many forces as it cooled and the vertical cracks or joints seen on the tors today resulted from that activity. The cracks have become more pronounced due to the action of acid rainwater, frosts and chemical reactions. The large joints of the most coarse granite made it comparatively easy for water to enter and when the water froze and expanded the exposed rock was split off creating the clitters.

Dartmoor, and westwards to the Scillies through Bodmin, there are six outcrops of granite masses. There is no granite east of Dartmoor. The south west peninsular has ‘tilted’ away from the rest of Britain. Granite has those big white ‘matchbox’ shaped crystals of felspar, the glassy crystals of quartz, known by geologists as ‘giant granite’. It is a well known fact that the slower a material cools the bigger the crystals grow. It is those white felspar crystals that have ‘rotted’ to form china clay. There are about 400 variations in the kinds of granite to be found. The finer granite known as elvan contain smaller crystals, and where the first coarser granite weathers easily the finer stone tends to shatter as pressures are applied either internally as it cools or externally by man’s activity. Near Haytor or is it Heytor or perhaps High Tor some of these variations of form can be found. Continual heating and cooling all those years ago effected the ‘natural stone’ and chemical activities altered the base rock into different types of stone and this can be seen, for example, at Meldon, where British Rail originally Great Western Railway produced their ballast for under the rail tracks, this is very hard and ideal for the job.

Reference was made to the Sticklepath Fault, this line, marked on some maps, runs from Torbay, through Bovey Tracey, Moretonhampstead, Meeth, Sticklepath and Bideford is a geological fault which continues across the Bristol Channel to Pembrokeshire. Created probably at the same time as when the Alps etc were formed (the Tertairy era). It can be seen as a wide trench, (a rift valley in the old school textbook sense), caused by cracks/faults in the underlying bedrock some of these cracks run parallel some diagonally, and it is interesting that at the southern side Bovey Basin and the northern side Meeth deposits of the coarser ‘Ball Clay’ are found. In the Bovey Basin Lignite, a crude coal-like fuel can also be found, this can be burnt but it was also used to produce things like glue. It is due to the warm rain water seeping into the cracks of this fault that has led to the deterioration of the bedrock to form the clay. At Lee Moor the better quality China Clay is found, this is used in many manufacturing processes like paper, toothpaste, cosmetics, even spark plugs for vehicles and of course china commodities.

Dartmoor was once an island and the water level, reached what is today 700 feet above sea level. These can be noticed by flat areas such as Hanger Down, Roborough Down, Plaster Down and also above Moretonhampstead. Dartmoor has such a selection of Tors and the ‘clitters’ of stones that surround so many are evidence of the action of ice and freezing periods followed by thaws. Examine a gravel pit and the structure of the soil or strata can be appreciated. In the Neolithic ages, there were trees up to 1500-1600 feet above sea level. The dark soil formed as a result of vegetation or leafmould that has rotted down, turned into soil, below this there is the subsoil. Above 1600 feet there is peat, for example at Redlake, 4-5 feet deep, and the stain from that and the effect of iron ore has led to the colouration of the sub-strata.

Returning to the Tors of Dartmoor, the example of Bench (Benjie) Tor spreading about a half a mile with all its associated bits added in, its strata starts horizontally and then bends down into the gorge. As the top granite is removed by weathering etc the internal pressures of the deeper granite ‘sprang in relief’ and so that, and the excavation of the Dart Valley over 1000 of years, has created this situation. This can be explained by the mining of granite in New York State U.S.A. The granite has to be mined from 2000 feet below the surface and if it is raised too quickly the rocks can explode as the pressure is released. This too can be witnessed by examination of thin slices of stone under a microscope when the crystals can be seen to alter and fracture.

Dartmoor Tors are quite unique so that is an extra reason why so many tourists came to the area to see them, they are uncommon in many areas. Tors come in all sorts and sizes, Great Mis Tor the largest on Dartmoor and its associated clitter which covers a considerable area. Tall Tors like Vixen Tor, Tors that are visible from afar, like Haytor which can be seen from the South Devon coast, east of Moretonhampstead, and even seen from out to sea. A landmark of significance to many cultures. The Tors are what has survived of the original rock after the exterior blocks have been prised off by natural processes. There are larger clitters to the western side of the moor due to the coarseness of the original stone, and it is debatable whether it is also partly due to the Southwest prevalence of the weather, washing the soil away and revealing the mass of stones below.

Taking a look at Holne Moor which over the years has been completely cultivated, the evidence of this can be noticed by the field patterns still there today. An archaeological dig on Holne Moors found old mole runs, no longer used, as there are now no earthworms at that depth but in perhaps 4000 years ago there were, vegetation = earthworms = moles.

