Dartmoor Granite, Tor Formation and the Haytor Granite Tramway 12th Sept, 2010

Key Features. Haytor (or Heytor) is a good example of an 'avenue' or 'valley' tor where 2 bosses of granite have been left after weathering and erosion of the core. The tor is composed of megacrystic granite, arguably with evidence of flow alignment, and xenoliths of country rock are incorporated in places. The 'tor' granite is underlain by a fine grain sheet intrusion and the junction can be clearly seen in the north-west face of the western boss of the tor, where the effects of periglacial weathering are also in evidence. Further weathering effects on both vertical and horizontal joints can be seen along with rock basins and other periglacial features such as clitters. The wider area is also underlain by poorly megacrystic 'blue' or 'quarry' granite, which has been extensively quarried. In this is exposed a third set of joints with evidence of late stage mineralisation, and aplite intrusions, evidence of late stage magma evolution. Stone from the quarries was transported to the lowlands and ultimately on to Teignmouth Port on the unique Granite Tramway.

Emplacement of the Dartmoor Granite. The History of the Dartmoor granite began in the Devonian, 400Ma, when the area was a vast flood plain. In time as the coastline moved northward the area was inundated by a shallow tropical sea whilst the area to the north remained dry and arid. The ocean contained coral reefs around which mud sediments and volcanic deposits accumulated. Shallow sea conditions persisted well into the Carboniferous when the swampy coastline moved steadily northward leaving coal deposits; only poor quality in North Devon, and interbeded with mudstones. Subsequently as offshore conditions became prevalent the sediments became increasingly sandy. Towards the end of the Carboniferous these sediments were driven up into a mountain chain by continental collision and thrusting from the south and magma was intruded into the basement of the mountain chain approximately 290 – 285Ma. The manner of the actual intrusion of the magma is still debated. It rose through the Devonian rocks but was forced to spread out when it encountered the more competent Carboniferous sediments. There is some suggestion that the magma migrated northwards along the thrust weaknesses and it was long maintained that this occurred as a series of sheets, based largely on the evidence of feldspar phenocryst size layering and apparent alignment parallel to the direction of supposed flow. Both these features can be seen in the granite of the cliffs surrounding Land's End and were supposed to correspond to features in the Dartmoor granite. Seismic studies have indicated that the Dartmoor pluton is deep rooted and steep sided with the upper surface tilted, rising slightly towards the north. As the magma rose it incorporated a large amount of the surrounding rocks but on the northern margin it

OUGS 12 Sept, 2010 The Granite and Quarries at Haytor Page 1 appears that the granite is only 6 miles or so thick. Some maintain that this is evidence of a sheet intrusion but another string of evidence points to a massive shelf of country rock that broke from the chamber roof and sank into the solidifying magma at a late stage. A further line of evidence points to the tilting of the granite as a consequence of the much more recent alpine orogeny at the same time as the sticklepath fault was active. Variation of texture and composition within the granite. Figure 1 shows a view of the western face of Haytor Rocks.

OUGS 12 Sept, 2010 The Granite and Quarries at Haytor Page 2 There are 2 textural varieties of granite commonly found on Dartmoor. 'Tor Granite' as the name suggests occurs principally around the high points of the moor and is regarded as remnants of the batholith roof. The ground mass typically has grains of quartz and feldspar in excess of 3mm with phenocrysts principally of orthoclase often exhibiting carlsbad twinning and typically 5-7cm in length. It is consequently classed as coarse megacrystic. The tor granite incorporated a high proportion of country rock as it was intruded but though the present day upper surface of the granite may represent a surface that has been eroded with the loss of as much as 50m of granite, xenoliths occur frequently at Haytor. They are usually highly altered having indistinct contact surfaces with the granite. The assimilation of the xenoliths altered the magma composition from sodium rich increasing the potassium content so that it is sometimes viewed as a sub-type and called contact granite. [Locality1 The flat granite surface leading up from the avenue between the 2 bosses of Haytor Rocks to just below the steps cut into the sharp rise of the western boss. This is a clean exposure of tor granite and shows clear megacryst alignment. A large lichen coated xenolith can be seen almost at the foot of the steps another about fist sized is at the top of the steps where it has been polished by passing feet.]

