49

FIELD MEETING, WIllTSUN 1951,AT TINTAGEL, NORTH CORNWALL

Report by the Director: Gilbert Wilson, Ph.D., F.G.S.

INTRODUCTION

THE Tintagel area of North Cornwall was last visited by theAssociation at Easter 1914, under the leadership of Mr. Henry

Dewey, and the report of that excursion appeared in the PROCEEDINGSin 1915 (26, pp. 24-33). Much the same ground was covered by thepresent Field Meeting, but greater emphasis was laid on the structuraldetails around Tintagel than on the former occasion.

By courtesy of the Council of the Geological Society of London,the twenty-five members attending were loaned galley proofs of theDirector's unpublished paper on 'The Tectonics of the TintagelArea, North Cornwall'. This provision was greatly appreciated byeveryone present.

Friday (evening), 11 MayThe party assembled at Tintagel, and at 8.15 p.m. the Director

gave a talk, illustrated by a coloured 25in. to one mile map, on thegeology of the area. Certain aspects of the structure outside the im­mediate area were briefly outlined, and the programme was dis­cussed.

Saturday, 12 MayThe party travelled by coach to Boscastle to commence a traverse

southwards along the cliffs to Tintagel, across the strike of the Culmand Upper Devonian beds. Penally Head on the north side of theharbour inlet was first visited for the excellent exposures of contortedCulm beds (Plate 4A), which indicate a north-north-westerly sense oftranslation of upper beds over lower, i.e., down the gentle dip of thestratification.

After crossing to the south side of the drowned estuary of theRiver Valency to see further examples of zig-zag folding, the profileof Penally Point (Plate 2A, facing page 32) was discussed. It was re­marked that the prominent 'nip' on the headland coincided with ahorizontal quartz-vein, and it was decided that the feature was moreprobably the result of present-day storm-wave erosion than an indi­cation of an earlier sea-level.

The structures showing on the walls of Eastern Blackapit wereexamined as the party followed the cliffpath and the Director pointedout that the chasm was largely due to erosion along vertical joint­planes. Further folding of the Culm was seen near Western Blacka­pit, and the contact between these beds and the top of the Devonian

PROC. GEOL. Assoc., VOL. 63, PART I, 1952. 4

50 GILBERT WILSON

was observed near California Quarry. The same contact repeated byfaulting was seen more clearly in the wall of Grower Gut.

The influence of inclined jointing and bedding planes in the de­velopment of hog's-back cliffs showed clearly from Boscastle OldQuarry (Fig. 5, page 31). Profiles due to inclined jointing may beseen towards the north, and those formed by dip-slopes can be ob­served by looking westerly. At Ladies' Window it was noted how flatprojecting ledges of Tredorn Phyllites were etched by weathering ontheir under surfaces. In some cases holes had been eaten from belowthrough beds two or three inches thick. This, it was suggested, mightbe due to the action of salt-spray or rain being driven violently upfunnel-shaped gullies in the cliffs during storms. The drops wouldthus strike the undersides of projecting strata with a much highervelocity than could be achieved by rain falling vertically. An alterna­tive suggestion put forward by the President (Mr. T. Eastwood) wasthat the circular hollows might be the result of solution by drops ofwater held by surface tension and hanging below the projecting sur­faces.

From Ladies' Window southwards the main units of the UpperDevonian sequence were crossed in descending order. These are theTredorn Phyllites, Trambley Cove Beds (at Trambley Cove), theVolcanic Series, now altered to chloritic schists, Barras Nose Beds,and the Woolgarden Phyllites. The Delabole Slates which lie belowthese last were not encountered on this day's traverse. The nine-inchband of creamy limestone, which occurs at the base of the VolcanicSeries in a prominent exposure south-west of Trewethet Gut, wasfound to be fossiliferous, and Dr. Scott Simpson collected some un­identified planispiral shells, possibly cephalopods. Incipient boudin­age structures in the same exposure were examined with interest(Wilson, G., 1951, fig. 3).

