CHAPTER 2
The Physical Anatomy of Dartmoor
A landscape is the function of structure, process and time.
W. M. Davis, 1884
THIS CHAPTER ANALYSES DARTMOOR’S whole terrain – the stage on which all things ecological, cultural and economic have been acted out over the last 10,000 years – according to Davis’s all-time truth. Only it survives of Davis’s great body of theoretical work, which dominated geomorphology for more than 50 years, but that troika – structure, process and time – is as critical to the appreciative understanding of any landscape now as it always was. Rocks, once emplaced on or within the earth’s crust, may be modified by geologic process and temperature change. When eventually exposed to the air they are attacked by all that the weather can throw at them, both directly, and through the water regimes it sustains, at the surface. All of those processes have varied in type and intensity through time. In Dartmoor’s case it has taken a lot of time, so long that rocks intruded into others have been exposed, buried again and re-exhumed. It is then that some evidence of their earlier exposure becomes part of the contemporary scene. As in any landscape, the nearer in time a particular process has been at work the fresher is its visible effect and at the same time the chance that it has removed or masked the effect of earlier processes is higher.
THE SOLID GEOLOGY
The granite
So Bill Dearman could write on the first page of a volume of Dartmoor Essays published in 1964: ‘The granite has itself determined naturally and almost exclusively the character of the [National] Park…even when it is not present at the surface its influence is obvious.’ Essays that follow his in that volume deal in the work of processes culminating (as far as the physical surface is concerned) in those of the Pleistocene ‘Ice Ages’ which produced the dramatic detail that takes the immediate attention of most casual observers of Dartmoor. That visible detail is on all steep slopes and mostly round the edges of Dartmoor’s two plateaux; but the intervening essayists also apply their reasoning to the wide landscape of long, low profiles and hidden hollows of the hinterlands leading to the plateaux’ summits, and the ‘flats’ of the upper Dart basin which lies between them. In both these cases the product of much older processes dominates the surface.
It better be said now that apart from the acts of faith which laymen must inevitably indulge in dealing with geological time, and despite much improved radio-chemical means of measuring the age of rocks and their derivatives, imagination and controversy about the origin of particular surface forms, even by field scientists, increase in proportion to the distance back in time of the events under discussion.
However, even since Dearman wrote, we have established a new and more precise age for the emplacement of the Dartmoor granite. Its first intrusion came to rest, still superheated and plastic, 280 Ma, nearer the surface of the earth’s crust than my school textbook had me believe. It turns out that Dartmoor is the youngest of the six upward intrusions from the single pluton that lies under the southwest peninsula and westward, from Dartmoor to beyond the Scillies.
In the Dartmoor case, three main phases of intrusion of molten magma were originally recognised. The first was thought to have produced the ‘giant’ granite characterised now by scattered large whitish crystals (megacrysts) of orthoclase feldspar up to 170 mm in length (but most of ‘side of matchbox’ size) embedded in the ground-mass of quartz, feldspar and mica (mainly dark biotite) typical of most granites. Though these latter crystals average 2–3 mm in diameter they are still at the coarse end of a granite spectrum. The megacrysts may comprise anything up to 30 per cent of the volume of the giant granite as Figure 26 shows. It is the main granite of most of the tors, though percentages of megacrysts by volume are low in an area of the northern plateau bounded by High Willhays, Cosdon, Cut Hill and Brat Tor, and again around Trowlesworthy, Shaugh and Lee Moor in the south, with odd small patches elsewhere. The whole intrusion should be visualised as a thick domed sheet spreading outwards, but mainly northwards, from a ‘feeder’ column centred under Ryders Hill (the highest point of the southern plateau). It was followed sooner rather than later by a second sheet of what was to become the ‘blue’ granite, of more
FIG 26. A weathered face of giant granite. Felspar phenocrysts, the size of the side of a matchbox, dominate the matrix of quartz, mica and small felspars. A felspathic vein runs diagonally across the face.
evenly distributed crystals of similar size of the three common granite minerals. It is now thought that the giant and blue granites are so closely related that they may be simply phases of the same intrusive event, and together make up 90 per cent of the area of the exposed granite. Both appeared to early fieldworkers to have suffered further intrusion by a finer-grained granite in dyke- and sill-like form (crudely: dykes appear to cut across existing structure, sills to follow it). However the origin of these finer granites has since been described as ‘enigmatic’ and include sheet-like bodies of aplite (rich in tourmaline and lacking biotite). Evidence suggests that they may predate the growth of the feldspar megacrysts, for some straddle the exposed boundary between fine and coarse granites. It is even suggested that they may have originally been sandstones that were caught up and melted in the molten magma as it arrived. Mineral veins are locally clustered throughout the whole pluton especially those bearing cassiterite or tin. For our immediate purposes the most important thing is that the considerable and easily recognisable variations in the whole mass of granite exist at Dartmoor’s surface and can make different contributions to the landscape detail (Fig. 27).
All this emplacement activity may have been topped off by further intrusion into the original roof over the main mass and even of extrusion in volcanic form at the aerial surface soon after that. There are detectable chemical affinities in the family of rocks from one magmatic source wherever they come to rest vertically in the ‘column’ from pluton to volcano. Volcanic fragments and crystal particles in surface deposits in the Permian beds of the Crediton trough just north of Dartmoor closely following the 280 Ma date suggest such a state of affairs, even demonstrating the order of denudation of the volcanic rocks, roof and granite in an inversion of the resulting deposits, i.e. volcanic fragments first,
FIG 27. The west side of Haytor. Rounded, chemically-weathered giant granite overlies the finer ‘blue granite’, or elvan, which – more densely jointed – has retreated faster under mechanical attack.
overlain by roof material and then by granite. So, though the granite was shallow to geologists but deep to you and me, very soon after emplacement (in geological time) it had already been unroofed and landscape-forming processes were at work on it. All this was a very long time ago.
(If you are still having difficulty with geological time, to reduce it to a relative matter compare the 600 million years of fossiliferous earth history (give or take some earlier algae) with a calendar year. Fish appear in early May in the Devonian, big reptiles in September (dinosaurs climax in October), grasses grow first at the end of November, and the main glaciations of the Pleistocene occur at about 1800 on 31 December. The Dartmoor granite was thus emplaced in early July and being eroded in the air before St Swithin’s, or even Bastille, Day.)
The country rock
Igneous rocks must obviously be younger than the rocks into which they have been intruded. The geological sketch map that is Figure 28 shows that our granites are in contact with the Devonian (Middle and Upper) shales, slates and grits and with the shales, slates, cherts and sandstones of the Culm Measures of
FIG 28. Geological sketch map of Dartmoor and its context (after Dineley, 1986).
FIG 29. Bulley Cleave Quarry, Buckfastleigh, in 1964 when the Devonian Limestone was still being worked.
the Carboniferous system. Outside the contact zone itself some of these rocks play a significant if local part in the National Park landscape. The Devonian limestones of the Ashburton/Buckfastleigh axis are perhaps the most intriguing, for they contain the whole span of the solid geological time scale of the National Park (Fig. 29). They are among the oldest solid rocks inside its boundary and like most limestones they contain underground passages and caves. Within the caves are the youngest mineral formations we have. They are interglacial in age, perhaps only 120,000 years old, dated by the fossil remains of many mammals in cones of talus or scree under solution holes in the original surface that clearly formed ‘elephant traps’. Their detail will be sorted out below. There are tiny patches of limestone in the Culm too, notably exposed in quarries just north of South Tawton and at Drewsteignton.
All of the country rocks, together with the Lower Devonian of the South Hams, Cornwall and Exmoor, were already intensely folded and physically metamorphosed. (Hence the slates, for instance, where compression has turned the flat mica crystals, abundant in the original clay, all on to the same axis and thus created a grain that makes for easy splitting.) For the late Carboniferous had seen the extensive mountain-building crustal movements most widely known as the Variscan, but also as Armorican and Hercynian. Armorica was Brittany, and the east–west trend (strike) of the folding in Devon and Cornwall is sub-parallel with the long axis of that peninsula. Geologists know the resultant mountains as Cornubian and the Dartmoor granite’s emplacement into the roots of the Cornubian Mountains might be regarded as the penultimate surge in this whole Variscan crustal disturbance. (The rich mineralisation which made southwest Britain a tin, copper and iron miners’ paradise was the ultimate phase.) Nevertheless, by the time these intrusions were taking place the mountain range was already being rapidly eroded and, as we have just seen, the granites themselves were soon exposed and attacked.
The basal beds of the New Red rocks, the Permian, which follow immediately in the geological sequence the uppermost rocks of the Carboniferous, are breccias and the evidence for that attack. They are coarse, iron-rich (thus bright red) sands containing a host of broken angular, many-sized fragments of other older rocks, including Dartmoor granite and its chemical derivatives. Their colour and angularity proclaim erosion and deposition on the edge of a desert basin, typical of the sludged deposits of desert-mountain gullies where, after 364 dry hot days but very cold nights – such temperature change continually cracking off angular fragments – on the 365th day the heavens open and all is sluiced down-valley. This was the start of a long period of arid conditions that begins the Mesozoic Era.