[Locality1a On top the eastern boss are the best rock basins and a view to the ball clay pits at Teigngrace and on to Teignmouth. There is also plenty of growan in the joints. Optional depending on weather and ability as it is a bit of a scramble.] Beneath the tor granite lies the 'blue' or 'quarry' granite. This has a moderately coarse ground mass with grains in the 2-3mm size range with feldspar phenocrysts typically 2- 3cm in length but they are very few and is consequently termed coarse, poorly megacrystic. There is a further minor compositional variation between these 2 types as the mica component of tor granite is almost exclusively biotite whereas the blue granite has a higher muscovite content. A third type of granite; fine, poorly megacrystic is seen at Haytor. This appears as a strikingly sharp boundary at the base of the north-western face of the tor, which has been seen as evidence supporting the theory for granite intrusion as several distinct sheets. [Locality2 The grassy area below the north-west face of the western boss. This shows clear vertical and horizontal joints and the very sharp boundary between the tor and the fine, poorly megacrystic granites. The fine granite has proved more susceptible to weathering and in places erosion has formed a substantial overhang or 'rock shelter'. There is extensive clitter of both tor and fine granite and some blocks show feather and tare grooves and there is a shallow quarry.] Finally in places there are intrusions of aplite with an enriched quartz content and sugary

OUGS 12 Sept, 2010 The Granite and Quarries at Haytor Page 3 fine grain texture. Aplite intrusions are common throughout Dartmoor usually appearing as narrow, high angle veins or dykes, though at Meldon the thickness exceeds 20m. The aplite veins often show chilled margins, evidence that they were intruded at a late stage after granite mass had already solidified.

Exhumation. The Dartmoor granite was intruded at a relatively shallow depth beneath as little as 8 – 10km of crust when the area was south of the equator. During the Permian and Triassic, as the land mass moved northward, there was rapid exhumation from beneath the semi-arid mountain range which was periodically eroded by flash floods. This led to extensive deposits of red sandstones and breccia on the lower ground. By Jurassic times the roof granite was exposed at surface in places though this was a short-lived situation. Rising sea level and crustal subsidence saw the area gradually inundated so that by the late Jurassic the high points of Dartmoor were a series of islands. Crustal subsidence continued during the Cretaceous until Dartmoor was completely submerged and became covered in an extensive thickness of chalk. During the past 50 million years the ocean retreated, warm sub-tropical conditions prevailed and Dartmoor was rapidly exhumed from beneath the chalk. However, 2 million years ago the ocean briefly returned leaving a shore line which is still visible around Dartmoor at about 200m above present O.D.

Weathering and Tor Formation. Weathering of the Dartmoor granite began almost as soon as it had solidified. Greissen formation occurred in places such as at Hemerdon where hydrothermal fluids decomposed the feldspars replacing them with muscovite whilst also depositing metalliferous mineralised veins and stockworks. Mineralised or not these hydrothermal veins can be readily identified on Dartmoor and across the south-west granite as a whole as a result of an unusually high boron content, which manifests as black deposits of tourmaline. The high boron content, relative to granites globally, indicates a mantle component in the magma. [Locality3 Middle Quarry to the north of Haytor Rocks. The main quarry face shows good clean vertical and horizontal joints in an aplite vein. Pieces of aplite are lose on the quarry floor as are clean pieces of blue granite. In the western wall of the quarry is a joint at an inclination of about 35degrees with a black tourmalinised centre and alteration of the granite for up to 3cm on each side.] Vein mineralisation is associated with tin, tungsten, lead, copper and zinc ores at various locations. Later stages of hydrothermal activity were responsible for the kaolinisation of large volumes of feldspar leaving workable china clay deposits in some areas.

