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Excursion to the St. Davids District, South Wales
21 5
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES. APRIL 13TH TO 22ND.
Directors:
J.
EASTER, 1911.
F. N. GREEN, B.A., F.G.S., AND PROFESSOR O. T. JONES, D.Sc., M.A., F.G.S.
Excursion Secretary: ARTHUR L. LEACH. (Rtport by MR. J. F. N. GREEN.) THE official party left Paddington at 8.45 a.m, on Thursday, April r jth, and arrived at 2.15 p.m. at Fisbguard Harbour; the long drive (16 miles) to the headquarters at the City Hotel, St. Davids, occupied nearly three hours. During the first four days of the excursion sixty-four members and friends were present. The principal object of the excursion was to study the complicated structure and inter-relations of the pre-Cambrian and Cambrian rocks of the St. Davids area; but the programme provided also for visits to younger rocks at Abereiddy, Llanvirn, Ramsey Island, and Newgale. The glacial and other surface geology of the district was incidentally studied during most of the tours. Upon the suggestion of Dr. J. W. Evans, who had undertaken the organisation of the Geological Section of the Coronation Exhibition, many of the members agreed to lend specimens obtained during the visit in order that a collection fully illustrative of the work done on a "long" excursion might form a special feature of the Association's display at the Exhibition. The arrangement of the maps, diagrams, photographs, rock specimens, and fossils for this exhibit was undertaken by Dr. Evans. The weather during the first four days was delightful, but on Tuesday a strong south-westerly gale sprang up and the projected visit to Ramsey Island had to be abandoned, while the Southern Coast and Newgale excursions were somewhat shortened in consequence of adverse weather conditions; otherwise the programme was fully carried out. April 14th. PRE-CAMBRIAN AND CAMBRIAN ROCKS OF THE CITY, WHITES AND BAY, AND PORTH SELE.
(Directorr J. F. N. GREEN.) The party first examined, in a quarry near the Church Schools, fine-grained spherulitic quartz-porphyry cut by basic
216
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
dykes, both intrusive in porcellanite and tuffs of Upper Pebidian age. The quarries lie in the great faulted anticline of preCambrian rocks which forms the backbone of the St. Davids promontory, and are excavated in both banks of the Alan, which flows here nearly parallel to the anticlinal axis. On the western side of the Alan, Lower Pebidian tuffs were examined in several excellent sections. The pale pulverulent tuffs on the east bank are regarded by the Director as part of the highest Pebidian series, and if this view is correct a strong reversed fault must separate them from the purple" halleflintas" in the crags near the Bridge. Above the latter come coarse felsitic agglomerates (Lower Pebidian) quarried near Penrhiw, which attracted much attention. From the top of Penrhiw the rocky gorge of the Alan was seen to open northwards into a broad, shallow valley, apparently continuous with a large drift-filled depression lying under the northern hills; the drift extends westward to Whitesand Bay. The investigation of this drift-filled basin should throw much light upon the age and mode of excavation of the Alan gorge. Following the path over the Burrows towards Whitesand Bay, many of the party were interested to see towering over them the crags of Carn Llidi, the parent mass, as the Excursion Secretary pointed out, of some of the large boulders which they had seen two years previously lying upon the extreme southern cliffs of Pembrokeshire. On the north side of Whitesand Bay a fine exposure of boulder-clay was seen, abutting on the north against the Upper Cambrians, and overlain by blown sand. Large boulders of coarse norite and gabbro were abundant in the lower part of the drift; amongst the smaller pebbles were flints and quartzite, and on a subsequent visit some members of the party obtained fragmentary marine shells. The slates to the north of the boulder-clay were seen to be capped by stratified sands and gravels, which were thought by some members to be referable to a raised beach, but on April 15th striated pebbles were found to be quite common in these gravels. The stratified beds do not seem to have been previously noted, and must be linked rather with the glacial beds than with the raised beach series. Further north the .Neseuretus-beds and other Arenig rocks were seen to be invaded by many sills, one of which formed a conspicuous wall-like mass on the beach, where the bedding was vertical. Leaving this flank of the bay, the party crossed the sands to the southern cliffs, noting on the way some deep bowlshaped hollows, resembling potholes, in the cleaved Cambrian rocks. These holes are probably caused by wave-action agitating pebbles in depressions, originally slight. The polished sides gave beautiful sections showing cleavage perpendicular to ribbon bedding. The beds were considered by Professor Jones to
PROC. GEOL.
Assoc.,
VOL.
XXII
PLATE
XXX.
/' /r,'/" "J' F . Ir. T1t n brid.:.· ~ .
FIG. I.-SECTION AT vVHITESAND BAY, PEMBROKESHIRE. BLOWN SAND, HEAD AND BOULDER-CLAY RESTING ON CLEAVED CAMBRIAN SLATE.
FIG. Z.-TRENCH NEAR PORTHCLAIS, SHOWING THE AND GRANOPHYRE.
(The hammer-head indicates the junction.) To face paJ;c 216.
EXCURSION TO THI£ ST. DAVIDS DISTRICT, SOUTH WALI£S.
217
belong to the top of the Solva (Lower Paradoxidian) beds. Their surface rises gently, forming the southern bank of a valley, now buried in drift, which must clearly extend to some depth below sea-level. (See the illustration, PI. XXX, Fig. 1.) An east and west fault brings up Upper Pebidian, consisting of highly sheared tuffs, at the southernmost part of the bay. A coarse conglomerate, the actual base of the Cambrian, lies here at a dip of about 50Q on the Pebidian. A small thrust, with quartzveining, comes in about two feet below the junction, under which a strong spring bubbles up through the sand. A little lower down in the series a basic sill with hematite specks, pseudomorphic after olivine, breaks through the tuffs. This complex is beautifully sectioned in the cliff. After examining the contents of the conglomerate, which contained pebbles, several inches in length, of quartzite, vein-quartz, and Pebidian tuffs, with smaller bits of jasper, porphyry, etc., the party climbed up to the sandhills, noting numerous springs at the junction between the sand and the underlying drift, and walked round into Porth Sele to view a basic dvke intrusive in the same conglomerate. The dyke had been greatly sheared and squeezed, showing movement later than the intrusion, which was itself later than the main faulting. Several hand-specimens were obtained, exhibiting perfectly the junction of the conglomerate and igneous rock. On the south of the bay were seen the Caerfai beds, green sandstone above the conglomerate, purple shale with thin felspathic tuffs, and purple sandstone. Part of the shale was faulted out, but the remainder was assiduously searched by the party for fossils, unfortunately without success. The use of a chapel at Rhoson Ganol had been obtained for tea, but it was much more pleasant to lay it out on the grass. After tea the next section visited was on Carn Rhoson, where a coarse agglomerate (B2) of Lower Pebidian age, composed chiefly of green vesicular lava in a red matrix with scattered boulders of fluidal rhyolite, was exposed, with a mass of olivinebasalt breaking across the bedding, thus clearly proving the intrusive character of the basalt, in spite of its close resemblance to a flow. A magnificent view was unfolded from the top. To the west, across Ramsey Sound, Ramsey Island presented a long platform, dominated by three piles of hard rock; to the south, beyond St. Bride's Bay, the platform could be seen, stretching for miles almost unbroken, except for Marloes Beacon, and passing on to Skomer Island; near the foot, Pwll Trefeithan lay in a shallow depression in boulder-clay. During the walk home most of the party diverged to the ancient camps of Clegyr-foia and Castell, and thus had an opportunity of viewing the Alan Valley, and a small exposure of the famous "Dimetian," which was to be studied more closely on Monday.
218
EXCURSION T O T HE ST. D AVIDS D ISTRI CT , SOUT H WA LES .
Apri! r 5th, LOWER AND MID DLE CAM BRIAN BEDS OF THE S OLVA VALLE Y,
Directors: M R. J. F. N. GR EEN AND PROF ESSO R O. T. J ON ES.
On th is morning Mr. Green .had to announce that, owing to the stat e of a tra ck along which it had been proposed to take the party, a detour would be necessary, "increasing th e walking distance to between nine and ten miles. T he news was received with groa ns. After walking through Caerfarchell, the party stopped at the edge of the Solva gorge to examine exp osures of th e lowest parts of the Caerfai beds, which are th ere thru st over the lower Solva. The Caerfai basal con glomerate and gree n sandstone are similar in all respects to the same beds at Po rth Sele. Professor Jones here explained his views as to the form ation of the gorge. He agre ed with Mr. Green that the original direction of drainage was westward, alon g th e slope of the Pli ocene platform ; but he considered that the di version was caus ed in preglacial times by th e cutting back of a later north to south drain age initiated by th e ad van ce of th e sea into St. Bride's Bay, and that th e absence of d rift in th e valley was due to the steepness of th e sides. H e pointed out that dri ft occurr ed in similar valleys elsewhere, as for example th e Cae rbwdy Valley. Afte r a long and int erestin g discussion a mong the memb ers, a move was made to th e ne ighb ourhood of Cae rforiog Bridge. Along the north side of th e gorge, which here runs east to west, are bold crags of Upper Pebidian, composed of fine-grained silicified ashes with bands of coarse congl omerate, dipping 35° north, containing pebbles of rhyolite and acid t uff up to a foot in d iameter. These pre-Cambrians were thru st over middl e Solva beds and the river ran along the thrus t-plane, cutting back the hard Pebidians as a stee p esca rpment, which overlooked a gentl e slope of Solvas, covere d with cultivation. R eturning down-stream , the part y (her e numbering sixty-nine) crossed the outcrop of th e Selva beds to th e horizon mark ed by a red band, in which P rofessor Jones demon strat ed a prett y example of a pitching anticline. Near this point and at a qu arry towards Mid dle Mill, th e exposures consisted of int erba nded shale and sandstone, exhibiting in great perfection the var iat ion of the slope of the cleavage-plane with the texture of the rock ; a nd some large spec imens were carried away. Opposite Mid dle Mill a laccolite-Iike mass of dolerite cam e in, th e ac tual junction, following a bedd ing-plane, being exposed for severa l yards in the quarries. About a mile further down-stream, Pr ofessor Jones d rew att ention to a han ging valley of unu sual type near Tre- Cadwgan. For some reason water had cease d to flow down in any apprecia-
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
219
ble quantity, and consequently wash and debris had accumulated in the bottom of the course, filling it up to a considerable depth, The party then crossed the river and walked along the southeast foreshore of the beautiful little harbour of Solva to the fossiliferous locality in the zone of Paradoxides hicksiz" discovered by Professor Jones and Mr. H. H. Thomas, where fossils were obtained by members in quite unexpected profusion. Numerous well-preserved specimens of 1J1icrodisC1ts punctatus, with Agnostus, Conocoryphe, and brachiopods, were found, but the prize of the day was a nearly perfect Anopolenus, not hitherto recorded from this place. The fossils occurred in a hard grey sandstone, classed by Hicks with the Menevian, but much more closely allied lithologically to the Solva series. Indeed the Directors thought it probable that a non-sequence occurred between it and the typical Menevian shales. Crossing to the north-west of the harbour, a large mass of dolerite was observed with junctions along bedding-planes, and it was pointed out that on the opposite side of the harbour nothing was seen but two petty sills, so that the laccolitic character of the intrusion was evident, Near this place, behind the lifeboat house, the Menevian was attacked with fair success, the black shale yielding Agnostus barrandei, and other agnostids, Obolina, and a fine pygidium, ascribed provisionally to Erinnys, Climbing up the cliffs above, an exquisite prospect of the harbour and coast lay before us. It was observed that the waveaction within the harbour had only cut a comparatively insignificant nick in the slopes of the drowned valley, which was merely a continuation of the Solva gorge. During the walk home, a short stop was made at a spot known as "Nine Wells," where quantities of water were thrown out near the meeting of the great Porthyrhaw and St. Davids faults. This has led to the establishment of a pumping station to supply water to St. Davids. Several good sections of Menevian were examined, and a visit paid to the grounds of the County School to see the specimens of acid volcanic conglomerate found in boring the well, which were obviously identical with the Pebidian ot the upper gorge of the Solva River. April 16th.
