INTRODUCTION

Location and Access

For purposes of location of Haytor (or Hay Tor) Rocks and geological setting there is shown above a general geological map of Devon and also an old geological map, dating from 1911 of part of eastern Dartmoor (survey of Ussher, Barrow, Sherlock, MacAlister, Dewey and Bromehead on a 1908 base map with railways inserted to 1911). Fortunately, development has been limited on Dartmoor and the road system is much the same. If possible, purchase modern geological and topographic maps and compare with the old maps.

To drive to Haytor Rocks proceed southwest from Exeter on the A38 Plymouth road. Turn off to the right when in the Newton Abbot area. Note that the roads near Haytor are rather narrow lanes. Coaches can get to the car parks.

There are two car parks from which Haytor Rocks are easily reached by short uphill walks. One of the car parks is directly south of the tor and is on a coll at SX 759767. There are no facilities here. Another is east of the tor at SX 765772. Here there is an information centre open at certain times. There are also toilets that are generally open. This is probably the best place for a coach party.

INTRODUCTION

Safety

There is little hazard in walking from one of the car parks to this tor, unless the weather conditions are very abnormal. However it can be cold and windy and suitable warm-weather clothing and sturdy footware should be worn. Extra clothing and a compass should be carried anywhere when out walking on the moorland of Dartmoor, because being caught out in the wrong place in the wrong weather can lead to risk of hypothermia.

Much care should be taken if any of the rocky outcrops are ascended. The northwestern rock has some convenient iron handholds. The southeastern rock is more difficult to ascend. It has some old steps which are cut into it. After about two thirds way up the stairway becomes less easy for the ordinary walker. Beyond a fissure the steps are not very safe and some iron railings or hand holds which were once present seem to have been cut off at rock level. Rock climbers scramble around the top but this is not recommended for the casual visitor and not necessary for geological or geomorphological study.

Introduction - Northwestern Haytor Rock - Geomorphology

This small supplement is primarily intended for use by Environmental Science students studying the River Teign at this locality.

This location map and aerial photograph shows the position of Dogmarsh Bridge, near Chagford. The River Teign can be studied here and there is a footpath alongside the river. The particular site is unusual in that there is a wide floodplain, even though the river is still in the Dartmoor area, and downstream there is a steep-sided valley.

Bibliography and References

(in early stages - references will be added)

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Blyth, F.G.H.B. 1962. The structure of the north-eastern tract of the Dartmoor Granite. Quarterly Journal of the Geological Society, 1962; vol. 118; issue.1-4; pp. 435-451. By Francis George Henry Blyth.
Abstract: The granite margin from Sticklepath to Bridford, of which the only map hitherto available has been the one-inch Old Series sheet 25 dating from 1866, has been re-mapped, together with the rest of the area lying north-east of the Lustleigh-Sticklepath fault-system (Blyth 1957). Complex faulting in the Lustleigh area is described. The fracture-pattern consists of a late NW-SE fault-system with associated NNW-SSE faults, which is superimposed on an earlier series of fractures including joints, aplites, and tourmaline and haematite veins. The northern part of the Dartmoor granite is visualized as a thick intrusive sheet with an outwardly dipping contact against Lower Culm sediments. More than one episode of movement on the NW-SE. and NNW-SSE. faults is postulated, and some part of it is thought to be of Tertiary age. This has resulted in an area of broken granite which has been more readily eroded than the area to the south-west of the faults.
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Durrance , E.M., Bromley, A.V., Bristow, C.M., Heath, M.J. and Penman, J.M.. 1982. Hydrothermal circulation and post-magmatic changes in granites of south-west England. Proceedings of the Ussher Society, 5, 304-320.
Abstract: Prolonged hydrothermal convective circulation within the granites of south-west England is indicated by metalliferous mineralisation and kaolinisation processes which have acted since the time of granite emplacement. The requirements for hydrothermal circulation - a sufficient heat source and suitable permeability - are found to allow modern convective flow within the granites. Radiogenic heat maintains the granites at elevated temperatures, and the results of an electrical resistivity survey of the top few kilometres of the Dartmoor granite show that the in situ permeability is likely to be high. 222Rn and U distributions in Dartmoor stream waters suggest that low values indicate sites of convective drawdown, with upwelling in adjacent areas giving high values. Residual compressive stress, acting mainly in an approximately NNW-SSE direction appears to control the high permeability pathways by opening fractures which trend in this direction. The present day heat flow pattern and distribution of thermal mine waters confirms the general character of the convective cells. The evolution of hydrothermal convective circulation since the emplacement of the granites suggests reactivation of convective cells by tectonic activity reopening fractures and the periodic input of enhanced mantle heat flow. Mapping of modern hydrothermal convection cells provides information which can lead to the prediction of hidden metalliferous mineralisation and china clay deposits, and has important implications for the efficient exploitation of geothermal energy.