There are tiny outliers of this material at Slapton and Thurlestone in the South Hams, and at Kingsand in southeastern Cornwall but the main outcrop of the Permian breccias lies just east of Dartmoor in a long north–south band from west Somerset to Torbay. During its deposition there was also more volcanic activity, some within that band and best expressed around Exeter, whose Roman remains and mediaeval fortifications are largely built of purplish red lava and related clastic rocks (breccias of volcanic origin). There is a long tapering tongue of Permian rocks extending westward from the main outcrop north of Dartmoor. It is clear that these rocks occupy a valley bottom of great age, at least contemporary with the Dartmoor granite’s first exposure, and its west-east trend is an indicator of an erosional and depositional direction that was to dominate southwestern landscape history for millions of years on. It is also, as it now lies
along the modern Vale of Creedy-with-Yeo, a nice demonstration of an ancient contribution to contemporary landscape via exhumation and re-working of a fossil surface feature.
But we must return to the granites and consider the effect of their intrusion on their ‘host’ rocks that have already been listed. They are often referred to collectively for convenience as the ‘country rock’ in circumstances such as these. The mechanical intrusive action pushes country rock aside, heaves it upwards and outwards, and prises its beds and the axes of its folds apart. The superheated fluidity of the intruder consumes and in many cases digests great chunks of country rock whose chemistry alone may remain as evidence within the granite of such activity.
On the grand scale, a long thin but distinctive band comprising outcrops of Lower Carboniferous shale, chert and limestone all intruded by dolerite lying just north of Dartmoor today displays in plan a shallow arc sub-parallel with the northern boundary of the granite. The pressure of the mobile granitic magma moving northwards from its feeder appears to have bent the east-west strike of the country rock northwards. A slight bend southwards in the band of Middle Devonian volcanic rocks which runs from Plymouth to Totnes immediately south of Western Beacon (Dartmoor’s southernmost point) suggests the same story at the opposite end of the pluton. Figure 28 displays both phenomena. In small-scale terms the faults and tiny but tight folds exposed in quarry faces in Meldon’s three pits near Okehampton (Fig. 30) and others just north of South Tawton have been interpreted by Dearman as evidence of the influence of the emplacement of the northern granite ‘toe’ on its closest contacts.
The metamorphic aureole
At close contact it is of course the heat of the magma that has greatest effect and in a zone all round the intrusion – the ‘metamorphic aureole’ – the host rocks have all been altered. Most have been baked, and thus hardened. Slates have become chemically ‘spotted’ and the spots get bigger towards the granite contact where the most intense conversion is into hornfels, a dark tough compact, flinty-looking rock. Sandstones’ quartz grains have recrystallised, and tourmalinisation is common in some places, notably at Leigh Tor above the Dart at Newbridge (Fig. 31) and in boulders on Mardon Down near Moretonhampstead.
At the present surface the aureole, which is shown in Figure 28, appears to average just over 1.5 km in width, though it reaches nearly 6.5 km briefly near Mary Tavy, and shrinks to 400 m near Wotter in the southeast and and a similar width near South Zeal in the north, though later faulting here may have affected such things. Where the actual surface slope of the granite contact can be observed or calculated, a more precise idea of the thickness of the aureole can be obtained.
FIG 31. Leigh Tor. Part of a reef of quartz schorl running across the Dart valley through Holne Chase, middle distance, to Ausewell Rocks on the skyline.
The slope of that contact near Mary Tavy is in fact very shallow at 10 degrees or so, and the true thickness of the aureole (at right angles to the granite surface) thus turns out to be only some 400–500 m there. Figure 28 shows three tiny outcrops of Culm rock sitting on the granite, remnants of its erstwhile roof, just in from Mary Tavy, in passing confirming the shallow angle of the contact just here.
The significance of the aureole in landscape terms is that it extends resistance to erosion and weathering beyond the boundary of the granite since both were exposed. Economically it has provided tougher, harder rocks that break up into more angular fragments than either the unaltered parent, or the granite next door. Granite, as we shall see, is physically strong but chemically weak – so the provision of hard angular, less soluble rocks as by-products of its emplacement has a nice utilitarian irony. Where the granite has succumbed to chemical disintegration totally the aureole rocks stand like the sponge fingers around a charlotte russe fencing in protectively the otherwise easily eroded kaolinite-mica-quartz sand mixture.
Kaolinisation and faulting
The chemical history of the granite is complex and still the subject of argument among those who delve into the mysteries of crustal chemistry, temperatures and pressures. Whether, for instance, the largest feldspar phenocrysts of the giant granite are an original feature, or the result of chemical change after emplacement, has not yet been determined.
I was taught that the slower the cooling the bigger the crystals (which helped separate plutonic (deep-seated) igneous rocks from extruded ones (volcanic glass has the finest grain of all). Equally, the lighter the colour the more acid is the rock. Silica and its derivatives provide the lightness, quartz and feldspars (aluminium silicates) dominate granite – and hence the physical hardness that quartz particularly provides. The feldspars, however, also lead to a chemical weakness because they are readily convertible under attack from the right liquids and gases into mixtures dominated by clay minerals. Kaolinisation is the process and it can occur at depth (through hydro-thermal activity and pneumatolysis) perhaps before, or as the magma cooled. It can also occur as chemical weathering at the surface, more effective in hot, damp tropical conditions, but wherever acid rainwater enhanced by acids in the soil and peat has free access to the crystalline surface. It is happening now, but has been even more effective when this granite has suffered tropical climates – as it has more than once during its long erosional history.
Both forms of kaolinisation have affected the Dartmoor granite, and two deposits in the southwest of the present outcrop (at Lee Moor and Redlake) appear to have been converted at depth, if only because there is still some
FIG 32. China clay being worked with a high-pressure water jet in Lee Moor pit on the edge of southwest Dartmoor.
250 m measured thickness of kaolinised granite (now china clay and micaceous sand) under Lee Moor, hardly a likely weathering depth (Fig. 32). There is much evidence of kaolinisation by weathering in the subsequent geological history of Dartmoor. Three substantial deposits of ‘ball’ clay (so-called because it was originally worked by hand and cut into spade blade sized cubes or ‘balls’) lie either side of the granite, one to the southeast and two to the northwest, on the same line which separates the middle east from the far east of Dartmoor.
FIG 33. Oligocene ball clay interbedded with lignite being worked with pneumatic spades in the Bovey basin in the 1960s.
The former lies between Bovey Tracey and Newton Abbot (Fig. 33) and the latter two at Petrockstowe in North Devon and in the Stanley Bank basin in the seabed just east of Lundy Island. They are related to each other and to the granite by the southeast-northwest line of the Sticklepath Fault – a major wrench fault with a horizontal throw of some 1.5 km – which runs from Torbay through the places already listed, out to sea west of Bideford and on between the Pembrokeshire islands and the Welsh mainland. Where the fault crosses the northern and southern boundaries of the granite (at Sticklepath and near Lustleigh) they are displaced as Figure 28 shows, and to the northwest both the narrow Lower Carboniferous chert outcrop and the tongue of New Red rocks in the Vale of Creedy already noted are dislocated by the same distance.
All the evidence points to the fact that this is not a simple single fault, but a zone of faulting, and there seems to have been sufficient vertical movement as well as the horizontal sliding to have created rift-like valleys (Fig. 34). Both the Bovey Basin and the Petrockstowe deposits (of sands, ball clay and lignite) have faulted boundaries and those dislocations north of the granite, because of the contrasted rocks involved, make it possible to map a myriad faults in close proximity. Within the granite such mapping is very difficult, but the fact that the pattern of parallel
FIG 34. The wide trough northwest of Moretonhampstead. Part of the Sticklepath Fault zone which runs from Torbay northwestward to beyond the Bristol Channel.
valleys, notably the Bovey, the Wray and those flooded by the Torquay reservoirs at Tottiford, conforms closely with the main fault direction amounts to circumstantial evidence for the zone rather than for a single fault. Between Sandy Park (near Chagford) and Whiddon Down, there are at least three deposits of pale clay and sand. With one there are quartz tourmaline and quartz grit blocks, the latter very similar to beds at the north end of the Bovey basin. These tiny outliers point to fluvial movement of Dartmoor-weathered material along the rifts.
The ball clay and its accompaniments are of Oligocene (or early mid-Tertiary) age, when a tropical climate held sway over what is now Britain. The lignite interleaved with the clay, as seen in Figure 33, is composed of up to 60 per cent fossil sequoia fragments, broken and battered, and few roots – although scattered rootlet beds occur at the margins of the Bovey basin. The picture then is of tropical forest timber and weathered granite products carried tumultuously but rhythmically into a rift-valley lake. The floor of the rift appears to have continued to sink as the deposits accumulated, making room for more deposition and suggesting a contemporaneous date for faulting, climate and deposition.
The faulting is the first crustal disturbance of any moment since the Variscan phase that ended with the granite’s emplacement. Some 240 million years had elapsed during which relatively stable conditions dominated this part of the earth’s surface and the deserts of New Red (Permo-Triassic) times had gently given way to invasive Jurassic and Cretaceous seas of varying depth (and to which we must return). But, the Alpine orogeny of the Oligocene changed all that. Southeast England was flexed into the broad folds of the Weald and the Hampshire and London basins, it gave us the Sticklepath Fault, sub-parallel ones to its northeast and from Prewley southeastwards across to Buckland (parts of the West Okement and the upper East Dart are close to the line). A significant fault also runs from Callisham Down to Harford in the extreme southwest (see Fig. 28). In addition Southwest England was almost certainly tilted to the south during the Alpine.