In the morning the party separated, some attending the Cathedral service, others going to the South Coast, Abereiddy or Penberry, and several to Whitesand Bay to study and photograph the glacial gravels. After lunch a party of about twentyfive went with Mr. Green to Carn Llidi, where several of the detached expeditions were met. In a quarry on the south-east flank above the farm fresh specimens of coarse norite were secured; and slightly to the north of this the junction of the igneous mass with Arenig shale was well exposed. The chilled
220
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
edge of the great igneous intrusion of Cam Llidi was fine-grained with large felspars, a porphyritic quartz-enstatite-dolerite, which had baked the shale to a splintery bluish rock. Clambering up to the top of the Cam, 595 feet above the sea, a splendid view of the northern and western coasts appeared. The summit was composed of a coarse gabbro with veins of aplite. An interesting problem which the party hoped to solve was that of the thickness of the ice from the Irish Sea. Though weathering had proceeded too far to leave definite traces of glacial action on the actual top, the general form suggested that the whole had been covered. Between 400 and 500 feet rounded hummocks, probably ice-shaped, were common; and strire were noted near the higher of these contour-lines. The general opinion was that ice had certainly overwhelmed the hill. Between the Cam Llidi mass and that of St. Davids Head a valley was found, truncated at both ends by the sea. The section at the Porth Melgan end showed that the original valleybottom must be below sea-level, and that the depression had been partly filled by boulder-clay, in which Mr. Leach had discovered shells and a large angular block of flint. Crossing this feature and a number of ancient stone walls, enclosures and circles, the party halted on the triple stone rampart fortifying the" Promontory Camp" of St. David's Head to listen to an interesting account from Mr. Leach of the Pembrokeshire camps and of Mr. Baring Gould's excavations, which proved this camp to have been in use well into the Iron Age. After a short examination of the gabbro forming the Head, which was found in places to be full of parallel veins of aplite, the members strolled back, enjoying a rich view of the city and surrounding country. April 17th. THE DIMETIAN GRANOPHYRE AND ITS RELATIONS TO THE CAMBRIAN AND PEIlIDIAN AT PORTH-CLAlS AND PORTH-LISKY.
Director:
MR.
J. F. N.
GREEN.
Before starting, the Director distributed copies of a map on the 25-inch scale of the area near Porth-clais Bridge, published in the Quarterly Journal of the Geological Soddy for August, 1908. On arrival at this spot, the first exposure visited was a quarry in granophyre, in which a vein of fault-breccia was being dug as road-binding material. The band of fault-breccia could be followed northward by a strip of cultivation, which had heen already pointed out during the walk down the Alan Valley. The cultivation followed the soft fault-rock between stony moorland. Southward the same fault-line could be seen as a depression run-
PROe. GEOL.
Assoc.
VOL.
XXII.
JOINTING IN VERTICAL SaLVA BEDS.
PLATE
XXXI.
OGOF-LLES:JG~. PEMBROKESHIRE
To race jag.:
220.
EXCURSION TQ THE ST. DAVIDS DISTRICT, SOUTH WALES.
221
ning Up the bank, where, indeed, exposures enabled it to be fixed within a few inches. A little corner of unaltered Cambrian sandstone was faulted into the floor of the quarry. Standing on this the Director described the character of the granophyre, drawing attention to its highly quartzose nature, the complete destruction of its ferro magnesian constituents, its peculiar fissile weathering, and the presence of brecciated bands, distinct from the later fault-breccias j he then expounded his views on the structure of the area, pointing out the features that defined the lines of the various faults. The party then walked down the north-east bank of the river to see two little leucophyre dykes, with prismatic jointing, about thirty and six inches thick respectively, intruded into the granophyre. Immediately south of this place the granophyre could be seen thrust over inverted Cambrian sandstone, belonging to the lower part of the Caerfai beds. Crossing the stream, the trench across the unconformable junction of the basal conglomerate of the Cambrian and the underlying " Dimetian " granophyre was found to be in good condition, having been much enlarged during a former visit by a party from Oxford. The appearance of the rocks in this excavation was extremely deceptive, as at a first glance there seemed to be coarse reddish conglomerate resting directly on pale decayed granophyre. On closer inspection, the coarse conglomerate proved to be washed-down blocks, and the actual junction to be in the middle of the pale face underlying it. The distinction between the basal grit and the igneous rock could, on close observation, be made by means of the pink quartzite pebbles in the former. Several excellent specimens were obtained showing the actual contact, and photographs were secured (see Pl. XXX. Fig. 2). Conscientious visits were then paid to a number of other exposures defining the fault-lines. One, a quarry by the roadside near the trench, showed a little wacke dyke along the thrust, enclosing lumps of fault-breccia j another, continuing the thrust previouly seen on the opposite side of the river, exposed the undulating slicken-sided floor of the thrust composed of grit, which, according to the Director, was much mylonised. Climbing the bank, the members overlooked the tiny harbour of Porthclais, in the cliffs of which the tuffy red shales of the Caerfai beds could be seen repeated by little thrusts. The cliffs were then followed to a point above Ogof-llesugn, where everybody sat down for rest and refreshment. The view here was of great interest. To the left stretched Solva sandstones, the bedding vertical and parallel to the coastline. They were intersected by three equiangular sets of joints, perpendicular to the strata, and the sea, working along these four planes of weakness, had fretted the rock into fantastic shapes. (Pl. XXXI.) To the right ran out to sea a fault face of moulded granophyre with many pegmatite veins j immediately
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Ig.-DIAGRAM EXPLANATORY OF THE SECTION ON PLATE
XXXII.-.J. F. N. Gran.
PROC. GEOL.
Assoc.,
VOL.
XXI£.
PLATE
XXXII.
PIIQ/ I} I>y 1:". /I ' . 7,fll bYl t{g c.
SECTION ON THE WEST SIDE OF ST. NON'S BAY.
(This view is explained by the Diagram, Fig. 19, on page
222.)
To face page
222.
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
223
below, wedged in between these dissimilar masses, lay a confusion of varicoloured rocks, which it was now the object of the visitors to examine. Some rock-scrambling was necessary, but on reaching the shore the climbers were repaid by a remarkable section. A mass of coarse Cambrian conglomerate, with a high dip and a little stained Pebidian still underlying It, had been faulted in between the Solva beds and the granophyre, the latter being intersected by a quartz-porphyry dyke. Subsequently basic igneous material had injected the faults and the neighbouring rock, so that now lenticular masses of the various components were imbedded, at first sight confusedly, in a continuous boss of greenstone. Crossing a ridge of granophyre, the party halted for a few minutes by the hornblende-picrite erratic described by Professor Bonney. The block is protected by an iron cage from earnest collectors. Owing to the peculiar petrographical characters of the rock, there was a good deal of doubt as to its origin, though the Director considered that it probably came from Anglesey. A hundred yards farther on, a steep track descended to Porth-lisky. This track was actually on the junction of the Dimetian, which ran seawards as a fault-cliff, with the soft pulverulent tuffs of the highest Pebidian, which had been scooped out by wave-action. As usual, a basic dyke had risen along the fault-line, enclosing masses of breccia. The party spent some time studying curious sheared tuffs, brightly-coloured pink, white, purple, yellow, and lilac, and some examples of ramifying wacke dykes. The west side of the bay was formed by a cliff of hard lower Pebidian, on the same horizon as the Penrhiw quarries, thrust against the soft upper beds. It was surprising to find, just behind the inner part of the bay, a knoll of Cambrian sandstone, whose unsheared condition, though caught between faults, suggested that most of the dynamic action on the Pebidians must be pre-Cambrian. In a field above Porth-henllys, numerous contacts could be seen of a coarse agglomerate with a pale flinty rock of rhyolitic appearance. The Director explained that this was one of the critical sections on which he had relied as evidence that the felsite, termed by himself the" schistose sill," was truly intrusive. Near the contact the felsite was fine-grained, apparently chilled, and it tongued in among the tuffs of its roof; but since Mr. Jones and Mr. Thomas had found rhyolites on much the same horizon at Newgale, he had begun to think that perhaps the field evidence in the 81. Davids area had been wrongly interpreted. Those present then examined carefully the sections before them, and the general conclusion arrived at was that the felsite was certainly intrusive at this point. Here and in other exposures seen a short way farther on, the felsite developed a schistose structure which seemed too regular for flow-structure. The Director
2z4
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
attributed it to pressure while the sill was still hot and viscous (protoclastic structure). Approaching the little promontory of Pen-y-foel, a pair of foxes were put up, the vixen bolting through the middle of the crowd. Mr. Fearnsides captured and, with due precautions against sharp teeth, exhibited to all a beautiful cub. The captive released, it was seen that at this point the beds of the lower Pebidian, classed as A3, Br, and Bz, were thinner, but at the same time coarser, than to the north, while the schistose sill had thinned out, so that this interesting part of the succession could be studied conveniently. A3 consisted mainly of broken quartz and felspar in an indefinite epidotic matrix, with scattered rounded pebbles of rhyolite of several varieties, and angular boulders of hornblende-andesite. The Director suggested that possibly this rock might throw light on the genesis of some of the quartz-epidote schists of the Highlands. Br and Bz, mainly composed of vesicular trachytic material in a chloritic matrix with large boulders of a fluidal rhyolite, resembled the same horizons as seen at Rhoson, but were coarser; some included fragments being over a foot in length. The end of the promontory was formed by a neck-like mass of augite-andesite, which gave off irregular veins into the neighbouring tuffs. A similar mass of augite-andesite, but on a much larger scale, was found to form the cliffs near Carnarwig, the next point visited. Part of this was, however, highly vesicular. Attention was also drawn to a curious honeycomb weathering of the basic rock. Portions of this cellular rock were used locally for ornamental purposes. It seemed only to develop on sea-cliffs. Scrambling down, masses of coarse Cambrian conglomerate were found, imbedded in the intrusion. One of these blocks, perhaps thirty feet thick, showed fluted sides where the igneous rock had been washed away. Comment was made on the similarities of this fine section to that at Ogof-llesugn, and it was pointed out that this sheet of lava-like rock must be, not merely post-Cambrian, but post-faulting. A picnic tea was spread on the grass at Treginnis-isaf farm, with geese and cows as spectators. On the way home some more exposures of the "schistose sill" were seen, and the Director gave an account of the field evidence for its supposed connection with the margin of the" Dimetian." April 18th. CAMBRIAN ROCKS OF THE SOUTHERN COAST.