OUGS 12 Sept, 2010 The Granite and Quarries at Haytor Page 4 Some geologists argue that the intruded magma was at a low temperature whereby it was already highly viscous. This coupled with intrusion at relatively shallow depth and relatively rapid removal of the over-lying crust lead to the formation of 2 joint sets through rapid cooling and decompression. Cooling and contraction resulted in vertical joints which in places propagate for long distances and to considerable depth in north-south and east-west directions. Locally joints were more plentiful in the centres of the tor masses as the extremities contracted away from the core. [Locality1 gives a good view of the vertical joints in the eastern boss of Haytor Rocks.] Exhumation lead to the second sub-horizontal decompression joint set. These 'floor' joints in places exhibit a domed profile in the tors and follow valley topography which has been interpreted by some as evidence that the joints mirror the topography of the original batholith roof. The floor joints produce such a marked layering that the resulting appearance is occasionally referred to as pseudo-bedding. [Locality2] The third set of joints are sub-vertical and typically inclined at angles of between 20 and 80 degrees and are developed only locally. These may have been the result of isostatic warping as the less dense granite achieved equilibrium with the denser surrounding crustal rocks. These joints are those that are associated with tourmaline-metalliferous mineralisation and the early chemical weathering. [Locality3] Further chemical weathering occurred during the Jurassic period when the upper surface of the granite was at or close to the surface. Environmental conditions had become milder and wetter than in the Triassic and the land was covered with sub-tropical vegetation. These conditions provided a plentiful supply of acidified circulating water causing chemical weathering particularly in the centres of the tor masses where the joints were more plentiful leaving 'softened' cores. During the Eocene the environment was particularly warm and wet with the granite mostly exposed at the surface. The biotite and feldspar minerals were particularly susceptible to the effects of chemical weathering and decomposed to leave a gravel (referred to locally as growan) of quartz, largely unaltered muscovite and part weathered feldspar. This is widely seen either loose on the surface [Locality1a] or still in situ where it is now sufficiently soft to be broken apart by hands alone. However, this sedentary growan does not show significant element leeching and much of the feldspar is unweathered, strongly supporting the notion of steady chemical weathering in sub-tropical conditions with only minor water circulation. In contrast, weathering in humid tropical conditions typically produces a sandy growan with a low clay content where virtually all the feldspar has been removed and the residue is significantly enriched in quartz sand. Where the Dartmoor growan was exposed it was eroded and deposited as ball clay in the lowland basins, which were forming due to subsidence along the active sticklepath fault (activated by the opening of the Atlantic Basin). Estimates of the ball clay deposit thickness varies from 400m to 2000m. [The view from Locality1a] Significant periglacial conditions have come and gone on at least 4 occasions over the past 2 million years and the associated physical weathering has heaved aside core stones and frost shattered blocks and tor surfaces. One clear consequence of frost action on the

OUGS 12 Sept, 2010 The Granite and Quarries at Haytor Page 5 granite has been the formation of rock basins [Locality1a] not only on the upper surfaces of the tor core stones but also on occasions on larger clitter blocks clearly indicating that basin formation must have occurred recently. Overall a combination of gentle chemical weathering and periglacial physical weathering followed by erosion has produced the distinctive valley tors exemplified by Haytor. Chemical weathering along the joints softening the tor centre forming growan and rounding the edges and corners of the core stones. Physical weathering has shattered the boulders on the flanks of the tors leaving clitter deposits, whilst solifluction during interglacial periods has rapidly eroded the growan to form extensive head deposits in natural hollows.

The Granite Tramway. Figure 2 shows a simple plan of the quarries surrounding Haytor Rocks.

[Locality4 Haytor Quarry. Note the jointing in the blue granite, evidence of working with feather and tare in the rock face, the winching mechanism and the ring bolts that anchored the guy ropes for the winch.] The Dartmoor Blue Granite has long been a valued building and ornamental stone as it could be worked with a low risk of fracturing but also had a sufficiently coarse crystalline