Within all that time Dartmoor granite was exposed to the air, drowned, covered with chalk, and its summits emerged again, for the last time, as far as we are concerned. Evidence of that first exposure we have already seen, in the upper parts of the New Red basement (Permian) whose outcrop has already been described. Characteristic grains and crystals from granite sources are also found in Lower Cretaceous rocks in what is now southeast Devon, which sit unconformably on the New Red, with no Jurassic remains in between. The earliest (New Red) phase was under desert conditions. The later transfer of Dartmoor detritus must have been by rivers flowing eastwards towards westward-advancing seas whose waves would have played their erosive role before the sea submerged the old Cornubian Mountain roots entirely.
Then began the warm-water deposition of the Chalk sheet that probably covered the whole of the southern half of what are now the British Islands and upon which inter alia elements of our present drainage pattern developed. In (younger) flint gravels on the Haldon Hills just east of Dartmoor and at Orleigh Court in North Devon there is fossil evidence from the Upper Chalk, and abundant chalk and flint deposits off the South Devon coast and in the Western Approaches add to the evidence for the westward extension of the Chalk ‘sea’ and thus beyond Dartmoor. But the uppermost beds of the European Cretaceous are missing from the British sequence, suggesting that that sea retreated more rapidly than it originally advanced and left us with a surface sloping gently eastwards (chalk easily trimmed by the waves of the withdrawing sea) upon which the landscaping of the Tertiary Era could begin. The Cretaceous alone had lasted 70 million years, within half that time again the Alpine orogeny would be in full swing, and that Sticklepath Fault would disrupt the developing Dartmoor landscape.
During the Eocene (the first 18 million years of the Tertiary Era) those flint gravels with their Cretaceous components were deposited throughout east Devon and beyond and now lie on the summits of that landscape, capping hills carved out of New Red and younger rocks. They also contain, on the Haldon, pieces of all manner of earlier rocks including fragments from the Dartmoor aureole and, most importantly for the Dartmoor story, granitic sand and whitish kaolinitic clay, a major symptom of granite decomposition. Some beds in these gravels contain abraded flint pebbles with rounded surfaces covered with ‘chatter marks’ (semi-circular fine cracks caused by flint pebbles continually hitting each other, as in a river or on a beach). It is argued that these must be fluvial in origin from the west because if they were deposited on a beach there would be other exotic material from the east, and there is none. Significantly kaolinitic components in these Tertiary gravels extend as far eastwards as the Hampshire basin (Dorset ball clays are worked near Wareham). There is, however, geological dispute about their origin and about the final modification of river courses that until then, having crossed proto-Dartmoor, extended eastward enough to have carried Dartmoor-derived material so far.
To that question and its context we will return when we consider the evolution of the surfaces we now see on Dartmoor, but before we complete this account of their geological basement and the processes and time involved in its history we have to consider the macro-structure of the granite itself
Joints
When a Dartmoor granite face is observed from some yards away, the thing that hits the observer first is the network of cracks and fissures in it (Fig. 35). They are ‘joints’ to a geologist and in most giant granite exposures have a most remarkably rectilinear pattern dividing the rock mass into large blocks. (Joints because they are where blocks join.) In most faces there will be two sets, those near the horizontal and those near the vertical, but there are many other joint directions and their origins explain some.
The pre-eminent cause of jointing in all igneous rocks is related to the cooling process. In all intrusive cases it begins at the contact with the country rock. Its first effect is to cause separations sub-parallel with the outer surface of the igneous body as cooling in layers proceeds, and thus over wide areas in a broad pluton the beginning of the near-horizontal joints. But remember that it is likely to be an irregular surface related to the way the country rock and the magma in our case have accommodated each other and the joints should be sub-parallel with that. Cooling close to that surface is fastest and slows as depth from it increases. Quarrymen go deeper to find bigger blocks and slabs because at depth the joints are further apart and that distance increases at least as far as the centre of the granite body. (Calculations point to a 9.5 km thickness for the Dartmoor granite.) Shrinkage goes with cooling too, so the layers formed by the
FIG 35. The classic joint pattern of the giant granite seen in Sharp Tor above the Dart.
FIG 36. The north end of Bench Tor. Joints sub-parallel with the valley side. Sharp Tor cuts the skyline on the far side of the Dart valley.
horizontal joints break up into blocks of a locally fairly consistent size by vertical jointing. Tensions and the fractures involved are themselves complex and patterns will also be geared to variations in the composition of the rock. Some extrusive rocks and those less deeply intruded than granite often exhibit very formal patterns in which the repeated block becomes a more striking feature than the joints. The Giants Causeway’s basalt and the Whin Sill’s dolerite are impressive examples where the tall vertical polygonal columns are what takes the eye.
The other main cause of jointing in granite is usually referred to as unloading. Granite was emplaced deep in the earth’s crust, that it is now exposed at the surface means that a great volume, and a great weight, of rock that lay on top of it and bore down upon it, has been removed. As that pressure is modified and eventually released so the granite springs apart to its own relief, and cracks develop parallel with the surface from which the weight has been lifted. That last is important because the phenomenon appears to work locally too, where a deep narrow valley or gorge has been cut rapidly in geological terms down through granite and pressure is thus released ‘sideways’ the granite springs outwards. Joints are then seen in valley side exposures sub-parallel with the surface slope (Fig. 36).
Since their own origin, joints have offered access to the granite interior to anything mobile enough to take advantage of them. Subsequent intrusions of later granites, the vapours and gases which brought about kaolinisation and tourmalinisation, and the mineral rich fluids, which Dearman described as arriving ‘in one great and prolonged exhalative’ spasm, all took advantage of the joints in the cooling or cold ‘giant granite’. So eventually would the acid water from rainfall, which passed down through soil and peat, once the granite was exposed to the air. Both the underground vapours and surface waters by chemical attack widen joints, round off the corners of blocks and leave the subsequent spaces full of ‘rotten’ granite. Surface waters also break down the surface itself entirely, into what is known locally, and now geologically, as growan. In growan in situ the constituent crystals of the parent rock have been separated but still lie in the original relationship with each other.
Kaolinisation has been dealt with, but tourmalinisation is also very common on Dartmoor, in it tourmaline (typically black and in the form of microscopic bundles of needle-like crystals) replaces micas and makes feldspars cloudy. Joints are often seen to be ‘armour-plated’ with tourmaline and separated boulders to be coated in a black layer 2 or 3 mm thick on one or more sides. Sometimes a quartz (white) and tourmaline (black) combination called schorl is produced and can form veins and reefs along joint spaces. It is very resistant and can form landscape features such as at Leigh Tor shown in Figure 30.
That the density of joint distribution throughout the granite varies in all three main dimensions must be emphasised. Their origins clearly presume that those sub-parallel with the surface will be closer together the nearer they are to the original roof over the intrusion. The Mary Tavy contact slope measurement and outliers of Carboniferous rocks on Standon Hill and near Hare Tor already mentioned suggest that the summits we see today are not far from where the roof once was. So in exposures on such summits joints can be very close together. Variation in the density of near-vertical joints is substantial and its explanation unclear. However, it is demonstrable that there are large-scale patterns of these joints best compared to the patterns which can be created by expanding and narrowing a wooden trellis. It is these that are reflected in the landscape of northeast Dartmoor at the gross scale, producing in their turn a surface pattern extending for many square kilometres which Ronald Waters analysed in a study of the differential weathering of ‘oldlands’ in the 1950s and which are detailed a little later.
The next chapter will deal with all weathering effects in detail but it is as well to emphasise here that it is as conductors of potentially corroding liquids that granite joints should be most carefully regarded by the student of the Dartmoor landscape. Their distribution, as we have just seen, can affect the large-scale form of the countryside in question, and with the blocks they individually circumscribe they can provide the constituents of the dramatic detail which may dominate the immediate view and punctuate the distant prospect.
GEOMORPHOLOGY – A HISTORY OF THE DARTMOOR SURFACE AND ITS SETTING
Between the North and the South Hams (for that is the ancient name) lieth a chain of hills consisting of a blackish earth, both rocky and heathy, called, by a borrowed name of its barrenness, Dartmoor…from these hills or rather mountains the mother of many rivers, the land declineth either way; witness there [sic] divers courses, some of which disburthen themselves into the British Ocean, others by long wandering seek the Severn Sea.
Tristram Risdon, A Survey of Devon, 1630
Risdon, whose ‘survey’ is largely about the landowners and their estates, for they sponsored his work, provided this remarkably economic but accurate summary description of the gross landscape of Dartmoor within Devon nearly 400 years ago. Needless to say the detail has been filled in, and scientifically analysed largely in the last 50 of those years. Just as with the solid geology, evidence about the evolution of the present surface of Dartmoor a) gets better – or at least is less contaminated or masked – the nearer relevant time approaches the present, and b) is subject to more imagination and theoretical controversy the further back we go. Davis’s theories referred to at the beginning of this chapter were still being applied to Dartmoor in the 1960s. What is observable on the ground now will be described and more recent theoretical explanations for it will be registered. As always there is probably a grain of truth in most ideas, and their combination is the most fruitful exercise for the newcomer attempting to understand a landscape.