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Durrance, E.M. and Laming, D.J.C. (Editors) 1982 (reprinted 1985, paperback, and 1993). The Geology of Devon . University of Exeter Press. 346 pp. ISBN 0 85989 247 6. This is a key publication on the area.
"Preface: Geological Field Work: It has often been remarked that geology is a subject best studied by actually looking at rocks, minerals and fossils, and their structures and relationships, in the field. Therefore, although this book mainly deals with descriptions from an interpretative viewpoint, at the end of each appropriate chapter a number of localities are listed which will serve to illustrate the main points dealt with in the text. The localities are mainly arranged in subject groupings, although some geographical subdivision is also present. Excursions to specific areas of Devon, to include visits to a number of sites of different character, may thus be constructed with the aid of the appropriate Ordnance Survey and Geological Survey maps, according to individual requirements. Excursion Guides to different parts of Devon are also published by the Geologists' Association:
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Exley , C.S. and Stone, M. 1964. The granite rocks of south west England In: Present views of some aspects of the geology of Cornwall and Devon. 150th Anniversary Volume. Royal Geological Society Cornwall.
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Hawkes , J.R. 1982. The Dartmoor Granite and later volcanic rocks. In: Durrance, E.M. and Laming, D.J.C. (eds.) The Geology of Devon. Exeter. 85-116.
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Perkins, J.W. 1972. Geology Explained: Dartmoor and the Tamar Valley. By John W. Perkins, B.A., F.R.G.S. Illustrations by the Author. 196 pp. David and Charles, Newton Abbot. [Like the other books in this series of "Geology Explained" this is an excellent geological guide book with superb, informative, labelled sketches].
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Reid , C., Barrow, G., Sherlock, R.L., MacAlister, D.A., Dewey, H., and Bromehead, C.N., with contributions by Flett, J.S. and in part from notes by Usser, W.A.E. 1912. The Geology of Dartmoor. Memoirs of the Geological Survey, England and Wales, Explanation of Sheet 338. London: Printed for Her Majesty's Stationery Office, by Darling and Son, Ltd., 34-40 Bacon Street, E. 102 pp. and 2 plates. Original price: Two Shillings and Threepence. By Clement Reid, F.R.S., et al.

Example extract from the Introduction:

"The following page describe the geology of an area of 216 square miles in Devon, including the greater part of Dartmoor. No coast line or tidal estuary cuts into this district, and the Moor, which is the highest land in south-western England, dominates everything. The geology we have to deal with is essentially the geology of Dartmoor; for the rocks outside the actual borders of the granite are either baked by the granite, or have been thrust aside by the intrusion of this granite. .

Large masses of Dartmoor rise 1,500 feet above the sea and much of it is over 1,700; at several points it exceeds 1,900 feet, but the only tors reaching 2,000 happen to lie just beyond our border. The highest points within the map described in this Memoir are Cut Hill, which reaches 1,981 feet, and Whitehorse Hill, which rises to 1,974 feet. These two hills, two miles apart, represent the watershed and central area, from which the rivers radiate north, south, east, and west; the somewhat higher tors further north are not on the watershed. This central and northern area is also the wettest and most peaty part of Dartmoor, for it has an annual rainfall of about 60 inches. All the land above 1,600 feet rises into the region of clouds; so that not only has it an exceptionally heavy rainfall, but the constant mists and slight evaporation tend to make it a far more efficient feeder of the streams than its area alone would suggest.