The path thence led down into Rocky Valley where pot-holesdevelop in the stream near its mouth, and several members recog­nised the scene as one of the better-known Geological Survey photo­graphs (No. A864).When the party had climbed out of the valley,theposition of the syncline of Barras Nose Beds between this locality andBossiney was indicated. This fold may be accurately located on thetrack which leads from the main road at Bossiney village down toBossiney Haven. Woolgarden Phyllites containing chloritoid werefound lower down the same track in front of a small ruined hut.

The plunging nose of the Davidstow Anticline was crossed alongthe cliff-path above Bossiney Haven to Willapark. From the latterthe relationship of the inclined jointing to the hog's-back cliff couldbe seen in the face and profile of Lye Rock; and on Wtllapark itselfevidence of incipient rock-falls was noted at the top of the verticalsouth-west face of the -promontory. Some members descended thesteep grass-slope below Willapark towards Gullastem to see the nor­mal step-faulting which cuts the Tredorn Phyllites and the under-

PROC. GEOL. Assoc., VOL. 63 (1952). PLATE 4

A.-CONTORTED CULM BEDS, PENALLY POINT, BOSCASTLE

B.-DRAG-FOLDS ASSOCIATED WITH NORMAL FAULTING,TINTAGEL.

[To face p. 50.

FIELD MEETING, WHITSUN 1951, TINTAGEL 51

lying Trambley Cove Beds. The unsheared epidiorite at the east endof Smith's Cliff was then examined and a return was made to Head­quarters.

Sunday, 13 May

The traverse recommenced at the east end of Smith's Cliff. After abrief explanation had been made of the manner in which the epi­diorite and Woolgarden Phyllites had been thrust over the VolcanicSeries and Trambley Cove Beds, the party descended to the foot ofthe cliff, noting features indicative of normal faults throwing downtowards the west, and the manner in which the epidiorite becamemore and more sheared as the bottom of the cliff was approached.A gouge-filled normal-fault fracture was exposed just above sea-levelin the re-entrant. As recent rock-falls had occurred hereabouts theparty was not permitted to linger, and were hurried around the cliffcorner westwards on to the broad ledge of sheared volcanic rockswhich forms the sole of the 'Flat Thrust' (T,). The ledge is withinreach of storm-waves which have eroded the actual thrust-zone into athin horizontal saw-cut. Cleavage within the thrust-zone, slicken­sides, elongation of spots, and the orientations of tension-cracksshowed that the upper rocks had moved north-north-westerly re­latively to those below. The welded contact of the epidiorite sheetwith the overlying Woolgarden Phyllites was observed in the craggyslopes immediately west of two big epidiorite boulders betweenwhich the party had to climb one at a time. Thence it became im­practicable to follow the thrust further, and the party scrambled up asteep grass-slope to rejoin the cliff-path.

Numerous examples of marine erosion along joints were notedwest of Smith's Cliff, and the importance of normal fault-zones inguiding wave-attack was pointed out in East and West Barras Gugs,Near the latter the conformable succession of Barras Nose Bedsabove the Woolgarden Phyllites of the Tintagel Thrust Unit was ob­served on the top of a small rise. A descent was thence made downthe grass-slope at the north-east corner of Barras Nose to see thethrust (T2 ) which brings the Barras Nose Unit with WoolgardenPhyllites at the base, on top of sheared Volcanic Series of the TintagelUnit. The cleanly-cut nature of this thrust aroused considerable com­ment, and the lack of shattering seen here was contrasted with thecrush-zone of the Flat Thrust (T I)'

From the path between Barras Nose and the east side of TintagelHaven, the importance of normal faulting was demonstrated. Thetrend of the Caves Fault Zone from West Barras Gug, under B1acka­rock peninsula and across the Haven to form the great shear-zone,which runs diagonally up the cliffs of the Island (Wilson, G., 1951.pl. 30), was clearly seen. The sloping face of the hillside here is largelyformed by gently inclined fault-planes which are the up-dip continua­tion of the Caves Fault Zone (Plate 3A, facing page 36). Barras Nose

52 GILBERT WILSON

Beds strongly contorted by down-dip movements and forming thehanging-wall rocks of these faults may be seen near the top of theslope to the west of the Castle Hotel (Plate 4B). The party then turnedinto the Trevena Valley and saw the line of the Castle Fault which, tothe north of the stream, is well marked on the valley side. The forma­tion of the 'Trevena Window', as a result of the stream cutting downthrough the Flat Thrust (T I ) into the Foundation Unit below, wasdemonstrated while walking up the valley; below the thrust is aprominent line ofTrambley Cove exposures; above it is the 'WindowFrame' ofsheared epidiorite and Woolgarden Phyllites .