Director: ]. F. N.
GREEN.
Rain and a heavy gale upset the arrangements for crossing to Ramsey Island, so an attempt was made to carry out Wednesday's
PROC. GEOL.
Assoc.,
VOL.
XXII.
PLATE
XXXIII.
Thoto hy E. TV. T1fnbril~~)·e.
FIG. I.-THE STORM-BEACH AT NEWGALE, RESTING ON THE MAIN ROAD.
1'Iw h b) E .
tr.
TII" b n ,(s:~ .
FIG. 2.-BROKEN OVER-FOLD IN CAERllWDY BAY.
To face fage
224'
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
225
programme instead. Walking to Pont Clegyr, a mile east of the city, a large exposure of boulder-clay was seen, resting against volcanic conglomerate with bands of silicified ash. It was pointed out that this was the same horizon of the upper Pebidian as that seen in the upper Solva gorge, the County School Well, and Treginnis-isaf. Turning down the valley, the party entered the halleflinta quarry. This quarry is in the horizon C4 of the Caerbwdy series, and showed bedded tuffs, very fine-grained, with numerous colour varieties, and some felspathic bands. The rock had a perfect conchoidal fracture, some specimens resembling blue glass. Farther down the valley the basal Cambrian conglomerate appeared, in a rampart-like mass, very coarse in places, with quartzite and porphyry pebbles over a foot in length. The wind was now so strong that it was decided to make for Porthyrhaw by an inland route. It was hoped that the eastern cliffs of the inlet would be sheltered, but on arrival the spray was seen flying over them in showers. However, four intrepid spirits crawled warily over the slippery rocks to investigate the historical localities, and, in spite of the wet, were rewarded by several Agnosti, a Lingula, and fragments of Paradoxides davidis. A splendid specimen of the latter was noticed, with a headshield nearly eight inches in diameter, but it was impossible to get it out without crumbling. All these fossils occurred in a band about the middle of the black shale of the Menevian. The higher fossiliferous bands, just below a little sill, could not be reached. It was too wet to see Protospongia, of which several fine specimens had been secured a few days before. Meanwhile the gale increased and the party separated, some returning to St. Davids, some going round to Caerfai, while nine disciples followed the Director over the cliffs. After crossing a glimpse was obtained of a fine broken overfold, dissected out by the sea so that a folded surface ran out into the waves like a huge cylinder. (See PI. XXXIII, Fig 2.) The view-point was a promontory fortified by three lines of earthworks. The main party avoided Caerfai Bay, where, unknown to them, several members were fossil-hunting, and passed on to an exposure of Cambrian conglomerate, containing a lenticle, seven inches thick, of a heavy sandstone, against which the coarser pink conglomerate was false-bedded. This black sandstone has been shown by Mr. W. Jones, of the Royal College of Science, to consist of clastic titaniferous hematite. (See post p. 232.) Beyond this point, quartz-porphyry and silicified ashes appeared on the cliff-track; but the latter were better shown in the track down to the basin in St. Non's Bay. The strata here, though much shattered, were obviously identical with the bedded halleflinta of the Caerbwdy; but instead of some 500 feet of Pebidian intervening between the halleflinta band and the basal Cambrian, only a few yards came in here. The conglomerate,
226
EXCl'RSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
dipping vertically, lay in contact with stained tuffs, the red ferruginous staining extending about six feet from the junction. Scrambling round into the little inlet west of the track, a remarkable geological illustration was displayed. The inlet was shaped like a cube, less the top and one side, the rocks being fully exposed on the remaining three sides and floor, so that the complicated structure could be seen in the solid as in a model. The halleflinta and tuffs, penetrated by a dyke of quartz-porphyry, were thrust over the conglomerate and vertical green sandstone, the fault-plane cutting across both. A thick fault-breccia occupied this plane, and up this a basic dyke, now altered to wacke, had risen. In the floor the porphyry could be seen running up to and terminating abruptly at the fault. (PI. XXXII.) Climbing back to the top of the cliff, the Director pointed out the .arch so often figured in geological publications. Its components stood out very clearly, when compared with the photograph. PI. XX (ante p. 124.) Attention was drawn to the fine cliff-section in front, along which the following rocks could be seen in order :-Quartz-porphyry, unstained tuff, stained tuff, basal conglomerate, green sandstone, red shale, purple sandstone, Solva beds. After visiting the ruins of St. Non's Chapel, the party walked to Penygarn, the type exposure of the Dimetian, where some examples of micropegmatite veins and a chloritic brecciated band were noted. An attempt, doubtfully successful, was made to find the marginal porphyry in a wet road, after which the members reached home in a draggled and disreputable condition, but full of energy. During the evening a hearty vote of thanks to the Directors was passed by the assembled party, on the motion of Professor Watts and Dr. J. \-V. Evans; and to the Excursion Secretary on the motion of Mr. Whitaker and Mr. VassalI. April 19th.
During the wet morning, several hours were spent in examining, under the guidance of the verger, the Cathedral, the Close Gate, and the ruins of the College and Palace. The chief building stone is a handsome purple sandstone from the upper Caerfai beds, which, when used in internal work, acquires a curious white pulverulent surface. In the afternoon the weather was worse than ever, and though an effort, prompted largely by a desire to enjoy the kind hospitality of Mrs. Baker, of St. Justinian, who had invited the whole party to tea, was made to see some exposures on the west coast, nothing much couldbe observed but mud and swamped boats.
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
April
227
20tft.
THE ORDOVICIANS OF LLANVIRN AND ABEREIDDY.
Director:
PROFESSOR O. T. JONES.
The party drove five miles to Mr. Perkins' farm at Llanvirn, and began operations in a small quarry near Nant, yielding a pretty ash-spotted slate, but no fossils. Proceeding northwards to Hicks' historical quarry, about a quarter of a mile north-east of Llanvirn-y-fran, fossils were soon found in iron-stained slates. They included Didymograptus bijidus, D. nanus and D. mi:holsoni, Climacograptlls confertus, Diplograptus dentatus, Trinudeus, Ampyx, Caiymeue, Placoparia, Orthoceras, Orthis, and Pleuratomaria. Towards the shore, opposite the mill, quantities of fossils were to be found in an old tip, including DIdymograptus bijidus, D. acutidens, and another species, perhaps D. patulus; very perfect Ctimacograpti and dendroid graptolites. Close to this point was a little quarry (with a bonfire premonitory of tea), in which Professor Jones explained the stratigraphy of the neighbouring sections, pointing out the various fossiliferous zones as exposed along the cliff. The quarry cut the base of the ash separating the btjidus and murchisom:zones. The former fossil occurred plentifully just below the ash-bed, which contained angular vesicular lapilli and flakes of slate. The Director then led the way to the shore, where Mr. Leach pointed out a remarkable old storm-beach observed by himself a few days before. It consisted of rolled flakes of slate with a few pebbles and shells, all cemented together by iron derived from the slates. The old beach was overlain by talus, and both had been eroded by a recent storm, leaving fine sections. South of this point the cliff was formed of Upper Llanvirn black slates, containing an amazing abundance of graptolites. Slabs lay everywhere covered with hundreds of Didymograptus murchisoni and of a Climacograptus, and all fell to picking out the richest and most perfect specimens. Other organisms found were Cryptograptus tricornis, dendroid forms, Ogygia, Trinudeus, Orthis, Lingula and other brachiopods. Above these slates Ogygia buchii and Favosites ftbrosus were found in harder calcareous beds. Walking northwards across the bay to the great Porthgain slate quarries, the trilobite beds were attacked, producing Ogygia, Asaphus, Trinudeus, Agnostus, Obolus, Conularia homfrayi and a gastropod. Looking north from this position, a bare cliff-section could be seen, composed of two very dissimilar rocks, one a black slate, the other, weathering yellow, obviously intruded into the first. As it would seem that the igneous rocks of Porthgain had hitherto been supposed to be contemporaneous, this relationship was unexpected, especially as the fragments lying about were PIWC.
GEOL. Assoc., VOL. XXII, PART 4, 19II.]
19
228
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
bighly vesicular; but Mr. Green took occasion to observe that, contrary to the usual view, he had found that in Dewisland highly vesicular rocks were invariably intrusive. A small party walking round to examine the junction, this suggestion was confirmed. The basic, yellow-weathering rock was found to be clearly transgressive at all boundaries, with a chilled edge baking the contiguous shale. Nevertheless, it was coarsely vesicular almost throughout. While this question was under examination, Professor Jones picked up a bit of slate with Didymograptus bijidus. The neighbouring slopes were immediately attacked and the zone-fossil was found plentifully in situ, proving repetition on a large scale. Ogygia was also found, and tuffs precisely similar to those in the junction-quarry on the south of the bay. Considerably excited by this unexpected discovery, the group hurried on in the direction of the headland, Trwyn Castell. A trench had been cut across the promontory, exposing boulder-clay with many erratics (including Old Red Sandstone and flints), resting on what appeared to be a conglomerate, really a compacter variety of the vesicular andesite with spheroidal weathering. The same material continued to the end of the headland, where it invaded a beautifully banded pink rhyolite. As far as could be seen in a hasty glance, this rhyolite strikes with the slates and ashes. Returning to the mill, a gipsy tea was taken and specimens compared. The opportunity was seized to give a hearty vote of thanks to Professor Jones. Thanks were also voted to Mr. Perkins of Llanvirn farm, who had greatly assisted the Association in identifying fossil localities and in making arrangements for tea. April zsst, THE PRE-CAMBRIAN AXIS OF NEWGALE AND BRAWDY.
Director:
PROFESSOR
O. T.
JONES.