The granite-with-aureole massif is nearly 37 km north to south and 33 km east to west, but those two longest axes cut each other only a third of the way in from the north and the west respectively. Dartmoor, especially its granite core, is essentially a great asymmetric, inverted right-angle triangle, with rounded angles and long sides tapering irregularly to the south. The metamorphic aureole smooths out those sides somewhat, and the National Park boundary extends the smoothing, at least of the whole ‘cultural’ unit. The greater granite bulk in plan forms an 18 x 18 km northern plateau, separated from a smaller, 13.5 x 9.6 km, southern version by the broad basin of the upper River Dart and its headwaters. To the east of the northern plateau, the ‘far east’ of the topography, is a slightly lower block, 16 x 6.5 km, separated from the middle east by the valleys following the Sticklepath Fault line.
The highest point of the whole, as we have seen, is that short Yes Tor-High Willhays ridge reaching 621 m OD and the southern extremity of the southern plateau, Western Beacon above Ivybridge, is 334 m OD. The summits between
FIG 37. Consolidated north–south profile of the surface of the Dartmoor mass. Significant summits are shown in their latitudinal position, but some are off the line of the virtual profile. Vertical scale exaggerated.
these two accord in the main with a remarkably consistent slope southwards of 1 in 132, or 40 feet to the mile when it was first worked out! It is portrayed in generalised form in Figure 37. The significance of the consistency of that slope must be examined, but to complete the altitudinal data set a reminder that the River Dart leaves the National Park at 30 m OD and that this is the lowest of the river exits, is apposite. So, the total relief within the National Park is 587 m in a relatively small, 953 square km, landscape unit.
Development of the drainage pattern
The contemporary drainage pattern within the park is of a dominantly southerly orientation as Figure 38 indicates. Its rivers have been listed already but arranged for these geomorphological purposes: from the southern plateau the Plym, Yealm, Erme, Avon and Harbourne radiate round from west to east. The Plym has collected the Meavy before it leaves. Further up the west side but out of the northern plateau the Tavy flows south from the moment it enters Tavy Cleave collecting the similarly southward flowing and long Walkham on its way to join the Tamar. The Lyd has substantial southerly reaches within the granite and through its own spectacular gorge before turning west to join the Tamar in due course. The West and East Okements and the Taw are the only substantial rivers (in Devon terms) rising on Dartmoor that flow, significantly quite briefly, northwards and off the northern edge. The Dart basin’s main river however is the major exception to the southerly trend, meandering across the central basin, as the West Dart, on a course just south of east from near Princetown (adding in the Black-a-Brook) and as the Double Dart as far as Buckfast. Even then it has
collected all its main headstreams on its left bank; the Blackbrook, Cowsic, upper West and East Dart, Cherry Brook, Walla Brook, West and East Webburn all flowing southwards out of the northern plateau to join it and the Ashburn repeats the direction if not the source. This eastward direction of the West and Double Dart in the central basin has a historic significance of its own, as will be seen. But up the east side of the northern Moor the Sig, the Lemon, the Bovey and the Wray flow just east of south and join the Teign. Another Wallabrook, the North Teign and the main Teign in tandem flow for some 20 km slightly north of east (sub-parallel with the main Dart) before the latter turns sharply southward just east of Dunsford and runs for 8 km more down the eastern boundary of the National Park, and for a further 8 km beyond that.
Putting together this dominantly southerly directed drainage on Dartmoor and that mean southward slope of the summits is the equation which led to the theory about a crustal southerly tilt during the Alpine orogeny which was referred to earlier. Off Dartmoor but close at hand on either side, the Exe and the Tamar both rise quite close to the northern coast of the Southwest Peninsula (7 km and 4 km respectively) and flow all the way across it to the English Channel. The Fal and the Fowey in Cornwall flow south from source to mouth and the Camel and the Torridge both have extraordinary courses with long southerly reaches before they turn on their heels and flow out from the north coast into the Atlantic. All this adds to the circumstantial tilting evidence.
But, we must recall quickly that fluvial deposits in the Eocene (before the Alpine earth movements) had been brought eastwards from across the granite and, in the same direction but long before that, into the proto Vale of Yeo-and-Creedy just north of Dartmoor from the Cornubian mountains. So, to most authorities, a dominantly eastward drainage pattern is, geologically, of long standing. It had developed during or soon after the granite’s origin, was submerged under a Cretaceous sea, buried under the resulting chalk and exhumed as rivers which also flowed eastward on the chalk slowly removed that cover. Eocene gravels with granite-derived minerals lie close to Dartmoor’s eastern boundary on the Haldon Hills, and on eastwards as far as the Hampshire basin – though some think the kaolinite there may not necessarily have its entire origin in Dartmoor. Nevertheless all this strongly supports largish rivers flowing eastwards in the early Tertiary Era. We have just recorded that on Dartmoor, now, 20 km of the Teign’s head-streams are in eastward-directed valleys, and 8 km of the West Dart and its tributaries flow east in a wide, shallow vale – that upper Dart basin already mentioned, and the Double Dart continues this line for the same length again, though now in a deep, narrow gorge of incised meanders.
FIG 39. Looking east down the valley occupied now by the West Dart. The wide floor and extensive ‘flats’ above it proclaim the elderly, even fossil, nature of the Moor’s central basin. The distant flat is c. 350 m OD around Prince Hall.
Of these easterly directed drainage landscape units it is the basin containing reaches of the West Dart and two of its feeders, the Black-a-Brook and Swincombe, which has been most carefully examined by geomorphologists. Within its confines there is a series of well-developed ‘flats’ ranging from 280 to 420 m OD (Brunsden, 1963) and their appearance as ‘steps’ related to each other on substantial spurs suggests strongly that they are remnants of former valley floors (Fig. 39). While, in modern thinking, the whole business of relating such features to distinctive former river base levels is now a dubious art, in practical terms and especially under conditions of diminishing discharge, fragments of one-time valley floors of bigger rivers are bound to be left on the later valley sides of their smaller successors. These examples lie in a wide shallow basin with a long axis roughly aligned with the equally wide vale in which the Teign estuary now sits (strangely at right angles to its main supply river). In between is a broad-bottomed trough between Ashburton and Bickington now carrying the A38 (Fig. 40), and then a segment of the River Lemon valley north of Newton Abbot. All this adds up to a corridor in the landscape of some age clearly not related to any present-day through stream. Moreover, the valley floor remnants up on Dartmoor are still higher than the flat-topped hills with Eocene fluvial deposits
FIG 40. The wide dry trough carrying the A38 between Bickington and Ashburton on a line running through central Dartmoor and the Teign estuary.
further east. The breadth of the valley floors that they define must also imply a catchment extending ‘upstream’ well west of the present Dartmoor limits. It was developed in weaker rocks long since reduced below the levels preserved now only in the tough granite-with-aureole mass.
Just to the north, head-streams of the Teign are sub-parallel with the direction of the proto-Dart, and to the south is a fainter line through a col near Erme Head and Redlake, seen partly in Figure 205, to the upper Avon at Huntington Cross on the southern flank of Dean Moor (a route nicely taken by the mediaeval Abbot’s Way from Buckfast to Tavistock).
These lines, on a moment’s lateral reflection in Dartmoor’s British context, are themselves sub-parallel with the Thames, the middle Trent, the Calder/Aire/Humber, the Tees, the Tyne, the Forth and the Tay. All are probably the successors of Eocene rivers initiated on that chalk sheet and flowing east to join a proto-Rhine, itself flowing north up a basin whose position is now occupied by the North Sea. This strategic lay-out is impressive enough, but the tactical scene supports its message. Why should the Double Dart, now deeply entrenched, follow meanders with an amplitude of more than 2 km between Sharp Tor and Holne Park which cut across the structural trend of tough rocks so dramatically? The reef of quartz-schorl from Aish Tor through Leigh Tor to Ausewell Rocks, for instance, runs straight across the Holne Chase meander as though it had had no influence at all upon the processes at work (Fig. 30). The answer must lie in the elementary landscape textbook, those meanders were superimposed on the present rock outcrops from some other substrate which overlaid them then and on which the meandering pattern was initiated…in this case, an elderly stage of the landscape developed on the chalk cover already described.
A gentle southerly tilt applied to such a pattern would disturb least those river courses running across it at right angles to the tilt. A bias in favour of left-bank tributaries would be likely, and captures by south-flowing streams with new incentives a distinct possibility. The upper Dart exhibits the former state and the strangely rectilinear pattern of the modern Teign from Castle Drogo to its mouth the latter. The disruption of the original pattern by a northwest/southeast fault zone, complex enough to throw rift valleys across west–east paths and divert stream loads into them, is almost certainly part of the mechanism for the major change in the proto-Dart at least.
Before pursuing that change – and those which followed – we should register one more thing about the uppermost – and thus the oldest – surfaces of Dartmoor now. That still-visible segment of the broad valley of the proto-Dart – which flowed from somewhere west and through the Princetown ‘gap’ – sat between chunks of landscape that also still exist. It had its own context then and some of that persists today. Correct the Alpine southerly tilt back to near the horizontal and the present flats of high northern Dartmoor that lie between 580 m and 520 m OD accord roughly with the present 493-463 m surfaces of the southern plateau in a north–south cross section. The levels, of which they are but remnants, are variously interpreted as the legacy of very mature landscapes related to base levels associated with long-term crustal stability under warm, humid climates, and with contemporary African analogies of the inselberg-and-pediplain type.