Because of the state of the tide it was decided to visit TrebarwithStrand that afternoon instead of continuing investigations aroundTintageI. The party therefore walked southwards along the cliffsabove the old quarries in the Delabole Slates of the Tintagel ThrustUnit. At the top of Lill Cove some good, but distorted, specimens ofSpirijer verneuili were found in a little excavation in the TredornPhyllites. The Lill Cove thrust-zone (Wilson, G., 1951, pl. 29,fig. 1) was then visited by traversing horizontally for a few yardsacross the top of the grass-slope in the north-east corner of the re­entrant. The evidence of the north-north-westerly thrust-movementpresented here was acknowledged by all. The party then descendedinto Trebarwith Strand, and, while waiting for the tide to retreatfurtber, followed the road into Port William to note the normalfaults which cut the cliffs of Dennis Point, and their influence on thedevelopment of slope-over-wall cliffs (Fig. 1).

As the tide fell it became practicable to work northwards along thebeach. Deformed lava-fragments or pillows in the rocks wereexamined at several localities, and their directions of elongation andthe orientations of joints in the less plastic inclusions of the shearedvolcanic rocks were shown to be in conformity with the generalnorth-north-westerly movement. Below Lill Cove the Directorpointed out the longitudinal edge of a partly exposed boudin in thecliff-face. North of Vean Hole the Flat Thrust (T,) was seen in thecliffs, and the prominent zone of boudinage structure, which lies be­low it, was examined with interest (Wilson, G., 1951, pI. 29, fig. 2).The tide did not allow the party to reach Hole Beach where theThrust can be examined on the shore, so a return was made to Tin­tagel via Treknow. This enabled members to see once again the generalsuccession of the Foundation rocks in the Trebarwith Valley. Boudin­age structure and drag-folding were also seen in the old quarry androad cutting at the top of Treknow Hill.

Monday, 14 May

The morning was spent in Tintagel Haven and on the Island.Small-scale structures-tension-gashes, cleavage, drag-folding, etc.­associated with the normal faulting movements were demonstrated

FIELD MEETING, WHITSUN 1951, TINTAGEL 53

FIG. I.-Normal Faults on Dennis Point seen from the top of Lin Cove. (Drawnfrom a photograph).

on the Haven cliff near Merlin's Cave, but as the tide was high, it wasimpossible to pass through the Cave into West Cove. The structureof that Cove was therefore explained by the Director from the Castleon the Island; he stated that the present shape and the forms of thecliffs surrounding it were due to erosion undercutting the hanging­wall rocks of the Castle Fault, and that these rocks had broken awayalong the inclined fault-plane on one side, and along vertical jointson the other (Plate 3B, facing page 36). Other fault zones, such as thatin Merlin's Cave, had also been attacked by the waves. While somemembers examined the Castle ruins others descended to the IronGate where the rock-sequence seen on Barras Nose crosses theHaven and can be picked up again on the Island. The general line ofthe thrust (T3) between the Volcanic Series of the Barras Nose Unitbelow, and the overlying phyllites which form the main mass of theIsland, was also indicated. Comment was made on the manner inwhich the phyllites west of the Castle undulate in wide gentle folds.The westerly regional dip does not show here, and the central part ofthe flat-topped plateau, which is partly covered by head, is synclinalin character. A rainwater catchment-area has thus originated, in thecentre of which the Castle's well had been sunk.