A damp drive of five miles took us to the neighbourhood of Pointz Castle, where a quarry had been opened in a quartzporphyry much resembling that found n.ear the cathedral at St. Davids. Close to this were numerous crags of banded rhyolite, weathering a dead white. The relationship between these two types was doubtful, but probably the porphyry was intrusive. The Director explained that these ancient rocks came up in a horst-like mass of pre-Cambrian and lower Cambrian, separated from the upper Cambrian and Ordovician by faults. Driving on again, the party descended to walk along a lane, in which the green sandstone of the Caerfai beds was repeatedly exposed, into Brawdy farmyard. The section here had been somewhat obscured by mud, but the line of junction of a granitic rock resembling the "Dimetian" with a rough grit could be traced without difficulty. In places quartz-veins occurred at or
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
229
near the junction. A few yards from this line a wacke dyke cut the" Dimetian," and it was clear that if the strike of the dyke were prolonged it would abut against the junction. Proceeding towards Knaveston, gravel-pits were seen in decomposed granophyre, the peculiar weathering of which exactly reproduced that of the similar rock of St. Davids. At Knaveston itself a quarry occurred in a handsome hornblende-diorite. The rock was remarkable for the way in which fine and coarse varieties and types with varying proportions of the ferro-magnesian The relations to the constituent were mixed up together. neighbouring rocks were completely obscured, as the quarry lay in the middle of featureless fields, but the Director considered that the diorite undoubtedly belonged to the pre-Cambrian series of intrusions. From this point the party tramped back to the beach at Newgale Sands, where a most astonishing sight appeared. The main road, with stone walls on either side, ran directly up to a huge pile of pebbles, disappeared under it, and did not come to sight again for half-a-mile. (See PI. XXXIII, Figv r.) Formerly the road lay on the land-side of a storm-beach closing the mouth of a broad silted valley; but the great storm of December last had shifted the beach bodily on to the top of the road, so that mails for Solva and St. Davids had to make a detour of some miles. Some men were employed in clearing a path to a little inn, which had narrowly escaped complete destruction; but as the whole basis of the beach had shifted it seemed doubtful whether, even if the vast labour of moving the pebbles, lying twenty feet thick on the track, were undertaken, the restoration would be at all permanent. The beach itself was of interest owing to the variety of petrographical specimens to be seen in it. Many were pre-Cambrian or Arenig igneous rocks, and others were derived from the drift. Some very striking boulders were derived from the neighbouring Millstone Grit, their curious appearance being due to irregular flakes of black shale interspersed among the light-coloured constituents of a conglomerate. Most, however, of this formation, which built the cliffs to the north of the bay, consisted of bedded green sandstone, with delicate honeycomb jointing. It so closely resembled the Caerfai sandstone in places that the Director said it could only be distinguished in the field by the occurrence of plant-fragments. These similar sandstones are faulted together in the little bay of Cwm Mawr, where the fracture is marked by a broad band of shattering, several hundred feet in thickness. The breccias produced did not show evidence of much movement. One in particular, formed in green sandstone in a deep red matrix, showed so little displacement that it was compared to a jig-saw puzzle. Small basic intrusions meandered among the breccias, but gave no clue to the character of the faulting.
230
EXCURSION TO THE ST . D AVID S DISTRICT, SOUTH WALES.
An energetic controversy took place as to the true nature of the fault. The Director considered it to be probably normal and unconnected with the great faults and folds, which he held to be pre-Old Red Sandstone and post-Silurian in age. He pointed out that the thrusts were in the opposite sense to the Carboniferous thrusts of the coalfields, and that the Silurian and Ordovician beds gradually disappeared under the Carboniferous, so that it was qu ite probable that here the Millstone Grit actually rested on Lower Cambrian. In this case the fault would be small, perhaps a few hundred feet. Mr. Green, on the other hand, said that the presence of great thicknesses of Upper Cambrian and Ordovician strata a short distance north, west, and south pointed to a displacement of at least 10,000 feet, and that there was certainly some rever sed faulting, though slight, to be identified near the main fracture. He thought that the evidence was in favour of post-Lower Coal-Measure age for the thrusting. April 22nd.
It had been the intention to drive in early to Fishguard and to spend some hours exploring the neighbourhood, but delays in starting and on the journey left a short time onl y. On the way the" Pilgrims' Well " was seen. This used to be the last halt on the old pilgrimage to St. Dav ids; near the sprin g a fine Celtic cross, disco vered on the spot, had been built into the wall. On arrival at Fishguard Station, the cliffs in which th e station had been constructed were found to be composed of an acid agglomerate, resembling in some respects that of the Pebidian. Climbing up, the part y came to a great exposure of a silicified, exquisitely banded tuff, much folded ; north-we st came banded and nodular rhyolites. Bould ers were seen of a vesicular and esite, closely resembling that of Porthgain. The general view was th at, as a consequence of the discovery on Thursda y of a thru st at Abereiddy, th ese volcanics also would have to be removed from th e Llandeilo, and would probably be found to lie between th e bifidus and murchisoni zones of the Llanvirn. POSTSCRIPT.
A special interest possessed by this excursion was that much of the geology, both solid and drift, was little known, so that the work had much of the charm of exploration. In particular numerous observations were made on the drift and surface features. The glacial gravels at Whitesand Bay (ante, p. 2 1 6 ) and the ancient storm-be ach at Abereiddy (allte, p. 227) do not seem to have been previously recorded. The discovery of a thrust at Abereiddy (ante, p. 228), probably continuous with that in the north of Ramsey Island, is of great importance in the
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
231
tectonics of Pembrokeshire, Valuable fossil records were made both on the north and on the south coasts. The preparations for the excursion led to some new and unexpected observations with regard to the sequence and structure of Ramsey Island. A few words must be said with regard to the courtesy and kindness shown by the people of St. Davids and its neighbourhood to the visitors. Where all were so good, it is difficult to select names, but those responsible for arrangements feel that they should at least mention Mr. Davies, of Rhoscribed Farm, Mr. Perkins, of Llanvirn Farm, the Dean's hearty welcome, and the kindness of Mr. and Mrs. Baker, of St. Justinian and Ramsey Island. Mr. H. P. Jackson, Master of the Secondary School, gave constant assistance before and during the visit. REFERENCES. MAPS. Geological Survey Map, Sheet 40, I in. (Old Series). (Out of date.) Geological Survey Index Map, Sheet 13, :\ in. (New Series). Price 2S. 6d. Ordnance Survey Map, Sheet 209, I in. Price Is. LOWER P ALiEOZOIC ROCKS. SALTER.-Quart. yourn. Geot. Soc., vol. xx, p. 233. SALTER and HICKs.-Quart. yourn. Geoi, Soc., vol. xxi, p. 476
1873. 1875·
(Menevian), HARKNESS and HICKS.-Quart. yourn. Ceo!. Soc., vol. xxvii, p. 384 (Lower Cambrian). HICKS, H.-Quart. Yourn. c,«. Soc., vol, xxix, p. 39 (Tremadoc). ----.-Quart. Your1,. Geol, Soc:, vol. xxxi, p. 167 (Arenig and Llandeilo). ----.-Popu!ar Saence Review. ----.-Ceo!. Mag., p. 2I (Olenellus zone). ELLES, MISS.-Ceo!. Mag., p. 208 (Graptolite zones).
P ALiEONTOLOGY. See above papers and also : SALTER.-Quart. yourn. Geoi. Soc., vol. xix, p. 274. SALTER and HICKS.,..-Quart. you,'n. Geol, Soc., vol. xxiii, p. 339. HICKS and RUPERT J ONES.-Quart. :!ourn. Geo]. Soc., vol. xxviii , p. In 1875. HOPKINSON and LAPWORTH.-Quart. yourn. Geoi. Soc., vol. xxxi, p.63I. 1880. LAPWORTH.-A1l11. Nat. Hist., vol. v, series v, pp. 275 and 280. 1900. COWPER REED.-Ceo!. Mag., p. 250. ELLES and WOOD.-" Monograph of British Graptolites" (Paleeonto graphical Society). LAKE, P.-Monog,'aph of Pal. Soc., vol. lx, pt. 5, and vol. lxi, pt. 8. 1877. 1883. 1883. 1884. 1884. 189°· 190 8.
PRE-CAMBRIAN AND STRUCTURAL. HICKS.-Quart. Yourn. Geol, Soc., vol. xxxiii, p. 229. GEIKIE, A.-Quart. yourn. Geol, Soc., vol. xxxix, p. 261, etc. HUGHES.-CtO!. Mag., p. 306. HICKS.-Quart. yourn. Geoi. Soc., vol. xl, p. 507. BLAKE, J. F.-Quart. Yourtl. Geol. Soc., vol. xl, p. 294. LLOYD MORGAN.-Quart. Yourn. Geol. Soc., vol. xlvi, p. 241. GREEN, J. F. N.-Quart. yourn. Ceo!. Soc., vol, lxiv, p. 363.
232
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES. PETROGRAPHY.
See above papers and also:
1908.
DAVIES, T.-Quart. yourn. Geol. Soc., vol, xxxiv, p. 164. -.-Quart. Journ. Geol, Soc., vol, xxxv, p. 291. TAWNEY.-Proc. Nat. Hist, Soc., Bristol, vol. 2, part 2. DAVIES, T.-Quart. yourn. Geol. Soc., vol. xl, p. 548. TEALL.-" British Petrography," pp. 318, 334. BONNEY.-Geol. Mag., p. 357. ELSDEN, J. V.-Quart. Yourn. Geoi, Soc., vol, lxi, p. 579. - - - - - Q u a r t . yourn. Geol. Soc., vol, lxiv, p. 273.
1885. 1894.
BONNEY.-Quart. youn,. Geol. Soc., vol, xli, p. 518. (Picrite Erratic.) HICKs.-G/acla/irts' Mag., April. "Evidences of Ice-action in N.W.
19°4.
JEHU.-Trans. Roy. Soc. Edinburgh, vol. xli, p. 53, etc.
1909.
Deposits of Northern Pembrokeshire." THOMAS, H. H.-Min. Mag., vol, xv, p, 2+1. in Tertiary and Post-Tertiary Sands."
1878. 1879.
1879. 1884. 1888.
1889. 190;.
GLACIAL GEOLOGY.
Pembrokeshire ," "Glacial
"Detrital Andalusite
NOTE ON A HEAVY SANDSTONE FROM ST. DAVIDS, PEMBROKESHIRE. By WILLIAM
JONES.
THE rock occurs as a lenticle in the middle of the basal Cambrian conglomerate and close to the path at the top of the cliff of St. Non's Bay, St. Davids. Where exposed the lenticle is about two feet six inches in length and seven inches at its greatest thickness, stringing out at the ends in dark lines. Thin veins of infiltrated quartz traverse the rock and are cut off by the falsebedded conglomerate; Mr. Green has noticed that a certain amount of apparently similar material occurs as augen in the sheared tuffs of the Upper Pebidian and also in the conglomerate elsewhere, particularly at Treliwyd, two miles north of St. Non's Bay. The specimen examined is a dark-grey granular crystalline rock with sub-metallic lustre on a fresh fracture, and is duller but free from alteration products on the weathered surfaces. It is thinly veined with quartz in two directions, has a dark red streak, a specific gravity of 4'5, and a degree of hardness ranging from 6'0 to 6'5. The apparent evenness in the size of the rounded grains, to the naked eye and under a pocket lens, suggested the possibility of the rock being of oolitic origin, whilst an igneous origin would not be inconsistent with its megascopic character. Under the microscope however these two theories of its origin seem to be quite untenable. The irregularity in the forms
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES. APRIL 13TH TO 22ND.