Long-past climatic weathering influences
Such warm and humid climatic conditions certainly pertained from the late Mesozoic into the Eocene and beyond. It is worth remembering that grass (or better, grasses) appeared for the first time in the fossil pollen record in the Eocene. It is a reasonable assumption that the sub-aerial denudation of a landscape by running water, downslope sludging and even wind were very different, more rapid and more efficient processes, before grasses existed and thus before turf could become a cover. Leaps of faith previously exhorted, in this case about the earliest erosion of the surfaces we still see, may be more easily made given that information.
Throughout the first part of the Tertiary Era, and in fact later in that era and parts of the next, it was undoubtedly much warmer and wetter than now. It was to all intents and purposes tropical and under such conditions granite, mechanically strong though much divided, but chemically weak because of its unstable felspars, suffers deep weathering. The effect of that attack is still evident, though not to say a controlling factor, in the medium-level detail of the land surface we still move over. The intensity of the weathering breakdown of the granite and thus its depth varied according to the distribution of the partings – vertical joints and faults – both in density and direction. In the northeast of the northern plateau Waters showed that two trends: NNE–SSW and NNW–SSE can be ascertained in both positive (ridge) and negative (valley) elements. Moreover cols in the ridges and widenings to basin-like form in the valleys are evident where perceived lines of weakness cross stronger ones. Where strong lines cross, summits with tors are the marker and they can be plotted on those two direction lines (say, Little Hound Tor-Hound Tor-Wild Tor, and Oke Tor-Steeperton Tor-Wild Tor-Watern Tor and see Figure 58). In the southern plateau, where Waters also worked, the Plym’s long profile shows an alternation of narrow and broader cross sections coinciding with steep and gentler gradients. The pattern of the Plym’s headwaters is also instructive. Five of its 11 km-long tributaries join its left bank on sub-parallel courses from the southeast, and the other two join the right bank having flowed parallel with the mainstream for most of their route. All this suggests a substantial degree of structural control involving first the pattern of partings in the granite and then the ‘etching’ of the granite on the basis of the density of the partings in that pattern. The significance of tropical phases in the history of the surface we now enjoy cannot be underestimated, even if as we shall see, cooler times have had a more recent and, some would say, more dramatic effect.
The story so far is thus of a landscape of low relief drained to the east and developed under the conditions just described over maybe 50 million years. It is suddenly, in geological terms, tilted south, dislocated from its original base levels, and 10 million years of turmoil ensues. Faulting probably continues and inter alia separates the Dartmoor mass from the rest of Cornubia at the same time. The differential strengths of granite-with-aureole and the country rocks begin to tell, now that the various ‘covers’ have been stripped away, and that differential is enhanced as the late Tertiary Era progresses. Dartmoor begins to stand proud; and its drainage begins to conform to something approaching a radial pattern albeit with that pronounced southerly bias.
The Dartmoor Island
Then, suddenly, the southwest of Britain along with the Mediterranean and the bulk of the North Atlantic basin sees a substantial end-of-Tertiary rise in sea level. All is engulfed to a height of about 210 m OD as we measure it now. Figure 41 reconstructs the scene. There is a widespread set of ‘flats’ at this height backed by a decayed low cliff in many cases, in both southwest (they surround Dartmoor) and southeast England. In the latter case there are marine
FIG 41. The virtual Dartmoor island, when sea level appears to have stood briefly at what is currently the c. 200 m OD contour given the disposition of flats in the surface right round the upland block.
deposits on some flats whose fossils give them a Lower Pleistocene age, i.e. early in the Quaternary. The classic sites in the Mediterranean give this hemispherical sea level its name – the Calabrian. True geomorphologists standing on Headley Heath in Surrey, Plasterdown near Tavistock, Hanger Down or on a bench in Whiddon Park near Chagford can hear the waves of the Calabrian sea breaking!
At the end of the Tertiary Era, then, Dartmoor was briefly an island, with wave-cut platforms and reefs cut into its coastal slopes (Fig. 42). Their best remnants are in the southwest of the National Park (where the modern wind and thus wave attack direction were already prevailing). Hanger, Roborough and Plaster Downs are good-sized samples. They recur up the western and right along the north sides, but up the eastern side too, round the embayment at Holne and on up the then ‘inlet’ of the Bovey Tracey – Moretonhampstead trough. There is a long peninsula extending westward from Okehampton, and small offshore islands like Great Haldon to the east and Kit Hill to the west.
Briefly was the geological word. The sea retreated quite soon under a generally cooling climate and while its level during that retreat oscillated with glacial and interglacial stages of the Great Ice Age, at each of the latter it returned to a lower position than those before. The stages of its withdrawal are well displayed south of Dartmoor throughout the South Hams in a series of horizontal platforms, sometimes with rearward (landward) bluffs most marked at 180, 130, 100, 85, 45, 15, 7.5 and 4 m OD. They are taken to represent pauses, or standstills, in that general, if wobbly, retreat when waves had time to cut the kinds of platform which result from a bandsaw-like attack of limited vertical dimension.
The Dartmoor Pleistocene
For the geomorphic timetable this is perhaps the logical place to re-register a Dartmoor detail. Between the last two cold phases of the Quaternary so far (we are after all, still in that Era now) – the Wolstonian and the Devensian – the interglacial is named the Ipswichian. The Ipswichian climate reached tropical quality and the proto-Dart was leaving Dartmoor though a limestone landscape where Buckfastleigh now stands. Beneath what is now an 83 m OD surface on a spur bearing inter alia what is left of Buckfastleigh church (Fig. 43), underground caverns, with sink-holes connecting their roofs to the open air contain the richest collection of Ipswichian mammalian remains in a British cave. The most significant cave is called Joint Mitnor, after the three men who discovered it in the 1950s via a cleft in the side of the abandoned Higher Kiln Quarry. They were Wilf Joint and his colleagues Mitchell and North. They found inside a scree or
FIG 43. The flat-topped spur at 90 m OD, carrying Buckfastleigh church spire and riddled with caves and passages containing Pleistocene fossils in quantity.
talus slope below a shaft, now blocked, containing in its surface layers the bones and teeth of the herbivores: straight-tusked elephant, rhinoceros and hippopotamus, giant red and fallow deer, bison and wild boar. The carnivores: brown bear, hyena, cave lion, wolf, fox and wild cat are also there; and then badger, hare, water vole and field vole for good measure. The interpretation of the situation here is that grazing animals fell in to the natural ‘trap’ and carnivores attracted by the smell of the dead and dying voluntarily followed and after a long or short gorge were unable to regain the surface.
Apart from the significance of the fossils to the interpretation of the Ipswichian climate and dependent vegetation, it is useful for us to note that the surface containing the sink-holes must have already been well above the river, otherwise the cave would have been full of water and no scree structure formed. Joint Mitnor Cave’s talus cone is regarded as one of the best in the world.
The Ipswichian ended some 70,000 years ago and the succeeding Devensian stage lasted more than 52,000 years as a ‘full glacial’ (the last real periglacial period in Dartmoor for our purposes). It thus brings us to about 18,000 BP but then takes at least another 5,000 years to fizzle out, and see the next chapter to continue the timetable. The retreat of the sea from Dartmoor during the Ice Ages, which the Ipswichian along with other interglacial stages and the Joint Mitnor Cave story clearly interrupted, persisted ultimately beyond the present coastline to somewhere near the present 30-fathom line. So the British Islands and thus Dartmoor were connected to the European mainland before the sea rose again through the Channel to a level close to its present, though still moving, position.
The whole sequence of the Quaternary so far has taken – according to careful examination of North Atlantic deep-sea deposits – a maximum of 1.7 million years. During that short geological time there have been at least 17 cold phases of which 8 or so occurred in only the last 850,000 years. They may not all have been glacial in the accepted sense of the word, and there is still difficulty in correlating what has been most recently calculated from those deep sea deposits with the land-based evidence of glaciations so long used by British and European interpreters. However, Dartmoor, an island at the beginning of the Quaternary Era, has since been part of continental Europe more than once – most recently as little as 6,000 years ago. Only 18,000 years ago it was within 100 kilometres of the edge of the last ice sheet to come south, and not long before that (geologically of course, say 130,000 years) was within 50 kilometres of the southernmost glacial extension over Britain when Irish Sea ice rode up on the shore of the north Devon coast.
Dramatic weather
So, on a landscape blocked out in the arid and tropical Tertiary Era from Palaeozoic and early Mesozoic rocks by riverine and marine action and earth movement, there is a series of final physical attacks by extreme forms of freezing weather and its gravitational consequences. Dartmoor lies during the latter glaciations well within what is known as the periglacial zone, currently best demonstrated in Northern mainland Canada and its Arctic archipelago, Spitzbergen, northern Scandinavia and of course northern Siberia. Vegetation is at most sparse – over large areas there is none.
In such places the earth’s crust is frozen to considerable depths – 600 m or more in Spitzbergen still. Whatever else that does it makes the crust watertight.
FIG 44. Sharp Tor: jointed blocks ready to fall as the intense freeze-thaw ceased – now a salutary reminder of the power of expanding ice crystals.