OUGS 12 Sept, 2010 The Granite and Quarries at Haytor Page 6 texture to be decorative. It was quarried at many locations beside Haytor: Merrivale and Foggingtor being some of the larger and longer lived operations and unfortunately in some places the landscape was massively altered. Luckily at Haytor the blue granite is separated from the tor granite by an intrusion of granite with a soft, fine crystalline texture and 'dirty' pink-yellow colour, which is unappealing as a building stone and both that and the underlying blue granite escaped quarrying directly from the tor. Quarrying in the sense of gathering fallen stone from the clitters and moorland surface has been going on since prehistoric times and must have dramatically altered the look of many of the tors. At Haytor whilst there is still a good deal of clitter on the northern side the southern flank and the avenue between the bosses has been stripped of almost all loose surface stone. However, even on the northern side many of the blocks show clear evidence of quarrymen's tools. [Locality2. There is a narrow fissure that cuts the eastern end of the boss and it is easy to scramble down this to the grass below. Walk straight out onto the moor about 15m and there is ½ of one large worked granite block.] The most distinctive marks are cylindrical grooves along the edge of a split surface. These grooves are typically spaced at about 6 inch intervals, are 4 to 6 inches deep and 1.5 inches in diameter and are the marks left by the use of feathers and tares. The holes were 'drilled' into the rock using a jumper bar. These bars were 5 to 6 feet long and the diameter of the hole. The working end was flattened to a chisel point and had a fatter collar along part of the length to give added weight. The bar was simply lifted and dropped to gradually drill the hole. The real skill came from turning the bar a few degrees every drop to get a round hole. This worked well in the south-west granite but in the Welsh slate mines the bar was turned further at each drop giving diagnostic 'triangular' holes. Once the line of holes had been drilled a cupped pair of copper or iron feathers were slid into the hole and an iron tare (chisel) was slipped down the hole between each pair. The quarryman then worked along the row of tares driving them home with a hammer until the rock parted. Copper feathers were preferred by some for though they were more expensive they were softer and so held in the rock better and held the tare, preventing it from bouncing loose, thus keeping up pressure on the rock. Feather and tare replaced the use of wooden wedges only during the late 1700s, so these marks give an idea when stone at a site was being worked. At Haytor there are a number of quarries of various sizes in an arc around the northern side and all were connected by a complex tram line network with rails and points uniquely cut from granite blocks. The most westerly: Ermsworthy and Horraburrow, were probably the first started, possibly in the late 1700s. However, much of the stone was the poorer fine granite and these quarries had been abandoned by 1840. Stone from these had been used to build Stover House, Teigngrace Church and much of Haytor Village. In the meantime Middle Quarry just north of the tor, Holwell Tor Quarries towards Hound Tor and the Haytor Quarries to the east had all started work in the more desirable blue granite.

OUGS 12 Sept, 2010 The Granite and Quarries at Haytor Page 7 Some researchers have suggested that the tramways were initially only in use to connect the quarries and provide a means to move rubble to the tips and blocks as far as the road for subsequent transport by horse and cart. Once the new quarries were in full production around 1820 it seems that the granite setts making up the rails of the tramway to the earlier quarries were taken up and reused on the extension to Ventiford. The quarries were in the ownership of the Templer family but it was the award of the contract to George Templer for the supply of stone for London Bridge (the one that is now at Lake Havasu in Arizona, USA) in 1819 that really got the business under way. Templer realised that he needed a more efficient way to transport the stone and this was after all the dawn of the railway age. A steam railway was out of the question, Haytor is 1300 feet above sea level and the gradient too steep but this was in itself the making of the granite tram way; gravity would do the work. The tramway runs 7 miles from Haytor maintaining a steady gradient looping around the contours of the hills until it reaches the level ground at Chapple Lane on the edge of Bovey Tracey. Along the whole length of the tram line there is only one cutting no more than 3 feet deep and 50 feet long through a slight rise on the open moorland between the Middle Quarry and Haytor Quarries. It then runs 3 miles further to the canal wharf at Ventiford where stone was offloaded to barges and carried 4 miles down the canal, which had been built by the Templer family in the 1790s to transport ball clay, and finally 4 miles on the tidal reach of the River Teign from Jetty Marsh at Newton Abbot to the Port of Teignmouth. [Locality5 The embankment close to Haytor Quarry. This is an opportunity to view the tramway and the tin streaming works.] The tramway is made of shaped granite setts about a foot square in cross-section and from 3 to 7 feet long. The raw material was obviously cheap, readily available and could simply be brought down from the quarries on the rails that were already in place and added to the end of the line. Some have assumed that the stone came from the actual quarries but inspection will quickly reveal that the vast majority is coarse megacrystic tor granite. It stands to reason, why use blue granite which had a resale value when the moor was littered with free stones of no intrinsic value. And fortunately unlike a wood and iron railway, when the line was abandoned there was so little value in the materials they were not robbed out. The tramway has a gauge of 4 feet 3 inches and the wheels of the wagons simply ran along flanges cut into the outside of the blocks. The points were also cut from stone blocks and were possibly fitted with a wooden or metal flange that could pivot on a pin slotted into a hole in the block, or possibly a wooden board with pins at either end could be fitted to guide the wheels. At the time the quarries were closing the wagons were described by a writer as being about 13 feet in length, flat topped on a 10 foot wheelbase. The trains were made up of as many as 12 wagons descending under gravity the speed controlled by wooden poles levered against the wheel rims as crude brakes. A team of 18 horses hauled the trains across the level ground to the canal basin and the empties back up to the quarries. As there are no passing places on the tram line some of branches on the moor were