Directors:
J.
EASTER, 1911.
F. N. GREEN, B.A., F.G.S., AND PROFESSOR O. T. JONES, D.Sc., M.A., F.G.S.
Excursion Secretary: ARTHUR L. LEACH. (Rtport by MR. J. F. N. GREEN.) THE official party left Paddington at 8.45 a.m, on Thursday, April r jth, and arrived at 2.15 p.m. at Fisbguard Harbour; the long drive (16 miles) to the headquarters at the City Hotel, St. Davids, occupied nearly three hours. During the first four days of the excursion sixty-four members and friends were present. The principal object of the excursion was to study the complicated structure and inter-relations of the pre-Cambrian and Cambrian rocks of the St. Davids area; but the programme provided also for visits to younger rocks at Abereiddy, Llanvirn, Ramsey Island, and Newgale. The glacial and other surface geology of the district was incidentally studied during most of the tours. Upon the suggestion of Dr. J. W. Evans, who had undertaken the organisation of the Geological Section of the Coronation Exhibition, many of the members agreed to lend specimens obtained during the visit in order that a collection fully illustrative of the work done on a "long" excursion might form a special feature of the Association's display at the Exhibition. The arrangement of the maps, diagrams, photographs, rock specimens, and fossils for this exhibit was undertaken by Dr. Evans. The weather during the first four days was delightful, but on Tuesday a strong south-westerly gale sprang up and the projected visit to Ramsey Island had to be abandoned, while the Southern Coast and Newgale excursions were somewhat shortened in consequence of adverse weather conditions; otherwise the programme was fully carried out. April 14th. PRE-CAMBRIAN AND CAMBRIAN ROCKS OF THE CITY, WHITES AND BAY, AND PORTH SELE.
(Directorr J. F. N. GREEN.) The party first examined, in a quarry near the Church Schools, fine-grained spherulitic quartz-porphyry cut by basic
216
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
dykes, both intrusive in porcellanite and tuffs of Upper Pebidian age. The quarries lie in the great faulted anticline of preCambrian rocks which forms the backbone of the St. Davids promontory, and are excavated in both banks of the Alan, which flows here nearly parallel to the anticlinal axis. On the western side of the Alan, Lower Pebidian tuffs were examined in several excellent sections. The pale pulverulent tuffs on the east bank are regarded by the Director as part of the highest Pebidian series, and if this view is correct a strong reversed fault must separate them from the purple" halleflintas" in the crags near the Bridge. Above the latter come coarse felsitic agglomerates (Lower Pebidian) quarried near Penrhiw, which attracted much attention. From the top of Penrhiw the rocky gorge of the Alan was seen to open northwards into a broad, shallow valley, apparently continuous with a large drift-filled depression lying under the northern hills; the drift extends westward to Whitesand Bay. The investigation of this drift-filled basin should throw much light upon the age and mode of excavation of the Alan gorge. Following the path over the Burrows towards Whitesand Bay, many of the party were interested to see towering over them the crags of Carn Llidi, the parent mass, as the Excursion Secretary pointed out, of some of the large boulders which they had seen two years previously lying upon the extreme southern cliffs of Pembrokeshire. On the north side of Whitesand Bay a fine exposure of boulder-clay was seen, abutting on the north against the Upper Cambrians, and overlain by blown sand. Large boulders of coarse norite and gabbro were abundant in the lower part of the drift; amongst the smaller pebbles were flints and quartzite, and on a subsequent visit some members of the party obtained fragmentary marine shells. The slates to the north of the boulder-clay were seen to be capped by stratified sands and gravels, which were thought by some members to be referable to a raised beach, but on April 15th striated pebbles were found to be quite common in these gravels. The stratified beds do not seem to have been previously noted, and must be linked rather with the glacial beds than with the raised beach series. Further north the .Neseuretus-beds and other Arenig rocks were seen to be invaded by many sills, one of which formed a conspicuous wall-like mass on the beach, where the bedding was vertical. Leaving this flank of the bay, the party crossed the sands to the southern cliffs, noting on the way some deep bowlshaped hollows, resembling potholes, in the cleaved Cambrian rocks. These holes are probably caused by wave-action agitating pebbles in depressions, originally slight. The polished sides gave beautiful sections showing cleavage perpendicular to ribbon bedding. The beds were considered by Professor Jones to
PROC. GEOL.
Assoc.,
VOL.
XXII
PLATE
XXX.
/' /r,'/" "J' F . Ir. T1t n brid.:.· ~ .
FIG. I.-SECTION AT vVHITESAND BAY, PEMBROKESHIRE. BLOWN SAND, HEAD AND BOULDER-CLAY RESTING ON CLEAVED CAMBRIAN SLATE.
FIG. Z.-TRENCH NEAR PORTHCLAIS, SHOWING THE AND GRANOPHYRE.
(The hammer-head indicates the junction.) To face paJ;c 216.
EXCURSION TO THI£ ST. DAVIDS DISTRICT, SOUTH WALI£S.
217
belong to the top of the Solva (Lower Paradoxidian) beds. Their surface rises gently, forming the southern bank of a valley, now buried in drift, which must clearly extend to some depth below sea-level. (See the illustration, PI. XXX, Fig. 1.) An east and west fault brings up Upper Pebidian, consisting of highly sheared tuffs, at the southernmost part of the bay. A coarse conglomerate, the actual base of the Cambrian, lies here at a dip of about 50Q on the Pebidian. A small thrust, with quartzveining, comes in about two feet below the junction, under which a strong spring bubbles up through the sand. A little lower down in the series a basic sill with hematite specks, pseudomorphic after olivine, breaks through the tuffs. This complex is beautifully sectioned in the cliff. After examining the contents of the conglomerate, which contained pebbles, several inches in length, of quartzite, vein-quartz, and Pebidian tuffs, with smaller bits of jasper, porphyry, etc., the party climbed up to the sandhills, noting numerous springs at the junction between the sand and the underlying drift, and walked round into Porth Sele to view a basic dvke intrusive in the same conglomerate. The dyke had been greatly sheared and squeezed, showing movement later than the intrusion, which was itself later than the main faulting. Several hand-specimens were obtained, exhibiting perfectly the junction of the conglomerate and igneous rock. On the south of the bay were seen the Caerfai beds, green sandstone above the conglomerate, purple shale with thin felspathic tuffs, and purple sandstone. Part of the shale was faulted out, but the remainder was assiduously searched by the party for fossils, unfortunately without success. The use of a chapel at Rhoson Ganol had been obtained for tea, but it was much more pleasant to lay it out on the grass. After tea the next section visited was on Carn Rhoson, where a coarse agglomerate (B2) of Lower Pebidian age, composed chiefly of green vesicular lava in a red matrix with scattered boulders of fluidal rhyolite, was exposed, with a mass of olivinebasalt breaking across the bedding, thus clearly proving the intrusive character of the basalt, in spite of its close resemblance to a flow. A magnificent view was unfolded from the top. To the west, across Ramsey Sound, Ramsey Island presented a long platform, dominated by three piles of hard rock; to the south, beyond St. Bride's Bay, the platform could be seen, stretching for miles almost unbroken, except for Marloes Beacon, and passing on to Skomer Island; near the foot, Pwll Trefeithan lay in a shallow depression in boulder-clay. During the walk home most of the party diverged to the ancient camps of Clegyr-foia and Castell, and thus had an opportunity of viewing the Alan Valley, and a small exposure of the famous "Dimetian," which was to be studied more closely on Monday.
218
EXCURSION T O T HE ST. D AVIDS D ISTRI CT , SOUT H WA LES .
Apri! r 5th, LOWER AND MID DLE CAM BRIAN BEDS OF THE S OLVA VALLE Y,
Directors: M R. J. F. N. GR EEN AND PROF ESSO R O. T. J ON ES.
On th is morning Mr. Green .had to announce that, owing to the stat e of a tra ck along which it had been proposed to take the party, a detour would be necessary, "increasing th e walking distance to between nine and ten miles. T he news was received with groa ns. After walking through Caerfarchell, the party stopped at the edge of the Solva gorge to examine exp osures of th e lowest parts of the Caerfai beds, which are th ere thru st over the lower Solva. The Caerfai basal con glomerate and gree n sandstone are similar in all respects to the same beds at Po rth Sele. Professor Jones here explained his views as to the form ation of the gorge. He agre ed with Mr. Green that the original direction of drainage was westward, alon g th e slope of the Pli ocene platform ; but he considered that the di version was caus ed in preglacial times by th e cutting back of a later north to south drain age initiated by th e ad van ce of th e sea into St. Bride's Bay, and that th e absence of d rift in th e valley was due to the steepness of th e sides. H e pointed out that dri ft occurr ed in similar valleys elsewhere, as for example th e Cae rbwdy Valley. Afte r a long and int erestin g discussion a mong the memb ers, a move was made to th e ne ighb ourhood of Cae rforiog Bridge. Along the north side of th e gorge, which here runs east to west, are bold crags of Upper Pebidian, composed of fine-grained silicified ashes with bands of coarse congl omerate, dipping 35° north, containing pebbles of rhyolite and acid t uff up to a foot in d iameter. These pre-Cambrians were thru st over middl e Solva beds and the river ran along the thrus t-plane, cutting back the hard Pebidians as a stee p esca rpment, which overlooked a gentl e slope of Solvas, covere d with cultivation. R eturning down-stream , the part y (her e numbering sixty-nine) crossed the outcrop of th e Selva beds to th e horizon mark ed by a red band, in which P rofessor Jones demon strat ed a prett y example of a pitching anticline. Near this point and at a qu arry towards Mid dle Mill, th e exposures consisted of int erba nded shale and sandstone, exhibiting in great perfection the var iat ion of the slope of the cleavage-plane with the texture of the rock ; a nd some large spec imens were carried away. Opposite Mid dle Mill a laccolite-Iike mass of dolerite cam e in, th e ac tual junction, following a bedd ing-plane, being exposed for severa l yards in the quarries. About a mile further down-stream, Pr ofessor Jones d rew att ention to a han ging valley of unu sual type near Tre- Cadwgan. For some reason water had cease d to flow down in any apprecia-
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
219
ble quantity, and consequently wash and debris had accumulated in the bottom of the course, filling it up to a considerable depth, The party then crossed the river and walked along the southeast foreshore of the beautiful little harbour of Solva to the fossiliferous locality in the zone of Paradoxides hicksiz" discovered by Professor Jones and Mr. H. H. Thomas, where fossils were obtained by members in quite unexpected profusion. Numerous well-preserved specimens of 1J1icrodisC1ts punctatus, with Agnostus, Conocoryphe, and brachiopods, were found, but the prize of the day was a nearly perfect Anopolenus, not hitherto recorded from this place. The fossils occurred in a hard grey sandstone, classed by Hicks with the Menevian, but much more closely allied lithologically to the Solva series. Indeed the Directors thought it probable that a non-sequence occurred between it and the typical Menevian shales. Crossing to the north-west of the harbour, a large mass of dolerite was observed with junctions along bedding-planes, and it was pointed out that on the opposite side of the harbour nothing was seen but two petty sills, so that the laccolitic character of the intrusion was evident, Near this place, behind the lifeboat house, the Menevian was attacked with fair success, the black shale yielding Agnostus barrandei, and other agnostids, Obolina, and a fine pygidium, ascribed provisionally to Erinnys, Climbing up the cliffs above, an exquisite prospect of the harbour and coast lay before us. It was observed that the waveaction within the harbour had only cut a comparatively insignificant nick in the slopes of the drowned valley, which was merely a continuation of the Solva gorge. During the walk home, a short stop was made at a spot known as "Nine Wells," where quantities of water were thrown out near the meeting of the great Porthyrhaw and St. Davids faults. This has led to the establishment of a pumping station to supply water to St. Davids. Several good sections of Menevian were examined, and a visit paid to the grounds of the County School to see the specimens of acid volcanic conglomerate found in boring the well, which were obviously identical with the Pebidian ot the upper gorge of the Solva River. April 16th.