All those joints in the granite are sealed; water that does arrive at the surface cannot descend nor move sideways within the rock. It is a time of long winters and very short summers. Spring (late May) through to autumn (late August) is the active season when some thawing occurs at the surface in the daytime and refreezing occurs at night. This alternation of expansion and contraction – the freeze-thaw process – generates forces, as ice crystals in cracks grow and melt, or grow and grow on, which split off fragments of all sizes (down to that of silt particles: 0.002-0.06 mm), thus widening joints and levering off the outer blocks of granite from every available face (Fig. 44). That last mode leaves, of course, a new set of ‘outer’ blocks for the next leverage, maybe the next day. Periglacial weather is windy (cold air sinks over adjacent ice sheets and winds blow outwards from there at strength), snow is picked up and blasted into exposed joints and other fissures where it is packed tight, becomes white ice where otherwise water would not be retained, and adds to the levering potential.
The brief summer daily thawing provides lubrication at the surface, so all the material split away becomes part of a mobile mass of sludge containing all sizes of rock fragment (from silt particles to boulders of considerable tonnage) quite randomly distributed through it – most unlike a waterlain sediment. On any slope, the mass moves. Indeed ‘mass movement’ is one of those useful generic terms for the processes that include this one – solifluction. Solifluction can be observed in motion on slopes of as little as two degrees. New additons of weight upslope transmit power through the sheet of sludging debris – a boulder falling on to the top end of the sheet causes the toe of it to move instantly forward. In Yarner Wood National Nature reserve (NNR), flanking the valley of the River Bovey in eastern Dartmoor, granite blocks lie now (where they last stopped) on an aureole sub-surface more than half a mile from the solid granite outcrop boundary. Sheets moving down valley sides can coalesce at the bottom and run down the valley. Such streams of rock fragmental mixtures are often called ‘rock rivers’ or ‘rock glaciers’ and play their own part in eroding the surfaces over which they move.
It is important however to recognise before we press on with the periglacial story, that when conditions favourable to movement change, such sheets and ‘rivers’ stop dead in their tracks and become static parts of the scene themselves. The deposit that they have then become is called ‘head’ – wherever it is in the landscape and whatever rocks provided its parentage. Head is what quarrymen shifted before they got to what they wanted, so Victorian geologists, surveying our rocks for the first time, adopted the term, for at that stage they too were only interested in the solid stuff and all else was in the way. So close in time are we to those periglacial events that head is a significant feature of most of the southern landscapes of Britain – not least on the coastline and in the uplands and its contribution to Dartmoor landscapes is immense. Often unobtrusive where its effect has been to smooth out profiles, sometimes spectacular where its halting has choked a valley or subsequent erosion has exposed its make-up in a modern cliff
The western coasts and uplands of Britain have had in common for a long time in our geological context, the fact that much rock is already exposed, and thus immediately vulnerable to the kinds of mechanical attack that invest the periglacial time and place. In the case of Dartmoor, as we have seen, the granite had been exposed and chemically corroded for a large part of the Tertiary Era, under warm, damp tropical conditions. Granite had been rotted, joints widened and rock kept bare in that environment. Contemporary granite under those conditions has been studied to check, among other things, the theoretical considerations surrounding historic chemical weathering. Granite ‘bosses’ and other tor-like features abound in Malaya and Borneo for instance and the processes of chemical change which inter alia have render the rock between them friable to some depth are now well understood. All this to underline the fact that the recent periglacial processes did not ‘start from scratch’, some advantage was offered to them by this long chemical preparation.
To illuminate the Dartmoor situation in these circumstances it is worth contemplating the outcome of the first onset of periglacial conditions. (There must have been at least eight such occurrences and between some of them a reversion to near tropical conditions, so repetitive attacks on the same surfaces alternating with chemical rotting was the norm for nearly a million years.) For our purposes, the arctic weather descends upon a Dartmoor hillside, the soil thaws diurnally in the appropriate season and is made mobile because its parent material remains frozen and sealed against water penetration. The resulting mud slithers off and down every sloping surface. The next thawing sees the parent material itself made mobile. In a normal southwestern granitic situation that material is ‘growan’, thoroughly rotten granite where the constituent crystals have all been separated but still lie as they did in the solid rock. (Growan is a Cornish miners’ term for useless material and has a Germanic origin meaning sand.) The mobile growan thus follows on, and over the mud already downslope. Then the parent of the growan itself, the joint-separated granite, is exposed, blocks are levered away and slither down, then over-top the mud and the growan. There are many observed instances in contemporary periglacial zones of blocks sliding down the frozen surface, on the rollers provided by smaller grains and of course, when it is available, on snow and ice. Remember the Yarner Wood blocks.
Low down our slope there thus occurs a complete inversion of the original sub-surface profile: now it is raw granite on rotten granite on soil. A more
sophisticated sequence downwards in the lower slope position has been described as boulder head, main head (from a growan origin with contorted streaming lines ), disturbed growan, growan in situ and back to granite, often exhibiting a platy division sub-parallel with the original surface. Back at the hill top, the summit is narrowing as more blocks are levered away and in extremis it narrows until no more levering is possible for lack of a fulcrum, but perhaps conditions change before such an idealistic climax is reached. Nevertheless most hill tops with decent slopes below them and at the edge of either Dartmoor plateau, or alongside narrow, deep valleys, exhibit the phenomena just described. Hence the Dartmoor tor – and its ancillary ‘clitter’: the boulder field that slopes away on some or all sides as Staple Tor demonstrates well (Fig. 45).
Tors and clitter
That is the simple story, and for lumpers it will do, but for splitters there is more than one path to tread. Tors occur at spur ends – e.g. Sharp Tor (Fig. 36), Western Beacon, Penn Beacon, Pew, Leather, Vixen (the tallest tor), Crockern and Fur Tor; at the lips of valley sides – e.g. Bench Tor, Hucken, Ger Tor (Fig. 46), Calveslake and Buckland Beacon; on valley sides themselves – Hockinston, Luckey, Sharp (at Drogo), Ravens, and in Wray and Lustleigh
Cleaves: and of course on true summits – Bellever (Fig. 47 – left), Laughter, Rippon, Hay, Saddle, Yar, Great Staple, Sheepstor, Sharpitor, Great Mis Tor (Fig. 47 – right), Belstone and Yes Tor. All these sites have in common at least the fact that they are the most exposed, the most vulnerable positions in our landscape – ripe for weathering and erosional attack. Gravity, the basic ingredient of all non-glacier surface erosion, transport and deposition, is able to offer its most effective contribution at all of them.
FIG 47. Summit tors: (above left) Bellever Tor from the south; (above right) Great Mis Tor overlooking the Walkham.
Tors themselves come in many guises, and inevitably the splitters have attempted to explain the differences and classify them. It is an exercise like crossword puzzle solving, a challenge only if there is nothing else to do. That is not to say that there are not some intriguing and repetitive peculiarities. One of the most challenging is seen in the so-called ‘avenue’ tors. These are usually in summit sites where two or more rock piles persist separated by a space floored by a smooth, now turfed, surface. The granite blocks making up the piles are clearly still in their original vertical and perhaps horizontal relationships, exhibiting the progression downwards of increased angularity, or the lessened rounding that exemplifies previous underground chemical corrosion. Great Staple Tor above Merrivale in the west, seen in Figure 45, has four corner piles. Hayne Down near Manaton in the east, with the famous but unique single column of Bowerman’s Nose on its flanks, exhibits a similar phenomenon at its southern summit, and Hay Tor on the extreme eastern edge not only makes an avenue with Haytor Rocks but classically demonstrates a correlation between closer joints and diminishing upper surface height (Fig. 48). What allowed the abstraction of the intervening granite leaving almost separate tor piles intact is a question still as open as these hill tops themselves. But the density of jointing, and thus variation in vulnerability to physical and chemical weathering, plays some part in the detail as it has already appeared to do at the landscape scale.
FIG 48. The eastern peak of Hay Tor from its western twin across the ‘avenue’.
FIG 49. Clitter slopes around the head of Tavy Cleave, where the Rattlebrook joins the Tavy from the north. The foreground is the western slope of Fur Tor.
Below tors there are invariably scatters of granite blocks. In many cases they are literally boulder fields with little or no space, soil or vegetation between the boulders themselves. The solifuction vertical profile that has already been discussed would have them classified as ‘upper head’, the resting place of the last blocks to be levered away from the summit and the immediate slopes around it. They form the ‘clitter’ or ‘clitters’ of local topographic description – and make the approach to some tors difficult, demanding continuous attention to foothold and stride. Clitter appears at first consideration to be much more common, extensive and ‘pure’ below western tors as Figure 49 suggests. Careful fieldwork in the east however reveals dense boulder distribution where woodland masks it from the casual view from outside. This situation is well expressed in the Dart gorge below Bench Tor, for instance, where the ‘pipe track’ from Venford Reservoir waterworks offers a splendidly carefree walk along a near contour-like line halfway down the steep valley side. Even more open slopes when traversed on foot exhibit an unexpected population of boulders disguised from above and below by bracken and scattered hawthorn. The Hayne Down slopes are a good example.
