Geology of the country around Snowdon. Memoir for 1:50 000 geological sheet 119 (England and Wales)

By M F Howells and M Smith

Bibliographical reference: Howells, M F, and Smith, M. 1997. The geology of the country around Snowdon. Memoir of the British Geological Survey, Sheet 119 (England and Wales).

Authors: M F Howells, BSc, PhD M Smith, BSc, PhD British Geological Survey. Contributors Palaeontology A W A Rushton, BA, PhD S G Molyneux BSc, PhD andS P Tunnicliff, B.Sc. British Geological Survey Mineralisation T B Colman M.Sc. British Geological Survey Geophysics R M Carruthers B.Sc. British Geological Survey

London: The Stationery Office 1997. © NERC copyright 1997. First published 1997. ISBN O 11 884523 3

The grid used on (Figure 8), (Figure 12), (Figure 13), (Figure 15), (Figure 17) (Figure 18) (Figure 19) (Figure 20), (Figure 23), (Figure 26) and (Figure 27 is the National Grid taken from the Ordnance Survey map. Figure 2 is based on material from Ordnance Survey 1:50 000 scale maps, numbers 107 and 116. © Crown copyright reserved. Ordnance Survey Licence No. GD272191 / 1997.

(Front cover) Cover photograph Snowdon viewed from the south-west at Moel Hebog.

(Back cover)

Other publications of the Survey dealing with this and adjoining districts

Books

Maps

Geology of the country around Snowdon—summary

The Snowdon district lies close to the centre of the Snowdonia National Park and is formed of Cambrian and Ordovician sedimentary strata with a wealth of extrusive volcanic rocks and igneous intrusions. The resistant Ordovician volcanic strata and the associated intrusions dominate the highest ridges of the Snowdon massif whereas the lower and less-rugged, but deeply dissected, areas in the south and west of the district are underlain by Cambrian sedimentary strata.

The detailed mapping of the district has allowed interpretation of the sedimentological and volcanological processes involved in the early evolution of this sector of the basin which was sited across Wales during Lower Palaeozoic times.

The lowest Cambrian sedimentary sequence is mainly of turbiditic sandstones and mudstones with intercalated laminated, black hemipelagic mudstones, and reflects a basinal depositional environment. Later in Cambrian times, the sandstone and silty mudstone associations indicate more open shelf conditions which were terminated in early Ordovician times by uplift and local emergence. The marine transgression and westwards overstep during Arenig times was followed by general subsidence and the widespread deposition of silty mudstone and fine-grained sandstone. In early Caradoc times, a central rift structure developed across the district which controlled the complex development of the Snowdon volcano, the distribution of the erupted materials and the patterns of the associated sedimentary rocks. The volcanic activity was dominated by voluminous, acidic, ash-flow tuff eruptions and a major caldera developed close to a shoreline. A sequence of basic volcaniclastic sedimentary rocks with associated basalt lavas and hyaloclastite has been interpreted to reflect the shallow-marine reworking of small basaltic island volcanoes. At the cessation of the volcanic activity there was rapid subsidence and widespread deposition of black mud.

The rocks which had accumulated in the Lower Palaeozoic basin were folded, cleaved and uplifted during end-Silurian to early Devonian times, at the height of the Caledonian orogeny; the rocks were metamorphosed to low greenschist grade. The later, Variscan (late Carboniferous) deformation cannot be distinguished and it was possibly restricted to movement along established faults. It is possible that the coastal faults which controlled the development of offshore sedimentary basins during Mesozoic times are represented below the tracts of estuarine alluvium in the south-west of the district. The spectacular cwms and U-shaped valleys, the moulded crags and plucked high ridges, the perched blocks and moraines all testify the Quaternary glaciation.

(Succession) Geological succession in the Snowdon district.

History of the survey of the Snowdon geological sheet (119)

The primary survey of the district, at the scale of one inch to one mile, was accomplished by A C Ramsay, W T Aveline and J B Jukes and the map, Old Series Sheet 75 NE, published in 1851, is close to the area of the 1:50 000 geological Sheet 119 (Snowdon). No sheet memoir was published to accompany this early map, but descriptions of the geology were incorporated into Ramsay's classic memoir on the geology of the whole of North Wales (1st edition, 1866; 2nd edition 1881).

The recent investigations by the Geological Survey into the Lower Palaeozoic rocks of North Wales began in the 1960s with the mapping of the Silurian strata in the Denbigh district Sheet 107, and later the Cambrian and Ordovician strata on the Bangor and Harlech districts, sheets. The Ordovician sequence in the Snowdon district 119 Sheet was mainly surveyed between 1982 and 1986 by M F Howells, A J Reedman, S D G Campbell and M Smith. In the north-east of the district, the area of overlap of the Betws y Coed and Capel Curig 1:25 000 Geological Sheet SH 75 incorporates the survey accomplished by M F Howells, C D R Evans, E H Francis and B E Leveridge between 1969 and 1973. In the north-west of the district, the overlap of the Llyn Padarn 1:25 000 Geological Sheet SH 55N, 56S, was surveyed by B D T Lynas between 1975 and 1980. In addition, we have incorporated the line- work from previous mapping in three areas: the northern edge of the Harlech Dome in the south of the district by C A Maley and T S Wilson (1946), north-west of Blaenau Ffestiniog by A V Bromley, Camborne School of Mines (unpublished) and the edge of the Migneint by B D T Lynas (1973).

The 1:10 000-scale geological maps included wholly or partly in 1:50 000 geological Sheet 119 (Snowdon) are listed below with the initials of the surveyors and the dates of the survey. Dyeline copies of these maps are available for purchase or public reference in the libraries of the British Geological Survey in Keyworth and Edinburgh.