In the morning the party separated, some attending the Cathedral service, others going to the South Coast, Abereiddy or Penberry, and several to Whitesand Bay to study and photograph the glacial gravels. After lunch a party of about twentyfive went with Mr. Green to Carn Llidi, where several of the detached expeditions were met. In a quarry on the south-east flank above the farm fresh specimens of coarse norite were secured; and slightly to the north of this the junction of the igneous mass with Arenig shale was well exposed. The chilled
220
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
edge of the great igneous intrusion of Cam Llidi was fine-grained with large felspars, a porphyritic quartz-enstatite-dolerite, which had baked the shale to a splintery bluish rock. Clambering up to the top of the Cam, 595 feet above the sea, a splendid view of the northern and western coasts appeared. The summit was composed of a coarse gabbro with veins of aplite. An interesting problem which the party hoped to solve was that of the thickness of the ice from the Irish Sea. Though weathering had proceeded too far to leave definite traces of glacial action on the actual top, the general form suggested that the whole had been covered. Between 400 and 500 feet rounded hummocks, probably ice-shaped, were common; and strire were noted near the higher of these contour-lines. The general opinion was that ice had certainly overwhelmed the hill. Between the Cam Llidi mass and that of St. Davids Head a valley was found, truncated at both ends by the sea. The section at the Porth Melgan end showed that the original valleybottom must be below sea-level, and that the depression had been partly filled by boulder-clay, in which Mr. Leach had discovered shells and a large angular block of flint. Crossing this feature and a number of ancient stone walls, enclosures and circles, the party halted on the triple stone rampart fortifying the" Promontory Camp" of St. David's Head to listen to an interesting account from Mr. Leach of the Pembrokeshire camps and of Mr. Baring Gould's excavations, which proved this camp to have been in use well into the Iron Age. After a short examination of the gabbro forming the Head, which was found in places to be full of parallel veins of aplite, the members strolled back, enjoying a rich view of the city and surrounding country. April 17th. THE DIMETIAN GRANOPHYRE AND ITS RELATIONS TO THE CAMBRIAN AND PEIlIDIAN AT PORTH-CLAlS AND PORTH-LISKY.
Director:
MR.
J. F. N.
GREEN.
Before starting, the Director distributed copies of a map on the 25-inch scale of the area near Porth-clais Bridge, published in the Quarterly Journal of the Geological Soddy for August, 1908. On arrival at this spot, the first exposure visited was a quarry in granophyre, in which a vein of fault-breccia was being dug as road-binding material. The band of fault-breccia could be followed northward by a strip of cultivation, which had heen already pointed out during the walk down the Alan Valley. The cultivation followed the soft fault-rock between stony moorland. Southward the same fault-line could be seen as a depression run-
PROe. GEOL.
Assoc.
VOL.
XXII.
JOINTING IN VERTICAL SaLVA BEDS.
PLATE
XXXI.
OGOF-LLES:JG~. PEMBROKESHIRE
To race jag.:
220.
EXCURSION TQ THE ST. DAVIDS DISTRICT, SOUTH WALES.
221
ning Up the bank, where, indeed, exposures enabled it to be fixed within a few inches. A little corner of unaltered Cambrian sandstone was faulted into the floor of the quarry. Standing on this the Director described the character of the granophyre, drawing attention to its highly quartzose nature, the complete destruction of its ferro magnesian constituents, its peculiar fissile weathering, and the presence of brecciated bands, distinct from the later fault-breccias j he then expounded his views on the structure of the area, pointing out the features that defined the lines of the various faults. The party then walked down the north-east bank of the river to see two little leucophyre dykes, with prismatic jointing, about thirty and six inches thick respectively, intruded into the granophyre. Immediately south of this place the granophyre could be seen thrust over inverted Cambrian sandstone, belonging to the lower part of the Caerfai beds. Crossing the stream, the trench across the unconformable junction of the basal conglomerate of the Cambrian and the underlying " Dimetian " granophyre was found to be in good condition, having been much enlarged during a former visit by a party from Oxford. The appearance of the rocks in this excavation was extremely deceptive, as at a first glance there seemed to be coarse reddish conglomerate resting directly on pale decayed granophyre. On closer inspection, the coarse conglomerate proved to be washed-down blocks, and the actual junction to be in the middle of the pale face underlying it. The distinction between the basal grit and the igneous rock could, on close observation, be made by means of the pink quartzite pebbles in the former. Several excellent specimens were obtained showing the actual contact, and photographs were secured (see Pl. XXX. Fig. 2). Conscientious visits were then paid to a number of other exposures defining the fault-lines. One, a quarry by the roadside near the trench, showed a little wacke dyke along the thrust, enclosing lumps of fault-breccia j another, continuing the thrust previouly seen on the opposite side of the river, exposed the undulating slicken-sided floor of the thrust composed of grit, which, according to the Director, was much mylonised. Climbing the bank, the members overlooked the tiny harbour of Porthclais, in the cliffs of which the tuffy red shales of the Caerfai beds could be seen repeated by little thrusts. The cliffs were then followed to a point above Ogof-llesugn, where everybody sat down for rest and refreshment. The view here was of great interest. To the left stretched Solva sandstones, the bedding vertical and parallel to the coastline. They were intersected by three equiangular sets of joints, perpendicular to the strata, and the sea, working along these four planes of weakness, had fretted the rock into fantastic shapes. (Pl. XXXI.) To the right ran out to sea a fault face of moulded granophyre with many pegmatite veins j immediately
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PROC. GEOL.
Assoc.,
VOL.
XXI£.
PLATE
XXXII.
PIIQ/ I} I>y 1:". /I ' . 7,fll bYl t{g c.
SECTION ON THE WEST SIDE OF ST. NON'S BAY.
(This view is explained by the Diagram, Fig. 19, on page
222.)
To face page
222.
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
223
below, wedged in between these dissimilar masses, lay a confusion of varicoloured rocks, which it was now the object of the visitors to examine. Some rock-scrambling was necessary, but on reaching the shore the climbers were repaid by a remarkable section. A mass of coarse Cambrian conglomerate, with a high dip and a little stained Pebidian still underlying It, had been faulted in between the Solva beds and the granophyre, the latter being intersected by a quartz-porphyry dyke. Subsequently basic igneous material had injected the faults and the neighbouring rock, so that now lenticular masses of the various components were imbedded, at first sight confusedly, in a continuous boss of greenstone. Crossing a ridge of granophyre, the party halted for a few minutes by the hornblende-picrite erratic described by Professor Bonney. The block is protected by an iron cage from earnest collectors. Owing to the peculiar petrographical characters of the rock, there was a good deal of doubt as to its origin, though the Director considered that it probably came from Anglesey. A hundred yards farther on, a steep track descended to Porth-lisky. This track was actually on the junction of the Dimetian, which ran seawards as a fault-cliff, with the soft pulverulent tuffs of the highest Pebidian, which had been scooped out by wave-action. As usual, a basic dyke had risen along the fault-line, enclosing masses of breccia. The party spent some time studying curious sheared tuffs, brightly-coloured pink, white, purple, yellow, and lilac, and some examples of ramifying wacke dykes. The west side of the bay was formed by a cliff of hard lower Pebidian, on the same horizon as the Penrhiw quarries, thrust against the soft upper beds. It was surprising to find, just behind the inner part of the bay, a knoll of Cambrian sandstone, whose unsheared condition, though caught between faults, suggested that most of the dynamic action on the Pebidians must be pre-Cambrian. In a field above Porth-henllys, numerous contacts could be seen of a coarse agglomerate with a pale flinty rock of rhyolitic appearance. The Director explained that this was one of the critical sections on which he had relied as evidence that the felsite, termed by himself the" schistose sill," was truly intrusive. Near the contact the felsite was fine-grained, apparently chilled, and it tongued in among the tuffs of its roof; but since Mr. Jones and Mr. Thomas had found rhyolites on much the same horizon at Newgale, he had begun to think that perhaps the field evidence in the 81. Davids area had been wrongly interpreted. Those present then examined carefully the sections before them, and the general conclusion arrived at was that the felsite was certainly intrusive at this point. Here and in other exposures seen a short way farther on, the felsite developed a schistose structure which seemed too regular for flow-structure. The Director
2z4
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
attributed it to pressure while the sill was still hot and viscous (protoclastic structure). Approaching the little promontory of Pen-y-foel, a pair of foxes were put up, the vixen bolting through the middle of the crowd. Mr. Fearnsides captured and, with due precautions against sharp teeth, exhibited to all a beautiful cub. The captive released, it was seen that at this point the beds of the lower Pebidian, classed as A3, Br, and Bz, were thinner, but at the same time coarser, than to the north, while the schistose sill had thinned out, so that this interesting part of the succession could be studied conveniently. A3 consisted mainly of broken quartz and felspar in an indefinite epidotic matrix, with scattered rounded pebbles of rhyolite of several varieties, and angular boulders of hornblende-andesite. The Director suggested that possibly this rock might throw light on the genesis of some of the quartz-epidote schists of the Highlands. Br and Bz, mainly composed of vesicular trachytic material in a chloritic matrix with large boulders of a fluidal rhyolite, resembled the same horizons as seen at Rhoson, but were coarser; some included fragments being over a foot in length. The end of the promontory was formed by a neck-like mass of augite-andesite, which gave off irregular veins into the neighbouring tuffs. A similar mass of augite-andesite, but on a much larger scale, was found to form the cliffs near Carnarwig, the next point visited. Part of this was, however, highly vesicular. Attention was also drawn to a curious honeycomb weathering of the basic rock. Portions of this cellular rock were used locally for ornamental purposes. It seemed only to develop on sea-cliffs. Scrambling down, masses of coarse Cambrian conglomerate were found, imbedded in the intrusion. One of these blocks, perhaps thirty feet thick, showed fluted sides where the igneous rock had been washed away. Comment was made on the similarities of this fine section to that at Ogof-llesugn, and it was pointed out that this sheet of lava-like rock must be, not merely post-Cambrian, but post-faulting. A picnic tea was spread on the grass at Treginnis-isaf farm, with geese and cows as spectators. On the way home some more exposures of the "schistose sill" were seen, and the Director gave an account of the field evidence for its supposed connection with the margin of the" Dimetian." April 18th. CAMBRIAN ROCKS OF THE SOUTHERN COAST.