Taken altogether therefore there is clitter throughout the Moor. Its exposure and apparently greater significance in visual landscape terms in the west may simply be that that is the ‘weather’ side of Dartmoor and has been so, ever since the end of the last periglacial period. Under a regime of westerly rain-bearing winds for 10,000 years the postglacial ‘fines’ (sand, silt and clay particles) which create the mineral frame for soil formation have always been readily washed away downslope; and thus a happy environment for low and tight vegetation to be established has never been achieved. Even in the east and despite the cover there, just described, it is clear that slopes facing the western half of the compass
FIG 50. Staple Tor from Cox Tor; the vestigial ‘net’ shows at the foot of the tor. In the foreground is the field of ‘periglacial goose-pimples’ – another variation on the patterned ground theme.
on the whole carry more boulders on the surface, or half buried, than those with other aspects. Down-wash has affected and still affects all such slopes, but best not to forget that in the dying days of the periglacial period these also would have been the sites of most effective thawing in the freeze-thaw phase. The sun strikes slopes more intensely than flats anyway, and strikes these west-facing ones after the day generally has warmed up. As ever, a combination of processes has almost certainly produced the effect in the landscape that we now see.
The flanks of the ridge crowned by Great Staple and Roos Tors bear wonderful examples of almost pure clitter. Viewed for more than a moment from Cox Tor (which is outside the granite) to the west, they can be seen to contain the vestiges of a pattern, or patterns (Fig. 50). Near the summit out of an apparently tumbled mass (the French call it a ‘chaos’ which is much more telling) there emerges downslope within the chaos a kind of net, and further down the meshes of the net open into strings running down to the bottom of the clitter. It is not a unique site – such ‘stone stripes’ emerging downwards from nets can be seen looking east from the B3212 across the headwaters of the River Meavy above Burrator Reservoir, and again on the slopes of Leaden the other side of the road (Fig. 51).
‘Patterned ground’ is a nice generic term for these and other phenomena that characterise most sub-arctic surfaces with little or no vegetation, and fossil versions are not uncommon in the highland zone of the British Isles. There are especially good examples above Applecross in Wester Ross, and on the Carneddau in Snowdonia, but Dartmoor is the repository of the southernmost we have. The physical process that produced the various patterns involved is (like the splitting off of small flakes from rock surfaces and the levering away of large joint-bounded blocks) largely a matter of the effect of expanding ice crystals – their actual formation and subsequent growth. Their expansion is universal in direction, i.e. as significant upwards and downwards as sideways or even more so. So particles from the whole of the clay-to-boulder spectrum can be heaved upwards and outwards and a kind of sorting happens which eventually moves the largest components of a mixture to its outer edge. Given the universality of the process over a flat or gentle slope, a ‘best-fit’ pattern of polygons is created. Hence the term ‘stone polygons’ which form the classic sub-arctic patterned ground in a wide variety of rock materials. You can perhaps imagine the different aspect of such phenomena in thinly cleaved slate with slabs lying in the vertical and in bulky granite in which no obvious long axis exists, but the gross pattern is the same in both. Hence also the nets of polygonal mesh on the upper slopes below tors, or even over the whole summit where no adequate tor remains.
FIG 51. Stone net running downslope into stone stripes below Raddick Hill. Cramber Tor is near the right-hand skyline.
Upper Dartmoor physical surface detail, then, is most often a series of fossil periglacial patterns and incidents. On occasion however the fossil detail is reflected and maintained by contemporary vegetation. If after viewing the Great Staple slope the observer walks towards it off Cox Tor he will soon find himself among what appears to be a continuous cover of 30 cm-high closely vegetated anthills touching each other at their bases, and almost as difficult to traverse as the clitter which he is approaching. Here is another periglacial pattern, common in the high arctic where strong winds are the norm blowing outwards from the cold air sump that exists over any ice sheet. Fine wind-borne particles accumulate as tiny dunes and mosses and arctic heather anchor them and grow upwards through the mineral mass trapping more. A cross between a molehill and a tussock emanates and is self-sustaining (Fig. 50 and compare Fig. 66).
If Cox Tor itself was originally approached from the car park to the south then our observer has climbed a series of benches or stair treads perhaps 20 m from front to back and with vertical risers of 10 m (Fig. 52). These ‘benched’ hillsides recur around the Dartmoor outer edge and are almost all on the aureole rocks. There are five more sets on the west side – at Peek Hill, Smeardon Down, Southerly Down, Lake Down and Sourton Tors. There are more on the north flank of Yes Tor, partly on the granite, and on East Hill above Okehampton. They are on Brent Hill in the southeast and Black Hill near Hay Tor further north. They appear to be structurally controlled, the treads of the stairways aligned with major weaknesses in the slates, sandstones or dolerites, close to near horizontal pseudo-bedding planes. They first remind us that the periglacial attack is comprehensive, i.e. not confined to the granite or to specific surfaces. Indeed, the finely divided aureole rocks offered to the freeze-thaw process many more tiny cracks on which to work and resulting slaty or slate-like fragments were easily transported across basal platforms developed on those dominant structural lines. Spring-sapping at the foot of each riser is now slowly destroying the benches, which at least confirms their relict status and our privilege in seeing them before they are no more.
That reminder of the universality of the periglacial process is emphasised by the implication already that the tors themselves are not confined to granite. Cox Tor and Sourton Tors have already been mentioned, but elsewhere substantial features occur in the landscape, like Brent Tor in the extreme west, Brent Hill (Figs 7 & 9) and Leigh Tor above New Bridge on the Dart. The latter exists in a reef of quartz-schorl which runs for two miles west to east, crossing the meandering Dart twice and with rock exposed at each of three high points (see Fig. 31). Neither are tors by name confined to Dartmoor, however rare they are elsewhere. The tors of Torbay are in Devonian limestone, Mam Tor in the Peak District is in Millstone Grit and High Tor in the Gower is in Carboniferous limestone, but all have a periglacial past. Origin and etymology nicely coincide – we have already seen that in the place name game physical features, rivers, hills and headlands have the oldest names. Tor is at least Anglo-Saxon, but the ‘Anglish’, in the highland zone at least, picked up the landscape names of their new hosts and, reminiscent of the Welsh ‘twr’, the Anglo-Saxon ‘tor’ meant tower as well as rocky summit.
The tor/clitter combination (with all the variations on patterned ground where visible) dominates the scene where local relief is substantial – hence the earlier reference to the plateau edges as a generic location. The type list of tor sites (see above) tells the more specific tale and almost all are in and alongside valleys. Even a few tors apparently well into the plateau interior, on more careful examination can be seen to relate closely to valley sides. Fur Tor is a classic example (Fig. 53). Perhaps the most remote tor of all (Fur was Vur or ‘far’ in early Devonian) – but when you are there it is clear that the land falls so steeply from the tor to the north, 120 m in less than 800 m in fact, that this is yet another valley-side site: the plateau edge is more dissected than you thought. Bellever Tor, on the other hand, looming over the central basin from its northern edge in Figure 47, is a summit tor unrelated to any really steep slope. From it, as from the south side of Fur Tor, and as from most plateau summits, long slopes of the
‘main head’ spread and clothe the interfluves, producing the typical ‘high land of low relief’ that is the basis of the Dartmoor ‘wilderness’, beloved of so many. Between them are the converse elements of such a landscape: the shallow ‘vales’ and basins where drainage is now impeded and very poor, and where mire and valley bog thrive, and remember the ‘oldlands’.
On the crests and flanks of the interfluves surrounding the central basin there are many shallow pits where commoners have long indulged their right to take gravel and stone for their own domestic use, as have Duchy tenants. Surviving faces in them have demonstrated in the last 50 years (and there are few left as clean exposures now) the once mobile nature of the head, as in Figure 54.
FIG 54. Periglaciation: (above left) Cryoturbation of granite head in a pit on the Cowsic-Black-a-Brook interfluve; (above right) fossil structure of growan dragged by downslope sludging on the side of the West Dart valley at Prince Hall.
FIG 55. Profile from contemporary soil (peaty, gleyed podsol) through head, to frost-split platy granite in situ: a pit face on Merripit Hill in the 1960s, now invisible.
Flow lines and ‘stirred’ involutions are the main symptoms detectable. In some the mixture of flattish granite fragments, crystal gravel and fines could be seen in downward sequence of surface boulder (or upper) head, soil profile, main head, disturbed growan, growan in situ, platy granite and blocks in their original positions below that; the now-complicated profile with which we started this periglacial story. Figure 55, taken in 1963 in a pit on Merripit Hill, shows most of the sequence.
In passing, a reminder that Bellever Tor, seen well in Figure 47, despite its present coniferous plantation backcloth, has a profile from the south against the sky reminiscent of the inselberg-and-footslope of the southern African landscape mentioned earlier. Of all the tors it offers most hope to those who remain convinced that tor development is of longer standing than the Pleistocene. It may well be. It is certainly clear from some shallow quarry faces – notably at Two Bridges in the upper Dart basin – that chemical decay via pneumatolysis and weathering can reduce immediately sub-surface well-jointed granite to tor-like shapes consisting of piles of round-cornered blocks increasing in size downwards (Fig. 56). They have been revealed in two dimensions by human
FIG 56. A classic ‘buried tor’ in the Two Bridges quarry with chemically rotted granite on either side.
excavation but otherwise remaining embedded in the growan and other products of chemical disintegration – in a sense appearing to sit in their own waste. However, it is hard to see such waste surviving the freeze thaw onslaught of the Pleistocene in situ in any exposed location, even to imagine pre-existing tors thus formed at the surface staying the same when the scale and intensity of the periglacial attack is so clear.
