Whitsun Field Meeting atAberystwyth

7-11 June 1957

Directors: ALAN WOOD, A. J. SMITH, N. J. PRICE

Report by ALAN WOOD

Received 4 October 1957

THE PARTY OF eighteen members and friends assembled on the evening ofthe 7th to hear an account of recent work in the area from Professor AlanWood and Dr. N. J. Price, which was illustrated by coloured lantern slides.

Saturday, 8 June

Cader Idris. The party drove to Llyn Gwernan (23/704159), south-west ofDolgelly, observing and examining on the way the glacial diversion of theRiver Leri, overflow channels along the course of the Dyfi, slump beds inthe Bala near Tre-Taleisin (22/668940), Borth Bog, the Lyfnant tear-fault,and the glaciated cliffs of Aberdovey in the distance. Near Corris rainbecame severe; nevertheless the party was able to leave the coach to takepictures of Tal-y-Llyn, to view the crags of the Upper Acid Series, thecontrasted screes of the Granophyre above and the rounded slopes ofthe Mudstones to the west. After arrival at Llyn Gwernan the weathercleared, and the party proceeded towards Cader Idris, turning eastwardsfrom the path about half a mile from the road, to observe the tuffs andlavas of the Lower Acid Group and adinolisation of the associated sedi­ments. Good volcanic bombs were collected. After observing a largedolerite sill, the party then examined the Lower Basic Series west ofLlyn-y-Gafr, where the whole range of variation within the series was seen,including vesicular and non-vesicular lavas, thin dolerite dykes, pillowlavas and thin bands of shaly sediment. At the moraine-dammed lake ofLlyn y Gader (23/708136)the great granophyre sill with giant columns wasclearly visible, but the beds above were shrouded in cloud. Here the partyseparated, one group keeping to the lower ground, going as far as LlynAran (23/734139) and observing excellent glacial features, including thetumultuous granophyre blocks of Cerrig Nimbwl, which Professor Woodsuggested might have slid over a stagnant corrie glacier to their presentposition. The larger group climbed Cader Idris by the Foxes Path,

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PR O C. GEO L. ASSO C. , VOL. 69 (1 958) PL A T E I

Photograph by the late John F. Hay ward

Te cton ic ripples in Aberystwy th Grits

Tectonic ripples on the sur face of greywacke beds, plunging parallel to the axis ofdr ag-folds. Half a mil e no rth of the end of the promenade, Aberystwyth

[To fa ce p . 28

FIELD MEETING AT ABERYSTWYTH 29

descending by the usual route, past the Upper Basic Series, the Llyn CauMudstones and the Upper Acid Series to Minfordd (23/732115),where theother party, refreshed by tea in Dolgelly, awaited them.

Sunday, 9 June

Aberystwyth to Wallog. Walking northwards from Aberystwyth along thebeach the party observed the sedimentary and tectonic structures of thetypical Aberystwyth Grits. Vortex casts, load casts, groove casts andtransverse rippling occur on the underside of beds, casts of organic tracksof various kinds also being seen. Within the beds convolute bedding,lamination and grading were distinguished. The Directors pointed out thethree key-beds which had enabled them to correlate across folds and faultsas far as Clarach (22/588837). Much discussion was aroused by the net-likemesh of Palaeodictyon (Retiofucus of Keeping), the general opinion beingin favour of an organic origin. The remarkable drag-folds and associatedshear structures near Cormorant Rock (Plate 1) were demonstrated byDr. Price, who showed that two periods of movement must be invoked.A peculiar saddle to the south and recent folding caused by landslippingwere also seen. As the party approached Clarach an ash band was pointedout, and the correlation with beds near Gilfach was discussed.