OUGS 12 Sept, 2010 The Granite and Quarries at Haytor Page 8 probably for marshalling the wagons and some wagons were clearly used for ferrying waste to tips as the tramways extend out onto the tips at Holwell Quarry. In most cases the lines extended up to the quarry faces and it appears that blocks were loaded and drawn away to be rough dressed on the wagons. The life of the quarries was quite short and by 1860 they were closed as better quality stone could be obtained more readily and cheaply from the likes of Foggintor Quarry near Princetown or the Cornish Quarries. Thereafter large stones were occasionally extracted for monuments, the last reopening in 1919 when stone was taken from Haytor Quarry for the war memorial in Rougemont Gardens in Exeter. This was probably when the lifting gear was installed, the mechanism for which is still in place, and a photograph taken around 1946 shows the gear still upright.

Metalliferous Mineralisation. There were a number of mines in the Haytor area. Close to the Haytor Quarries the tramline crosses a gulley on a low bank, this is not a natural ground feature but is a tin streaming works from which alluvial cassiterite was collected. On the edge of the moorland below the Haytor middle car park was Bagtor Tin Mine where lode cassiterite was worked in the granite. A little further down the hill were the Atlas and Albion Tin Mines and the Smallacombe cutting, all working lode tin in the Carboniferous sedimentary beds. The aureole around the moor was the source of a wide variety of commercial ores besides tin. In Haytor Vale a mine worked 3 lodes of iron ore containing significant amounts of magnetite from at least as early as 1600. In Lenda Woods below Ilsington Silverbrook Mine worked 2 lodes of galena with sphalerite. The majority of the ore bodies are indicated to be of hydrothermal origin though not the Haytor magnetite the origin of which is still hotly debated. As already discussed the majority of mineralised lodes are high angle sub-vertical but the 3 lodes at Haytor are inclined at roughly 30 degrees to the horizontal and the middle lode of the 3 is up to 13 feet in thickness. [Locality6 The Yarner Mine area. This has the remains of an engine house beside a shaft and extensive spoil heaps.] In Yarner Wood a copper mine worked through the middle part of the 1800s on 2 parallel lodes to a depth of 50 fathoms and produced around 2300 tons of copper ore. The shaft and ruin of the engine house is on the upslope side of the valley beside the forestry track. On the downslope side are a number of overgrown spoil heaps at the mouths of minor drainage adits. The main spoil heap is where a new turning space and workshop have been built and this has turned over a good deal of mine spoil. The more rust covered pieces of spoil often feel uncommonly heavy and breaking these open may reveal pyrite (iron sulphide, a bright brassy yellow colour), chalcopyrite (copper, iron sulphide, a richer

OUGS 12 Sept, 2010 The Granite and Quarries at Haytor Page 9 yellow colour than pyrite) and arsenopyrite (arsenic sulphide, silvery grey in colour).

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