After leaving the Island the party ascended from the narrowisthmus to the Outer Wards of the Castle situated on the mainland.While the long flight of steps was being climbed, attention wasdrawn to the normal fault-planes which locally coincide with theslope of the hillside. Drag-folding and boudinage structures were

54 GILBERT WILSON

both studied in the rock-face beside the Castle gateway (Wilson, G.,1951, fig. 9, p. 412). The top of the Castle Fault, with which is asso­ciated a plexus of quartz-veins, was also examined. From this point,on looking down the great slope into West Cove, it was clearly seenhow the dislocation had been utilised by the wave-attack which hadexcavated the Cove.

The contact between the epidiorite and the Woolgarden Phylliteswas studied near the Castle gateway. About fifty yards south of theCastle sheared epidiorite and overlying spotted Woolgarden Phyl­lites are exposed; the orientation of the elliptical spots in the phyllitewas observed, and numerous specimens were collected. Similar rockshad also been seen on the beach of Tintagel Haven beside the Water­fall, and, by comparing the difference in level between the two ex­posures, the throw of the intervening faulting was estimated asapproximately 250ft. towards the north-west.

After luncheon the party were conveyed by coach to Roughtor onthe Bodmin Granite. The route crossed Slaughterbridge, near Camel­ford, where the soft phyllites exposed by the roadside were pointedout as belonging to the lowest group of the Upper Devonian strataknown in this area-the Slaughterbridge Beds. They lie strati­graphically below the Delabole Slates. The coach stopped at Rough­tor Farm where Mr. C. E. Leese (of Camelford) introduced theowner, Mr. A. W. Patten, to the members. Mr. Patten was theninvited to give a talk on his experiences in counteracting mineraldeficiencies in pastures situated on the granite. He stated that it hadlong been realised that sheep and cattle (but not horses) could not begrazed continuously on the local granite-soil without contracting'moorsickness' or 'pine', which leads to rapid loss of weight anddeath; and, in order to keep the animals healthy, they had to be pas­tured periodically on killas or slate country for a while. Even sheepin the last stages of collapse would recover if they were carried downfrom the moor to the other pastures. Investigations indicated that theaisease was due to cobalt deficiency in the granite-soil, and experi­ments were conducted, as they had been in New Zealand, to see ifthis could be counteracted. This was first done by dosing (or drench­ing) once a week with a weak solution of cobalt chloride (7 mg. perhead). Lambs so treated rapidly put on weight, even though they werenot removed from the granite pasture-land. Twenty untreated lambswere kept as a control on the same grazing; of these nineteen died.Such treatment is impracticable with large numbers, so the pasturesthemselves are now dressed with cobalt salts which are extractedfrom the soil by the growing vegetation. The animals thus take inthe cobalt as they feed. Mr. Patten showed photographs of lambs fedon treated and untreated pastures, which clearly demonstrated thesuccess that he had achieved. The President thanked Mr. Patten forhis most interesting talk and congratulated him on the results of hisinvestigations.

FIELD MEETING, WHITSUN 1951, TINTAGEL 55

The party then walked to the foot of Roughtor where they weremet by Miss Dorothy Dudley, who had most kindly come fromexcavations farther south on the moor to conduct the party aroundthe 'Hut-Circles' which lie on the north-west slope of the Tor. MissDudley explained that she is excavating in three neighbouring sites ofhut-circles with which are associated field-systems. Her excavationsso far are not conclusive in this area, and as no pottery has as yetbeen found, a definite age cannot be assigned to the constructions,which possibly range from the Bronze Age to Medieval times. MissDudley pointed out that there were two types of hut-circle present.The first consists of huts 12ft. to 20ft.'in diameter, with fireplaces,saddle-querns, fire-cracked stones, and some flints. The second typeislarger, 30ft. to 40ft. in diameter, with well-madewalls. It is with suchhuts that the Celtic field-systems are linked. Miss Dudley also indi­cated two ancient earthworks or defensive lines on both sides of thesummit-ridge of Roughtor. The party then ascended to the top of theTor, where they looked at the view, investigated rock-basins hol­lowed into the upper surfaces of the granite-blocks, or discussed thegood examples of tor-weathering.