As is so often the case in the landscape it is as well to accept that all the processes so far observed, described and conceptualised by geomorphologists, have played some part in the achievement of the present surface form. It is reasonable to conclude that, however effective and efficient the final periglacial denudation of the Dartmoor surface, it and its underlying structures had been well prepared for it by the exploitation of their weaknesses by chemical means in interglacials. Much rock and rock waste was lying on that surface innocently waiting to be stirred up and swept downslope. As long as the climatic conditions pertained the processes would persist, exposing more and more rock on hill tops and valley sides to be translated downwards to fill up valley bottoms with gritty mobile sludge.
The final melt, the last big rivers
The critical conditions did of course change, and from sometime just before 14,000 years ago freeze-thaw ceased to be effective. That is a landscape understatement. Whether the change was sudden in our terms or in geological terms, one must try to imagine the effect of the final melting of all those ice crystals. The permafrost had probably penetrated to 600 or 700 m down. It all had to melt, most of it had to exude at the surface and, however briefly, torrential valley discharges were bound to occur. The positions of the tors, on valley sides especially, have one more, simple message for us – they underline the fact that the detailed valley pattern was here before the last periglacial phase, indeed before the Ice Age itself. Thus the meltwater at the end of that phase was channelled down the valleys we now see. So, at first each springtime, sluicing out of the valleys began. As the climate continued to warm up torrential rivers, far greater in depth and width than the present occupiers of our valleys, became all-year-round affairs, until there was nothing left to melt even at depth. Then the torrents stopped, whimpered and (metaphorically overnight) shrank to a discharge dependent on the seasonal rainfall regime which the new temperate climate of Northwest Europe dictated. Britain had entered the next period of the Quaternary Era, the Holocene. The next 14 millennia of Dartmoor’s history enjoyed a climate similar to that of our, or at least my, childhood with a few simple but significant temperature peaks and troughs.
However, in that geologically short meltwater time valleys were deepened. When ice advances from the poles globally sea level inevitably drops worldwide as seawater is locked up in ice. So, when they start running again long before the general polar ice retreat is complete, rivers have a base level far to seaward and much lower than the coastline that we know now. Their incentive to cut down is thus that much the greater. Remember also that this whole process was repeated at least four times in the Pleistocene Ice Age in the same valleys. For their pattern, as we have seen, was established before the onset of the first glaciation. Sea level, remember, offshore around the southwest peninsula at the peak of the last periglacial period, was at about the present 30-fathom mark. The lowest reaches of the rivers radiating from Dartmoor southward and their last reaches before leaving the plateaux in which their rivers had risen thus both became gorge-like, very narrow and very steep-sided. The former were eventually drowned by rising sea levels – the rias of South Devon. All the rivers that rise in a Dartmoor plateau display the same phenomenon as they leave it (Fig. 57). So much so that a topographer might use the phrase ‘tor-and-gorge’ to characterise this zone at the Dartmoor plateau edges (see Fig. 58).
FIG 57. (above left) The Dart gorge from Sharp Tor – 152 m deep from valley lips at 300 m OD; (above right) looking down the Teign gorge from above Castle Drogo – 170 m deep near centre of view. (T. Eliot-Reep)
FIG 58. The distribution of gorges and significant tors around plateau edges.
The Dart, the largest stream to leave Dartmoor still, and possessing the largest catchment within it, carries three ‘gorges’. The West Dart valley becomes gorge-like below Crow Tor and contains Wistman’s Wood. The East Dart’s last mile above Hartyland at Postbridge is in a steep-sided north–south slot and enters another just below Babeny. Its West Dart partner does the same below Huccaby House. They combine at Dartmeet and form the Double Dart that continues in the same mode. The whole of that gorge then runs for nearly 12 km to Holne Bridge, seen already in Figure 3, and under Bench Tor has reached 150 m in depth and the fragmented valley floor is still miniscule throughout.
But, when the source for torrential discharge (the melting permafrost) was finally used up, so the load of the moment, fines and boulders, was dropped (the load moved by any river is absolutely related to its discharge) and clogged the valley bottom wherever there was a natural sump – a sudden wider stretch of valley floor or the exit from a gorge on to a ‘plain’ for instance. That Dart gorge exemplifies the situation well. For three-quarters of its 12 km there is no valley floor to speak of, the present river – a puny inheritor of the gorge compared with its last real working occupant – still occupies the whole of the valley bottom. Suddenly, however, as at New Bridge, there is a widening (interestingly just above the point where the Aish to Ausewell schorl reef crosses the Dart for the first time). A small flat, perhaps a mile long, in three ‘beads’ – below Hannaford, at New Bridge itself and at Deeper Marsh – and never more than 150-175 m at its widest, provides space for human activity, odd tiny enclosures, even a dwelling or two, nowadays a car park and all that goes with it. It also provides, for our purposes here, a more important demonstration. Boulders lie on its surface, protrude through that surface from below and, in section at the modern river’s edge, are buried in it. It is clearly a jumbled deposit with none of the graded vertical order of a normal water-lain one. It arrived in a torrent and was dumped when the torrent finally failed.
Such phenomena occur throughout the modern exit gorges of the Dartmoor streams. In extreme cases and in narrower, tighter valleys there can be a complete choke of jumbled boulders as at Becka Falls just before the Becka Brook joins the River Bovey emerging from its own gorge – Lustleigh Cleave (Fig. 59). Here the Brook ‘falls’ properly sometimes in the winter, but all summer long filters white water through a boulder wall perhaps 12 m high. But everywhere over Dartmoor now, at all grades of downstream slope, the puny contemporary streams flow round the boulders moved into place by their mighty predecessors. Collectively the boulders may form islands or ‘aits’ – cf. the ‘eyots’ of more sedate rivers, and in extremis text book braiding of the streams occurs. A classic example is in the River Swincombe just below its exit from Foxtor Mire south of Princetown (Fig. 60).
FIG 60. The braided River Swincombe – just within the Foxtor newtake.
A final related phenomenon is where the torrent of meltwater and the Dartmoor massif parted company cleanly. Here a fan, almost a delta, may mark the exit. The most impressive lies south of Ivybridge, a settlement sited at a classic crossing point-cum-power source on the River Erme. Here the surface slopes gently outwards, fanning away from the moorland edge and now bearing fields, a tight housing estate and the A38 dual carriageway, all of which mask the dense deposit of rounded boulders of the Erme’s great parent stream. Before the last quarter of the twentieth century the space was called Newlands, a late enclosure of square fields bounded by walls and massive Devon hedge-banks, the latter typically 2 m thick and 2 m high and clearly faced by rounded granite boulders. Short sections of them are now incorporated into a number of housing estate garden walls. Where the fields still exist for their original function some straightforward single- or double-thickness walls also remain.
Granite core-stones are already rounded by chemical attack as we have seen, but these boulders have clearly been cleaned of anything rotten and further rounded by their transport downstream. They are now hard-surfaced balls. Indeed before 1975 whenever a public utility dug up Ivybridge High Street (the old A38) for one purpose or another, the road and pavement were littered with free-standing one-ton round boulders, for all the world as though a giants’ skittles match was in progress. All this to underline the power of the transfer
FIG 61. Wall of rounded boulders near Newlands Cross, south of Ivybridge.
system involved and the distance an unnaturally dense load of boulders, jostling each other the while, was transported. Normal riverine deposits consist of sub-angular fragments of whatever size. That is how they are distinguished, whether still free or lithified in the geological sequence, from marine deposits, where smooth and complete rounding is the norm.
So, the last contribution of the Pleistocene period to the modern Dartmoor landscape was brief, tumultuous and had dramatic effect. In its way it ranks with the other ‘spasms’ that have characterised the history of this singular bloc of country since the first granite emplacement all those 280 million years ago. Those spasms involve two more granite intrusions, Dearman’s ‘exhalation’ of metalliferous pneumatolysis and kaolinisation, the great faulting and tilting of the Alpine orogeny and even the brief survival as an island in the Plio-Pleistocene sea. They should all be remembered as making their own distinctive and long-lasting gifts to the Dartmoor surface as we know it. But the spasmodic ‘periglaciations’ at the end of the Pleistocene earn their place in this premier league of dramatic interludes partly because they are so recent and their effects all still so impressive to any contemporary observer.
Fifteen thousand years of soil and vegetation development and the occasional scour and flood of shrinking, and thus weakening, successor streams, and 7,500 years of human interference as the second half of that 15,000, have not seen any real diminution of the sculptural and moulded detail provided by those arctic spasms. Indeed it might be argued that much organic activity has enhanced their scenic effect. The lasting natural large-scale additions from that activity – mires, bogs and valley-side woodlands – have increased the variety at the surface without masking (honourably excepting blanket bog) any of the geomorphological detail. Human manipulation has modified these natural organic contributions somewhat, most dramatically in the alteration of soils and the accidental invention of inland moor and heath after woodland clearance. But despite exploiting something of each of the hard and soft products of the granite face itself, man has left, and still leaves, only incident (pits, quarries, buildings) to punctuate, and thread-like nets (of walls and banks, tracks and lanes) to accentuate, the grand scale of the whole countenance of the high moor.
The rest of this book is about the way all forms of life have worked out their relationship with that physical landscape, and with each other, upon it.