A vegetation map of Dartmoor can show many variations. The two plateaux, north & south with their peat, the valley bogs with their characteristics, some very distinct areas of heath, heather & gorse, the modern plantations, grassland and even the bracken, all visible in a 1965 aerial photograph. High Willhayes and Yes Tor on the highest ridge on Dartmoor, but not far away a vast area of plateau without tors can be seen. The southern plateau is similar, smooth with few tors in some areas. Between these two is a central basin of the river Dart, Widecombe is situated on one of the main tributaries of that river. Between the two plateaux is a central gash, something must have created that. Possibly a much larger river which due to the ice age had changed its course. Dartmoor did freeze to perhaps 2000 feet down but it was not glaciated. When about a mere 20,000 years ago the thaw took place, all this frozen water had to come out of the ground and find its way to the sea, that created other valleys. Some of these level areas part way up a valley are the old river beds, each making its own river cliffs and beds, then over the years the rivers of today have cut further into the ground making its own comparatively newer and deeper valleys. Some old valleys now obsolete from the river point of view but making a speculation panorama, were perhaps the old courses of "an early Dart"? This can be supported by the presence of Dartmoor rock pebbles and china clay in the Hampshire Basin. How did that get there? So thousands of years ago ‘did a river start, way out to the west of the present South West, somewhere in the Atlantic of today, cross Dartmoor, through the Princetown central basin and out to the east perhaps through what we now call Teignmouth, a majestic river, the forerunner of todays rivers? ‘Tis possible! The Dart Gorge under Sharp Tor and Bench Tor. It cuts through that hard stone on the edge of Dartmoor created by heat and chemical activity, a fine example of this rock is Leightor a mixture of Granite and Quartz and Tourmaline, known as ‘Schorl’. A reef running from Aish Tor to Ausewell Rocks near Ashburton through Holne Chase. Very hard rock but the Dart cuts its way through that mass, possibly following the course of an old river that originally ran through softer rock above.

The uses of granite was considered by looking at the Dartmoor stone walls, the buildings like churches and old houses, it has been used from the very beginning of man. Some was used to supply, buildings and shelter, some used, like in hedges, clearing the land to make it more easy to cultivate. When there was a specific need, stone was chosen, cut, shaped and brought back to the homesteads to be used, troughs, gateposts, millstones, the list in considerable. Many broke during ‘manufacture’ and their remains are still on the moors to be seen, the evidence of the cutting of the stone by drilling, and the use of ‘feathers & tares’, to cut and shape, the evidence is there to see. Then there were the commercial activities of quarrying, Foggingtor, Merry Vale, Haytor and Blackingstone to mention but a few. This was an industry that lasted just 150 - 200 years and the Merry Vale Quarry was the most commercial as it had the equipment to cut and polish the stone for various uses as well as to supply the rough hewn stone for local use. Mining and quarrying encouraged the need for gunpowder and at Powdermills near Postbridge, gunpowder was produced, but dynamite was invented soon afterwards and so that industry did not last long. All this has resulted in evidence of the industrial use of Dartmoor, which is now being preserved as part of our heritage. Waterwheels to drive the mills, pumps etc for these enterprises, lead to leats and so the history of Dartmoor unfolds.

The white spoil heaps of Dartmoor are in contrast to the black heaps of the ‘Black Country’. Controversy is also part of the history of Dartmoor, the claypits, military activity, mining and quarrying, reservoirs, forestry, but this provides employment, so this too needs careful consideration. Where there is activity of man, this creates a ‘scar’. Should it be infilled? Should it be left for nature to colonise? Should it be left to be studied? Or should it be left as an obstacle for nightime adventurers - a boobytrap!

The intense use of Dartmoor as a place of recreation has increased the pressures on the moors, erosion, ever evident at Haytor, pony trekking, rambling, letter boxing, hang gliding, rock climbing all part of tourism and increasing due to the ease of modern travelling. This leads to litter and at one time litter bins were placed around the moors. This proved to be counterproductive as the bins soon overfilled, and people left their rubbish piled around them, the ponies, crows, foxes etc, saw this as a source of food and pickings, and the litter began to get spread everywhere. The Dartmoor National Park removed the bins and the majority of people took their litter home with them - what a success story that was!

Agricultural activity in recent years, has sometimes exposed previous activity that no-one knew were still there, early field systems and remains. Management agreements have been drawn up in several places, with several farmers and landowners, to preserve archaeological interesting features, wildlife habitats of interest and importance, creating a compromise to avoid damage, conserve, and still help the farmer to maintain a viable business and earn a living. This is a continual learning curve for all concerned, the local people, the visitor, the DNP and others concerned with the welfare of Dartmoor and those that live and have businesses on the moors. Traditional methods of maintenance, burning(swaling), clearing leats, maintaining walls, and trying to reduce the erosion caused by the over use of some areas. The ESA agreements are put in place to help with this conservation, and well tried historical forms of management must not be lost.

Satellite photography permits an overall look at the country these days. From space ‘an eye’ can be kept on activities anywhere. Photographs show the vegetation, the effect that action can and has had on this, and for example light pollution, Dartmoor, Snowdonia and the Peak District, show clearly little light pollution but the rest of Britain certainly suffers from this.

This concluded a very interesting lecture, we hope for more another day, but for now our sincere thanks for an enjoyable and informative evening.

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The information on this page was last modified on March 18 2013 12:55:14.

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