On the return journey Mr. Leese took the party to an exposure atGreylake of the long quartz-porphyry or elvan dyke which runsroughly parallel to the edge of the Bodmin Granite. Some specimensshowed little rosettes of tourmaline. In the same general localityfragments of silvery andalusite- and spotted-schists were picked up.Thence, after thanks had been expressed to Mr. Leese for his helpfulco-operation, a return was made to Tintagel.

Tuesday, 15 May

On this day it had been arranged to visit the Old Delabole SlateQuarry, Helsbury roao-stone quarry and Pentire Head. At Delabolethe party was met by Mr. Joe Setchell and Mr. J. A. Kent, Directorsof the Old Delabole Slate Co. Ltd.; and the general setting of thequarry was then explained by Mr. Setchell. The original quarrieswereworking in the reign of Queen Elizabeth I, and were sited on thesides of a valleywhich drained southwards. This hollow is now largelyburied beneath great dumps of waste upon which stand the present­day quarry-buildings. The excavation is a roughly D-shaped holesome 500ft. deep, and a mile in circumference. The straight westernedge is determined by a north-south zone of tear-faulting which dipssteeply to the west and is known as 'The Clay Lode' because of itsgouge-filledcharacter. This fault brings Delabole Slates of the quarryon the east against valueless phyllites (probably Tredorn) on thewest. The northern end of the quarry cannot be extended because ofthe local railway; so activity is at present towards the east. The dip iswesterly at about 15°,bedding is difficultto identify, but in weatheredsawn blocks it can often be seen lying at 10-15° to the cleavage.

56 GILBERT WILSON

Usually bedding can only be recognised in the field where rare thingrit-bands occur. These are commonly discontinuous having beenwrenched apart during the tectonic movements. In the bottom of thequarry such a grit-band shows a sharp recumbent isoclinal foldclosing downwards to the west and plunging south-south-westerly atabout 8° with its axial plane parallel to the slaty cleavage. The slate­belt is crossed by faults of various trends. Those which are parallel tothe main north-south Clay Lode, are usually referred to by the sameterm because they too contain clay gouge. Others of similar strike,which dip eastwards, are called 'shorters', and are cleanly cut frac­tures without gouge. Steeply-dipping minor dislocations called'ratchets' trend at right-angles to the grain of the slate, and are ofassistance in the extraction of slate-blocks. They cut across all theother structures in the quarry, including several inclined fault-planeswhich dip northerly and westerly.

The party were then loaded into two quarry-cars and lowered downone of the steep inclines to a bench about half-way down where aworking face was examined. Thence they were taken to see a gooddip-section of shorter and clay-lode faulting in the south face of thequarry. Here also is an ancient drainage tunnel about half a mile longbeneath the level of the original valley floor. It had been driven alonga clay-lode fault and is still in use. During the descent to the bottomof the quarry there was considerable discussion on the subject of'cats-paws'. These are occasional buff or pale brown dolomiticblotches in the slate. They are paper-thin, and may be roughlyrounded or elliptical in outline; where marked elongation of a'cats'-paw' is observed, it is found to be parallel to the grain of theslate. This, and their similarity to phenomena observed elsewhere,led the President to conclude that they were squeezed and drawn-outconcretions.

When the tour around the quarry had been completed, the dressingsheds were visited. Here the slate-blocks are cut by circular sawsarmed with diamonds, and are then split by hand into roofing slates.Large slabs are mechanically planed and smoothed for tombstones,stone shelves, etc. Good grade slate which is unsuitable for splittingis crushed and ground to make slate-powders used as fillers in as­phalt, roofing-felt, paper, plastics, etc. The powders are air-elutriatedand the finest has a grade-size of which ninety-five per cent of theproduct is less than IOjL in diameter.

At noon, after the President and Director had thanked Mr.Setchell and Mr. Kent for their courtesy and kindness, the party wenton to the Helsbury Quarry which lies due east of the village ofMichaelstow on the Camelford-Bodmin road. The quarry is workedby the County Council of Cornwall for road-metal, and permissionto visit it had been kindly granted by Mr. E. H. Collcutt, O.B.E., theCounty Surveyor. The quarry is divided by the entry-road; the nor­thern section is recent, and the southern section, which is deeper, is

FIELD MEETING, WHITSUN 1951, T1NTAGEL 51

older. Both sections were visited. The rock quarried is shown on theI-inch geological Sheet No. 336, as a narrow north-south intrusionof diabase, dipping, according to the map-section, west at about 45°.The mass lies within the metamorphic aureole of the Bodmin Gran­ite, and is formed of a hard tough rock, unlike many of the unmeta­morphosed greenstone intrusions of South-west England. It is coarsein texture and in the hand-specimen resembles a gabbro.