The wide-spread upland peat-mosses of Dartmoor in which the rivers rise occupy most of the land above 1,600 feet, i.e., the area north-west of the Princetown and Moretonhampstead road. These mosses form continuous sheets clothing the highest land, and in this they are quite unlike the 'basin-peats' of the other and lower granitic areas in Devon and Cornwall. The growth of the peat is directly connected with the elevation; for the land above 1,600 feet, as already mentioned, is liable at all times of the year to be wrapped in cloud. Thus a peaty vegetation grows, and peat accumulates on slopes that at any lower elevation would show merely a rough boulder-strewn granitic soil.

Streams from Dartmoor feed all the principal rivers of Devon, and many of them have in Dartmoor their principal source, even though their catchment area on the Moor is not so large as tnat in other parts of the basin.

This radiation of the streams from a central point on Dartmoor., though conspicuous now, was formerly of far greater importance. It seems to date back to early Tertiary times, when probably the Moor was much higher, and the sea had not so greatly trenched into the surrounding lowlands. Eocene rivers flowed westward to Penzance, eastward across Devon into Dorset, and probably northward also to Bideford.

The streams that now descend the northern side of Dartmoor to reach the sea on the north coast are the Okement, one of the principal branches of the Torridge, and the Taw; both of which flow into Bideford Bay. Tributaries of the Teign also descend the northern slope of the Moor; but this river, after leaving the granite, takes a very strange course, for it flows eastward and then turns southward, receiving in its course to the sea at Teignmouth various eastward- and southward-flowing streams, which also rise on the Moor. The Dart and its numerous tributaries drain nearly half the Moor, flowing southward and eastward to combine near Ashburton, after which the river holds a southeastern course to Dartmouth. The south-flowing rivers, the Avon, Erme, and Plym, only flow for a short distance within our area, and then follow a normal course to the sea. Streams from the west side of Dartmoor form the head-waters of the Tavy and some of the smaller tributaries of the Tamar, both of which empty into Plymouth Sound.

It will be understood from the foregoing account that a large part of the area of which we treat is unenclosed moorland. A good deal of the rest is too steep for cultivation, being deeply trenched by the rivers and the slopes clothed with timber. The remainder, especially where greenstone and schalstein occur, is often good land, though naturally its close proximity to Dartmoor and the consequent heavy rainfall make it more suitable for dairy-farming than for cereals. On the Moor numerous ponies run half-wild, and excellent crops of potatoes are raised.

In addition to agriculture there is a certain amount of mining around Dartmoor, though in the heart of the Moor the mines are of little value. Copper, tin, lead, silver, iron, arsenic, and manganese have all been worked, as well as ochre and umber. Granite is quarried, and London Bridge was built from the granite of Haytor ; the cost of carriage however is a serious obstacle to extensive working, for the rock has to be carted or taken by tramway several miles to the barge, and there is no deep-water harbour within reasonable distance.

The peat-mosses, just now of little economic importance, will be worked again as fuel becomes more expensive and the better methods now in use are applied. The extensive peat-area north of Princetown and Postbridge is still however without roads and difficult of access. Peat was formerly used in the smelting of the tin-ores; but the alluvial tin has now all been removed, and the peat is only required for ordinary fires.

It may be said in conclusion, however, that Dartmoor and its surroundings owe much of their present importance to their value as a health resort, as a national playground, and as a region of great scientific interest. To these aspects of the geology special attention will therefore be paid in the following pages.

The geological formations represented on the Dartmoor Map are as follows:

RECENT - Peat and Alluvium
PLEISTOCENE - Valley Gravel and Head. .

CARBONIFEROUS -

Shale and Grit (Upper Culm ?).
Radiolarian Chert. (Lower Culm.)
Shale, LImestone, and Grit (Lower Culm).

DEVONIAN.

Upper - Slate and thin Limestones.
MIddle - Limestone.

CARBONIFEROUS IGNEOUS

Elvan or Quartz-porphyry.
Granite.
Mica Trap. [lamprophyre?]
Keratophyre
Diabase. [dolerite]
Spilite (Pillow-lava).

DEVONIAN

Diabase. Spilite (Pillow-lava).

In addition various types of metamorphic rock occur in the neighbourhood of the granite and others are found close to the masses of greenstone.

[end of Introduction. A section on Literature follows.]