Director: ]. F. N.
GREEN.
Rain and a heavy gale upset the arrangements for crossing to Ramsey Island, so an attempt was made to carry out Wednesday's
PROC. GEOL.
Assoc.,
VOL.
XXII.
PLATE
XXXIII.
Thoto hy E. TV. T1fnbril~~)·e.
FIG. I.-THE STORM-BEACH AT NEWGALE, RESTING ON THE MAIN ROAD.
1'Iw h b) E .
tr.
TII" b n ,(s:~ .
FIG. 2.-BROKEN OVER-FOLD IN CAERllWDY BAY.
To face fage
224'
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
225
programme instead. Walking to Pont Clegyr, a mile east of the city, a large exposure of boulder-clay was seen, resting against volcanic conglomerate with bands of silicified ash. It was pointed out that this was the same horizon of the upper Pebidian as that seen in the upper Solva gorge, the County School Well, and Treginnis-isaf. Turning down the valley, the party entered the halleflinta quarry. This quarry is in the horizon C4 of the Caerbwdy series, and showed bedded tuffs, very fine-grained, with numerous colour varieties, and some felspathic bands. The rock had a perfect conchoidal fracture, some specimens resembling blue glass. Farther down the valley the basal Cambrian conglomerate appeared, in a rampart-like mass, very coarse in places, with quartzite and porphyry pebbles over a foot in length. The wind was now so strong that it was decided to make for Porthyrhaw by an inland route. It was hoped that the eastern cliffs of the inlet would be sheltered, but on arrival the spray was seen flying over them in showers. However, four intrepid spirits crawled warily over the slippery rocks to investigate the historical localities, and, in spite of the wet, were rewarded by several Agnosti, a Lingula, and fragments of Paradoxides davidis. A splendid specimen of the latter was noticed, with a headshield nearly eight inches in diameter, but it was impossible to get it out without crumbling. All these fossils occurred in a band about the middle of the black shale of the Menevian. The higher fossiliferous bands, just below a little sill, could not be reached. It was too wet to see Protospongia, of which several fine specimens had been secured a few days before. Meanwhile the gale increased and the party separated, some returning to St. Davids, some going round to Caerfai, while nine disciples followed the Director over the cliffs. After crossing a glimpse was obtained of a fine broken overfold, dissected out by the sea so that a folded surface ran out into the waves like a huge cylinder. (See PI. XXXIII, Fig 2.) The view-point was a promontory fortified by three lines of earthworks. The main party avoided Caerfai Bay, where, unknown to them, several members were fossil-hunting, and passed on to an exposure of Cambrian conglomerate, containing a lenticle, seven inches thick, of a heavy sandstone, against which the coarser pink conglomerate was false-bedded. This black sandstone has been shown by Mr. W. Jones, of the Royal College of Science, to consist of clastic titaniferous hematite. (See post p. 232.) Beyond this point, quartz-porphyry and silicified ashes appeared on the cliff-track; but the latter were better shown in the track down to the basin in St. Non's Bay. The strata here, though much shattered, were obviously identical with the bedded halleflinta of the Caerbwdy; but instead of some 500 feet of Pebidian intervening between the halleflinta band and the basal Cambrian, only a few yards came in here. The conglomerate,
226
EXCl'RSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
dipping vertically, lay in contact with stained tuffs, the red ferruginous staining extending about six feet from the junction. Scrambling round into the little inlet west of the track, a remarkable geological illustration was displayed. The inlet was shaped like a cube, less the top and one side, the rocks being fully exposed on the remaining three sides and floor, so that the complicated structure could be seen in the solid as in a model. The halleflinta and tuffs, penetrated by a dyke of quartz-porphyry, were thrust over the conglomerate and vertical green sandstone, the fault-plane cutting across both. A thick fault-breccia occupied this plane, and up this a basic dyke, now altered to wacke, had risen. In the floor the porphyry could be seen running up to and terminating abruptly at the fault. (PI. XXXII.) Climbing back to the top of the cliff, the Director pointed out the .arch so often figured in geological publications. Its components stood out very clearly, when compared with the photograph. PI. XX (ante p. 124.) Attention was drawn to the fine cliff-section in front, along which the following rocks could be seen in order :-Quartz-porphyry, unstained tuff, stained tuff, basal conglomerate, green sandstone, red shale, purple sandstone, Solva beds. After visiting the ruins of St. Non's Chapel, the party walked to Penygarn, the type exposure of the Dimetian, where some examples of micropegmatite veins and a chloritic brecciated band were noted. An attempt, doubtfully successful, was made to find the marginal porphyry in a wet road, after which the members reached home in a draggled and disreputable condition, but full of energy. During the evening a hearty vote of thanks to the Directors was passed by the assembled party, on the motion of Professor Watts and Dr. J. \-V. Evans; and to the Excursion Secretary on the motion of Mr. Whitaker and Mr. VassalI. April 19th.
During the wet morning, several hours were spent in examining, under the guidance of the verger, the Cathedral, the Close Gate, and the ruins of the College and Palace. The chief building stone is a handsome purple sandstone from the upper Caerfai beds, which, when used in internal work, acquires a curious white pulverulent surface. In the afternoon the weather was worse than ever, and though an effort, prompted largely by a desire to enjoy the kind hospitality of Mrs. Baker, of St. Justinian, who had invited the whole party to tea, was made to see some exposures on the west coast, nothing much couldbe observed but mud and swamped boats.
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
April
227
20tft.
THE ORDOVICIANS OF LLANVIRN AND ABEREIDDY.
Director:
PROFESSOR O. T. JONES.
The party drove five miles to Mr. Perkins' farm at Llanvirn, and began operations in a small quarry near Nant, yielding a pretty ash-spotted slate, but no fossils. Proceeding northwards to Hicks' historical quarry, about a quarter of a mile north-east of Llanvirn-y-fran, fossils were soon found in iron-stained slates. They included Didymograptus bijidus, D. nanus and D. mi:holsoni, Climacograptlls confertus, Diplograptus dentatus, Trinudeus, Ampyx, Caiymeue, Placoparia, Orthoceras, Orthis, and Pleuratomaria. Towards the shore, opposite the mill, quantities of fossils were to be found in an old tip, including DIdymograptus bijidus, D. acutidens, and another species, perhaps D. patulus; very perfect Ctimacograpti and dendroid graptolites. Close to this point was a little quarry (with a bonfire premonitory of tea), in which Professor Jones explained the stratigraphy of the neighbouring sections, pointing out the various fossiliferous zones as exposed along the cliff. The quarry cut the base of the ash separating the btjidus and murchisom:zones. The former fossil occurred plentifully just below the ash-bed, which contained angular vesicular lapilli and flakes of slate. The Director then led the way to the shore, where Mr. Leach pointed out a remarkable old storm-beach observed by himself a few days before. It consisted of rolled flakes of slate with a few pebbles and shells, all cemented together by iron derived from the slates. The old beach was overlain by talus, and both had been eroded by a recent storm, leaving fine sections. South of this point the cliff was formed of Upper Llanvirn black slates, containing an amazing abundance of graptolites. Slabs lay everywhere covered with hundreds of Didymograptus murchisoni and of a Climacograptus, and all fell to picking out the richest and most perfect specimens. Other organisms found were Cryptograptus tricornis, dendroid forms, Ogygia, Trinudeus, Orthis, Lingula and other brachiopods. Above these slates Ogygia buchii and Favosites ftbrosus were found in harder calcareous beds. Walking northwards across the bay to the great Porthgain slate quarries, the trilobite beds were attacked, producing Ogygia, Asaphus, Trinudeus, Agnostus, Obolus, Conularia homfrayi and a gastropod. Looking north from this position, a bare cliff-section could be seen, composed of two very dissimilar rocks, one a black slate, the other, weathering yellow, obviously intruded into the first. As it would seem that the igneous rocks of Porthgain had hitherto been supposed to be contemporaneous, this relationship was unexpected, especially as the fragments lying about were PIWC.
GEOL. Assoc., VOL. XXII, PART 4, 19II.]
19
228
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
bighly vesicular; but Mr. Green took occasion to observe that, contrary to the usual view, he had found that in Dewisland highly vesicular rocks were invariably intrusive. A small party walking round to examine the junction, this suggestion was confirmed. The basic, yellow-weathering rock was found to be clearly transgressive at all boundaries, with a chilled edge baking the contiguous shale. Nevertheless, it was coarsely vesicular almost throughout. While this question was under examination, Professor Jones picked up a bit of slate with Didymograptus bijidus. The neighbouring slopes were immediately attacked and the zone-fossil was found plentifully in situ, proving repetition on a large scale. Ogygia was also found, and tuffs precisely similar to those in the junction-quarry on the south of the bay. Considerably excited by this unexpected discovery, the group hurried on in the direction of the headland, Trwyn Castell. A trench had been cut across the promontory, exposing boulder-clay with many erratics (including Old Red Sandstone and flints), resting on what appeared to be a conglomerate, really a compacter variety of the vesicular andesite with spheroidal weathering. The same material continued to the end of the headland, where it invaded a beautifully banded pink rhyolite. As far as could be seen in a hasty glance, this rhyolite strikes with the slates and ashes. Returning to the mill, a gipsy tea was taken and specimens compared. The opportunity was seized to give a hearty vote of thanks to Professor Jones. Thanks were also voted to Mr. Perkins of Llanvirn farm, who had greatly assisted the Association in identifying fossil localities and in making arrangements for tea. April zsst, THE PRE-CAMBRIAN AXIS OF NEWGALE AND BRAWDY.
Director:
PROFESSOR
O. T.
JONES.