Beneath the microscope the rock is seen to consist of large crystal aggre­gates of fibrous, pale green amphibole (actinolite), some chlorite which givesultra-blue polarisation colours, and sphene, alI embedded in a finer grainedgroundmass of altered felspar. The groundmass felspars are crowded withfinely granular zoisite and little flakes of actinolite. The shapes of theplagioclase crystals can be faintly distinguished, but the best evidence oforiginal texture is presented by their relationship to the sphene. This mineraloccurs as crystals up to 5 mm. across, and poikilitically encloses lath-shapedinclusions of altered felspar. In a few cases the latter are stilI represented byindividual plagioclases. The shape of the inclusions and their relationship totheir host-mineral are indicative of an earlier doleritic texture in the rock,now lost in its alteration to epidiorite. The present coarsely crystalline ap­pearance of the rock is not original and is mainly due to the development ofamphiboles during recrystallisation.

Mr. Leese, who was kindly acting as guide, pointed out the sedi­ments which underlie the sheet-like intrusion on the north-east sideofthe excavation. These contain fossils and several specimens in ratherpoor state of preservation were found. The igneous mass above thesesediments is much broken by crush-zones containing laminatedgouge. Felspathic, epidotic and garnet-bearing veins occur in therock and some interesting specimens were collected. Recently, whilestripping the overburden from this quarry, the workmen found alittle group of flints, each of which was broken as if it had beenchipped for flakes. In the old southern section of the quarry attentionwas drawn to the depth of weathering, which reaches 20ft. in places,penetrating down joint-planes and shear-zones, and thus enclosinglarge weathered boulders in reddish-brown clay.

From Helsbury 'Quarry the party was conveyed to the laneleading to Pentire Point with its great sea-cliffs mainly composed ofpillow-lava. Several exposures, including those illustrated by Deweyin the PROCEEDINGS (1914)and in the South-West ofEngland, RegionalGuide, were seen. After a brief visit to the extreme end of the Point.where dolerite-sheets cut sheared black slates, a return was made tothe coach. Here the President proposed a vote of thanks to theDirector, who, he also mentioned, had acted as Field Secretary forthe Meeting. The party then returned to Tintagel.

EXPLANATION OF PLATE 4A.-Contorted Culm Beds, Penally Head, Boscastle. The zig-zag folds were

formed by a movement of the upper beds towards the left (north-north­easterly) relative to the lower beds. The axial planes are nearly horizontal.

58 FIELD MEETING, WHITSUN 1951, TINTAGEL

B.-Drag-folds in Barras Nose Beds associated with normal faulting, west of theCastle Hotel, Tintagel, The folded beds form the hanging-wall of the faultswhich can be seen on the right-hand side of the plate. Quartz-filled fracturesshow up white against the dark Barras Nose Beds.

RELEVANT LITERATUREREID, c., G. BARROW and H. DEWEY. 1910. The geology of the country around

Padstow and Camelford. Explanation of Sheets 335 and 336. Mem.Geol. Surv.

DEWEY, H. 1909. On overthrusts at Tintagel (North Cornwall). Quart. Journ.Geo/. Soc. Lond., 65, 265.

---. 1914. The geology of North Cornwall. Proc. Geo/. Assoc., 25,154.---. 1915. Report of an Excursion to North Cornwall. Proc. Geol. Assoc.,

26,24.WILSON, GILBERT. 1951. The tectonics of the Tintagel area, North Cornwall.

Quart. Journ. Geol. Soc. Lond., 57, 263.---. 1952. The influence of rock structures on coast line and cliff develop­

ment around Tintagel, North Cornwall. Proc. Geol, Assoc., 63, 20.