After lunch, the party continued to Wallog (22/590858), observing thecorrelation by lithological pattern, the complicated tectonics and furthersedimentary structures. The fragmented (slump) beds, with vortex casts intheir bases, near Clarach, and the associated ripple drift bedding attractedmuch attention. Theories of the origin of the Sam were discussed atWallog, and observations made on the coastal geomorphology. BeyondWallog, attention was concentrated on the slurried beds here so well dis­played, it being pointed out that the slurried portion overlies the greywackein these northern exposures, while it underlies the greywacke portion in thesouth. The concept of these two types of density current chasing each otherdown the slope, and the possibility of a photo-finish in the central region,aroused some amusement.

Monday, 10 June

Aberystwyth to New Quay. The day was devoted to a study of coastalgeomorphology and the sedimentation of the southern facies of theAberystwyth Grits. South of Llanrhystyd (22/527686)the bevel and the twolevels of the solifluction terrace below were demonstrated. The party thenproceeded to Morfa Mawr (22/503657), and viewed the cliffsofthe northernpart of the Aberarth section. Here the bevel lies behind the remains of tworaised beaches, at about eighty feet and 200 feet, and these, as well as thefossil cliff of the submerged beach, could be clearly seen. The sedimentscontain a higher proportion of grit, in thicker beds, than at Aberystwyth,

30 ALAN WOOD

and composite beds with load-casting at the interfaces were commonlyseen. Tiny injection dykes within a greywacke bed aroused some discussion.After returning to the coach and reaching the top of the cliff, the partyviewed the whole sweep of Cardigan Bay, from Pembrokeshire to BardseyIsland , before proceeding to Cei Bach (22/410598). Here the fossil cliffwas excellently displayed; the present sea cliff being merely its strippedequivalent. Prolapsed beds, current bedding within thin greywackes whichat Aberystwyth would be merely laminated, and a greywacke 'frozen' inthe act of picking up portions of a bed beneath, were striking features of thesuccession . Some members of the party climbed around the headland ofCraig Ddu (22/425607) to examine a washout in the grits, while theremainder with Dr. Price observed the tectonics of this area. The party thenproceeded to New Quay, where Neptunean dykes and a possible washoutfilled with black shales aroused much discussion. The view from New Quaywas particularly fine, the whole bevelled coastal platform, with Cader Idris,the Rhinogs, Hebog and Snowdon beyond, standing out brilliantly in theevening sun.

After dinner the party visited the Geological Department at Aberystwyth.

Tuesday, 11 June

Devil's Bridge and Monks Cave. In the morning the party travelled bycoach towards Devil's Bridge, stopping at the quarry near Bwlch Farm(22/611796), where Mr. Challinor explained his theory of glacial over­turning of the highest beds. The lower beds by the road are upside down,their surfaces showing a variety of marks with which members were grow­ing familiar. Bounce casts were particularly well seen. At Devil 's Bridge,and on the road beyond, the probable old course of the Rheidol and thecauses of the river capture were discussed. The incised meanders in boulderclay were particularly fine. A further stop was made above Nant-yr-arian(22/718813), where glacial features and the remarkable leets were pointedout. At Aberystywth, where several members left the party, Dr. John F.Hayward moved a very warm vote of thanks to the Directors and particu­larly to the Secretary, Dr. A. J . Smith.

Lunch was consumed on the cliffs above Monks Cave (22/555745), andafterwards the cliff was safely descended. Egg Rock, an isolated stackdoomed soon to disappear, was duly admired. The party then moved south­wards, observing the coincidence of tectonic and sedimentary folds alongthe same axes a quarter of a mile south of Monks Cave. After passing thebase of a macro-graded unit of the series, attention was concentrated ontectonics, it being pointed out how the yielding of the thinner beds hadconcentrated pressure on the thicker ones, which eventually failed andbecame overthrust.

FIELD MEETING AT ABERYSTWYTH 31

Continuing south past a series of impressive cliffs the party reached anunnamed cove on the most deserted part of the coast (22/549731), wherecolonies of nesting sea-gulls and cormorants were seen. Here large vortexcasts and, in particular, remarkable slump masses like immense Swiss rollscaused animated discussion. The return was made by climbing the cliff,ropes having been provided, stakes driven in, and steps cut by the energeticSecretary and his assistant.