A damp drive of five miles took us to the neighbourhood of Pointz Castle, where a quarry had been opened in a quartzporphyry much resembling that found n.ear the cathedral at St. Davids. Close to this were numerous crags of banded rhyolite, weathering a dead white. The relationship between these two types was doubtful, but probably the porphyry was intrusive. The Director explained that these ancient rocks came up in a horst-like mass of pre-Cambrian and lower Cambrian, separated from the upper Cambrian and Ordovician by faults. Driving on again, the party descended to walk along a lane, in which the green sandstone of the Caerfai beds was repeatedly exposed, into Brawdy farmyard. The section here had been somewhat obscured by mud, but the line of junction of a granitic rock resembling the "Dimetian" with a rough grit could be traced without difficulty. In places quartz-veins occurred at or
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
229
near the junction. A few yards from this line a wacke dyke cut the" Dimetian," and it was clear that if the strike of the dyke were prolonged it would abut against the junction. Proceeding towards Knaveston, gravel-pits were seen in decomposed granophyre, the peculiar weathering of which exactly reproduced that of the similar rock of St. Davids. At Knaveston itself a quarry occurred in a handsome hornblende-diorite. The rock was remarkable for the way in which fine and coarse varieties and types with varying proportions of the ferro-magnesian The relations to the constituent were mixed up together. neighbouring rocks were completely obscured, as the quarry lay in the middle of featureless fields, but the Director considered that the diorite undoubtedly belonged to the pre-Cambrian series of intrusions. From this point the party tramped back to the beach at Newgale Sands, where a most astonishing sight appeared. The main road, with stone walls on either side, ran directly up to a huge pile of pebbles, disappeared under it, and did not come to sight again for half-a-mile. (See PI. XXXIII, Figv r.) Formerly the road lay on the land-side of a storm-beach closing the mouth of a broad silted valley; but the great storm of December last had shifted the beach bodily on to the top of the road, so that mails for Solva and St. Davids had to make a detour of some miles. Some men were employed in clearing a path to a little inn, which had narrowly escaped complete destruction; but as the whole basis of the beach had shifted it seemed doubtful whether, even if the vast labour of moving the pebbles, lying twenty feet thick on the track, were undertaken, the restoration would be at all permanent. The beach itself was of interest owing to the variety of petrographical specimens to be seen in it. Many were pre-Cambrian or Arenig igneous rocks, and others were derived from the drift. Some very striking boulders were derived from the neighbouring Millstone Grit, their curious appearance being due to irregular flakes of black shale interspersed among the light-coloured constituents of a conglomerate. Most, however, of this formation, which built the cliffs to the north of the bay, consisted of bedded green sandstone, with delicate honeycomb jointing. It so closely resembled the Caerfai sandstone in places that the Director said it could only be distinguished in the field by the occurrence of plant-fragments. These similar sandstones are faulted together in the little bay of Cwm Mawr, where the fracture is marked by a broad band of shattering, several hundred feet in thickness. The breccias produced did not show evidence of much movement. One in particular, formed in green sandstone in a deep red matrix, showed so little displacement that it was compared to a jig-saw puzzle. Small basic intrusions meandered among the breccias, but gave no clue to the character of the faulting.
230
EXCURSION TO THE ST . D AVID S DISTRICT, SOUTH WALES.
An energetic controversy took place as to the true nature of the fault. The Director considered it to be probably normal and unconnected with the great faults and folds, which he held to be pre-Old Red Sandstone and post-Silurian in age. He pointed out that the thrusts were in the opposite sense to the Carboniferous thrusts of the coalfields, and that the Silurian and Ordovician beds gradually disappeared under the Carboniferous, so that it was qu ite probable that here the Millstone Grit actually rested on Lower Cambrian. In this case the fault would be small, perhaps a few hundred feet. Mr. Green, on the other hand, said that the presence of great thicknesses of Upper Cambrian and Ordovician strata a short distance north, west, and south pointed to a displacement of at least 10,000 feet, and that there was certainly some rever sed faulting, though slight, to be identified near the main fracture. He thought that the evidence was in favour of post-Lower Coal-Measure age for the thrusting. April 22nd.
It had been the intention to drive in early to Fishguard and to spend some hours exploring the neighbourhood, but delays in starting and on the journey left a short time onl y. On the way the" Pilgrims' Well " was seen. This used to be the last halt on the old pilgrimage to St. Dav ids; near the sprin g a fine Celtic cross, disco vered on the spot, had been built into the wall. On arrival at Fishguard Station, the cliffs in which th e station had been constructed were found to be composed of an acid agglomerate, resembling in some respects that of the Pebidian. Climbing up, the part y came to a great exposure of a silicified, exquisitely banded tuff, much folded ; north-we st came banded and nodular rhyolites. Bould ers were seen of a vesicular and esite, closely resembling that of Porthgain. The general view was th at, as a consequence of the discovery on Thursda y of a thru st at Abereiddy, th ese volcanics also would have to be removed from th e Llandeilo, and would probably be found to lie between th e bifidus and murchisoni zones of the Llanvirn. POSTSCRIPT.
A special interest possessed by this excursion was that much of the geology, both solid and drift, was little known, so that the work had much of the charm of exploration. In particular numerous observations were made on the drift and surface features. The glacial gravels at Whitesand Bay (ante, p. 2 1 6 ) and the ancient storm-be ach at Abereiddy (allte, p. 227) do not seem to have been previously recorded. The discovery of a thrust at Abereiddy (ante, p. 228), probably continuous with that in the north of Ramsey Island, is of great importance in the
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES.
231
tectonics of Pembrokeshire, Valuable fossil records were made both on the north and on the south coasts. The preparations for the excursion led to some new and unexpected observations with regard to the sequence and structure of Ramsey Island. A few words must be said with regard to the courtesy and kindness shown by the people of St. Davids and its neighbourhood to the visitors. Where all were so good, it is difficult to select names, but those responsible for arrangements feel that they should at least mention Mr. Davies, of Rhoscribed Farm, Mr. Perkins, of Llanvirn Farm, the Dean's hearty welcome, and the kindness of Mr. and Mrs. Baker, of St. Justinian and Ramsey Island. Mr. H. P. Jackson, Master of the Secondary School, gave constant assistance before and during the visit. REFERENCES. MAPS. Geological Survey Map, Sheet 40, I in. (Old Series). (Out of date.) Geological Survey Index Map, Sheet 13, :\ in. (New Series). Price 2S. 6d. Ordnance Survey Map, Sheet 209, I in. Price Is. LOWER P ALiEOZOIC ROCKS. SALTER.-Quart. yourn. Geot. Soc., vol. xx, p. 233. SALTER and HICKs.-Quart. yourn. Geoi, Soc., vol. xxi, p. 476
1873. 1875·
(Menevian), HARKNESS and HICKS.-Quart. yourn. Ceo!. Soc., vol. xxvii, p. 384 (Lower Cambrian). HICKS, H.-Quart. Yourn. c,«. Soc., vol, xxix, p. 39 (Tremadoc). ----.-Quart. Your1,. Geol, Soc:, vol. xxxi, p. 167 (Arenig and Llandeilo). ----.-Popu!ar Saence Review. ----.-Ceo!. Mag., p. 2I (Olenellus zone). ELLES, MISS.-Ceo!. Mag., p. 208 (Graptolite zones).
P ALiEONTOLOGY. See above papers and also : SALTER.-Quart. yourn. Geoi. Soc., vol. xix, p. 274. SALTER and HICKS.,..-Quart. you,'n. Geol, Soc., vol. xxiii, p. 339. HICKS and RUPERT J ONES.-Quart. :!ourn. Geo]. Soc., vol. xxviii , p. In 1875. HOPKINSON and LAPWORTH.-Quart. yourn. Geoi. Soc., vol. xxxi, p.63I. 1880. LAPWORTH.-A1l11. Nat. Hist., vol. v, series v, pp. 275 and 280. 1900. COWPER REED.-Ceo!. Mag., p. 250. ELLES and WOOD.-" Monograph of British Graptolites" (Paleeonto graphical Society). LAKE, P.-Monog,'aph of Pal. Soc., vol. lx, pt. 5, and vol. lxi, pt. 8. 1877. 1883. 1883. 1884. 1884. 189°· 190 8.
PRE-CAMBRIAN AND STRUCTURAL. HICKS.-Quart. Yourn. Geol, Soc., vol. xxxiii, p. 229. GEIKIE, A.-Quart. yourn. Geol, Soc., vol. xxxix, p. 261, etc. HUGHES.-CtO!. Mag., p. 306. HICKS.-Quart. yourn. Geoi. Soc., vol. xl, p. 507. BLAKE, J. F.-Quart. Yourtl. Geol. Soc., vol. xl, p. 294. LLOYD MORGAN.-Quart. Yourn. Geol. Soc., vol. xlvi, p. 241. GREEN, J. F. N.-Quart. yourn. Ceo!. Soc., vol, lxiv, p. 363.
232
EXCURSION TO THE ST. DAVIDS DISTRICT, SOUTH WALES. PETROGRAPHY.
See above papers and also:
1908.
DAVIES, T.-Quart. yourn. Geol. Soc., vol, xxxiv, p. 164. -.-Quart. Journ. Geol, Soc., vol, xxxv, p. 291. TAWNEY.-Proc. Nat. Hist, Soc., Bristol, vol. 2, part 2. DAVIES, T.-Quart. yourn. Geol. Soc., vol. xl, p. 548. TEALL.-" British Petrography," pp. 318, 334. BONNEY.-Geol. Mag., p. 357. ELSDEN, J. V.-Quart. Yourn. Geoi, Soc., vol, lxi, p. 579. - - - - - Q u a r t . yourn. Geol. Soc., vol, lxiv, p. 273.
1885. 1894.
BONNEY.-Quart. youn,. Geol. Soc., vol, xli, p. 518. (Picrite Erratic.) HICKs.-G/acla/irts' Mag., April. "Evidences of Ice-action in N.W.
19°4.
JEHU.-Trans. Roy. Soc. Edinburgh, vol. xli, p. 53, etc.
1909.
Deposits of Northern Pembrokeshire." THOMAS, H. H.-Min. Mag., vol, xv, p, 2+1. in Tertiary and Post-Tertiary Sands."
1878. 1879.
1879. 1884. 1888.
1889. 190;.
GLACIAL GEOLOGY.
Pembrokeshire ," "Glacial
"Detrital Andalusite
NOTE ON A HEAVY SANDSTONE FROM ST. DAVIDS, PEMBROKESHIRE. By WILLIAM
JONES.
THE rock occurs as a lenticle in the middle of the basal Cambrian conglomerate and close to the path at the top of the cliff of St. Non's Bay, St. Davids. Where exposed the lenticle is about two feet six inches in length and seven inches at its greatest thickness, stringing out at the ends in dark lines. Thin veins of infiltrated quartz traverse the rock and are cut off by the falsebedded conglomerate; Mr. Green has noticed that a certain amount of apparently similar material occurs as augen in the sheared tuffs of the Upper Pebidian and also in the conglomerate elsewhere, particularly at Treliwyd, two miles north of St. Non's Bay. The specimen examined is a dark-grey granular crystalline rock with sub-metallic lustre on a fresh fracture, and is duller but free from alteration products on the weathered surfaces. It is thinly veined with quartz in two directions, has a dark red streak, a specific gravity of 4'5, and a degree of hardness ranging from 6'0 to 6'5. The apparent evenness in the size of the rounded grains, to the naked eye and under a pocket lens, suggested the possibility of the rock being of oolitic origin, whilst an igneous origin would not be inconsistent with its megascopic character. Under the microscope however these two theories of its origin seem to be quite untenable. The irregularity in the forms