Walking on Dartmoor. Earle John
or Plymouth was a once-a-year outing, who thought nothing of travelling to market in gig or cart, taking two or three hours there, and back again, whose slow, hard life revolved around the seasons and the harsh taskmaster Dartmoor.
Life and the old ways have changed from the days when every small village had its bakery and blacksmith, when the grocer, the butcher, even the fishmonger from Brixham, the haberdasher and tailor delivered to the door of remote farms by pony trap; when harvest suppers and whortleberry gathering parties were part of the year's major social events. Modern farmers on Dartmoor are a different breed, but just here and there are a few folk whose memories reach back into the old days and the ways of their fathers and grandfathers before them.
The wildlife on Dartmoor is not outstanding but when you do come in contact with the secret inhabitants, it is the more exciting. The buzzard, I suppose, with its moth-like wings and the mewing call, like a kitten, is the most common big bird on the margins of the moor and in many way epitomises Dartmoor with its soaring, wheeling freedom or sitting like a sentinel on a pole or bare tree surveying the world. Then there is the thrill as a couple of red grouse get up with a clatter and their loud ‘go back’ call, or the excitement or the brief glimpse of a reddy-black, arrogant hill fox loping off in no hurry. But it is the skylark, that minute speck in the blue summer sky, with its bubbling song, that brings back a surge of happy, childhood memories of walking or riding on hot, breathless days into the heart of the moor and I still scan the skies trying to find the little, soaring creature pouring out its ecstasy.
The Nine Stones lookng back to Cullever Steps with High Willhays beyond, Walk 34
I hope therefore to share with you, through this Guide, some of the magic and mystery of Dartmoor. I should like to show you places to visit that I think will interest and fascinate you, so that like me, you will become a person who loves and appreciates this lonely wilderness and will return to it again and again, for it has a haunting, almost hypnotic influence on those who walk there.
Geology and formation of Dartmoor
This Guide is no place to give a full and detailed description of the geology of Dartmoor. For those who would like more information, it can be found in some of the excellent books listed in the bibliography. However to appreciate and understand the moor and its landscape, it is interesting at least to know how Dartmoor was formed and something of what happened over the millennia since its creation.
At a time when the Earth began to look green as plants and even small trees evolved and the seas were full of vertebrate animals such as primitive sharks that eventually led to the existence of the first amphibians, where Devon and Cornwall are now was part of a huge flooded plain. The sediments of certain areas of this plain became the early rocks seen now, such as the Dartmouth slates and Devonian limestone. This time in the development of the Earth was known as the Devonian period and occurred 400 million years ago and lasted some 50 million years.
As the warm seas encroached the southern areas of what was to become the British Isles changes took place. Coral reefs grew in the seas and volcanoes erupted here and there. Sediments, mud and sand, accumulated round the coral reefs and volcanic debris and we have the beginning of the Carboniferous period, 345 million years ago, which overlapped with the previous Devonian period. It was, of course, the chief coal forming age associated with the coal seams and carboniferous limestone.
Then as this period came to an end, about 290 million years ago, both the Carboniferous and Devonian deposits were subjected to mountain building pressures and foldings known as the Armorican movements. The rocks and soils of Devon and Cornwall as we know them now were formed at this time. The limestones and sandstones appeared, caused by the great folds and upthrusts.
It was about this time too, 290 million years ago, that the granite of Dartmoor probably arrived from below the earth's crust, though dating such events is fairly difficult. Granite is an igneous (Latin: fiery) rock and is formed under conditions of intense heat. Dartmoor granite arrived as an igneous intrusion into those overlying sedimentary rocks which, because of the violent folding they had been subjected to, had many faults and cracks. Some of the granite was able to follow these weaknesses to the surface while in other areas the granite welled up under the Devonian and Carboniferous rocks like water in a blister. Some of the sedimentary rock even became absorbed into the granite itself because of the ferocious heat. So we have the characteristic dome-shaped mass of rock, in an area some 365 square miles, in the centre of Devon.
Because the granite was protected by the layers of rock above from the cold air, the molten rock cooled very slowly resulting in large visible crystals; the slower the cooling process, the larger the crystals. Gradually the protective layer was eroded and destroyed by the weather during the Permian period and in time the granite boss was exposed.
Scorhill Circle, Walk 33
Later during the millions of years that followed a layer of granite itself between 50m and 200m (150–650ft) thick was also eroded, until we have Dartmoor as we know it today.
Dartmoor granite is composed mainly of three types of crystal. First quartz, which is the glassy grey substance that in its pure form produces the distinctive six-sided crystals with pyramidical points and striations on the sides. Next the small dark, glistening specks of black mica, a crystal that occurs in many of the massive rocks, and lastly felspar, which gives granite its colour: red, white or grey. These are the larger crystals seen in granite. It is felspar, when it has decayed or been decomposing by weathering, that becomes kaolin or china clay.
For several different reasons there are many varieties of granite to be found on Dartmoor.
Surrounding the moor like a ring is an area called the metamorphic aureole where the encircling rocks have been changed in composition by the intense heat of the igneous intrusion. (I shall be writing briefly below about the ancient tinners of Dartmoor and other mining activities. Suffice to say, at this stage, that most metallic ores are associated with plutonic rocks or areas of metamorphosis.)
On the margin of such granite masses as Dartmoor, superheated water or gases forced their way into the cracks and deposited layers of crystalline minerals including the ores of metallic minerals. The way this process led to the present position of the various areas of minerals is very complicated. Put in simple terms, the mineral-bearing fluids and gases started deep down in the granite mass and as they were forced up to the surface the minerals crystallised in the order of their crystallisation temperatures. So obviously minerals with the high crystallisation temperature became solid near the hot granite and the others followed in order as the temperature decreased towards the surface. However, the erosion mentioned earlier has resulted in the mineral deposits on and around Dartmoor, shown in the mineral digram on the previous page.
Simplified mineral diagram
This period of mineral deposition probably took place 190 million years ago but the process might well have been spread over as long a period as 115 million years.
A final word must be written about the tors (Celtic twr, a tower) of Dartmoor. They are after all its most distinctive feature, sometimes described as ‘cyclopean masonry’. It has even been suggested that they were put up by the Druids! They are, of course, residual features left after both chemical and mechanical weathering has taken place. Controversy surrounds their origins and which of the two methods of weathering is the most important.
(i) Chemical weathering is the actual rotting of the rock itself and depends on the composition of the crystals in the rock and how they react within themselves.
(ii) Mechanical weathering is straightforward erosion by water, frost, freezing and heat.
But however they reached their present state, they are fascinating features that are fun to explore and scramble on, each one being different from the next.
Surrounding many of the tors are large areas of