SH 53 NW MS 1986–1987
SH 53 NE MS 1987
SH 54 NW MS, SDGC 1984–1985
SH 54 NE SDGC, MFH, AJR, MS 1984–1985
SH 54 SW MS 1986–1987
SH 54 SE MS, SDGC, MFH, AJR 1985–1986
SH 55 SW BCW, SDGC, MS 1980–1985
SH 55 SE SDGC, MFH, AJR 1983–1985
SH 63 NW MS 1987
SH 63 NW after CAM, TSW 1946
SH 63 NE MS after CAM, TSW 1946
SH 64 NW MFH, SDGC, AJR 1982–1983
SH 64 NE SDGC 1982–1983
SH 64 SW MS 1985–1986
SH 64 SW RVB 1963
SH 64 SW AVB 1963
SH 64 SE MS 1987
SH 64 SE after AVB 1963
SH 65 SW MFH, AJR, SDGC 1982–1983
SH 65 SE SDGC, AJR, MFH 1982–1983
SH 73 NW MS 1987
SH 73 NE MS, BDTL 1986–1987
SH 74 NW MFH 1987
SH 74 NW after AVB 1963
SH 74 NW BDTL 1973
SH 74 NE MFH 1986–1987
SH 74 NE after BDTL 1973
SH 74 SW MFH 1986
SH 74 SW after AVB 1963
SH 74 SW BDTL 1973
SH 74 SW CAM, TSW 1946
SH 74 SE MS 1988
SH 74 SE after BDTL 1973
SH 75 SW EHF, MFH, BEL, CDRE 1968–1969

Acknowledgements

This memoir has been written mostly by M F Howells and M Smith but, as in all BGS work, it has been to a very large extent, a group effort. The contributions of all our co-authors in the many previous publications is acknowledged and especially the contributions of S D G Campbell and A J Reedman. A W A Rushton contributed greatly to the Cambrian chapter, the Tremadoc Series section and, together with S G Molyneux and S P Tunnicliff, was responsible for the palaeontology. Mr S Jusypiw kindly provided palaeontological data from his unpublished research into the Tremadoc rocks of Ynyscynhaiarn. R B Evans and B Chacksfield carried out most of the geophysical investigations and R M Carruthers produced the account. T B Colman contributed the section on volcanogenic mineralisation and, from time to time, R J Merriman and T K Ball were consulted about petrographical and geochemical matters. The memoir was edited by A A Jackson.

Notes

Preface

This memoir describes the geology of the Snowdon district which lies in the heart of the Snowdonia National Park. It is an area of spectacular landscape, from the shore edge of the Glaslyn Estuary to the serrated high ridges about the summit of Snowdon, and it has been a permanent reference in the cultural and social evolution of Wales and the Welsh. Snowdonia has a special place for the geological community because of its Ordovician sequence, which figured in the great debate between Sedgwick and Murchison during the 19th century. The sequence was termed Upper Cambrian by Adam Sedgwick, mapping up from the Cambrian in the Harlech Dome, and Lower Silurian by Roderick Murchison, mapping down from the Silurian of the Welsh Borders. In 1879 Charles Lapworth resolved the controversy by defining the Ordovician System but, by that time, both the formidable protagonists had died.

The broad outline of the geological sequence was originally established by the publication in 1851 of the maps at a scale of one inch to one mile by the Geological Survey. These maps provided the basis for A K Harker's classic essay on the Bala Volcanic Rocks of Caernarvonshire, published in 1889, and further interest in the volcanic rocks was stimulated later, in 1927, by H Williams's map and account of the geology of the Snowdon. However during the early years of this century the work was mainly biased towards the stratigraphy as expressed in the papers of W G Fearnsides, on the Tremadoc district, and C A Matley and T S Wilson on the Cambrian of the Harlech Dome which encroaches the southern edge of the district.

The recent investigations by the Geological Survey into the Lower Palaeozoic rocks of North Wales began in the 1960s with the mapping of the Silurian strata on the Denbigh Sheet (107) and later the Cambrian and Ordovician strata on the Bangor Sheet (106) and the Harlech Sheet (135). In 1982 a multidisciplinary study of the Caradoc volcanic rocks of central and northern Snowdonia was begun. This study involved extending the mapping south from the Bangor district, on to the Snowdon Sheet (119), and a major enquiry on the volcanic rocks to interpret their physical characters, geochemical evolution and their sedimentary and tectonic setting. This work has been published in various specialist journals and all the strands of the enquiry have been brought together in a book Ordovician (Caradoc) marginal basin volcanism in Snowdonia, north-west Wales (Howells et al., 1991). This memoir considers the overall Cambrian and Ordovician sequence, which crops out within the district, and the processes involved in its accumulation. Particular attention has been placed on the contained faunas and the biostratigraphy, especially at the historical type section of the Tremadoc Series which lies close to Tremadog village.

The topography was profoundly affected by the Devensian ice sheet although the associated drift deposits are generally of restricted extent in the mountainous terrain. In the past there has been massive exploitation of slate, especially in the vicinity of Blaenau Ffestiniog, but the current extraction is very small. Because of the National Park there is little likelihood of extensive metalliferous mine development or of quarrying the vast supply of hard rock which lies within the district.

I hope that this succinct account and the accompanying maps will satisfy the need of the increasing number of people who are keen to understand the processes involved in the development of our landscape. In addition, I hope that it will form the firm base that is essential to more specialist geological studies.

I gratefully acknowledge the invaluable cooperation of local land owners in allowing BGS staff access onto their land in the course of the survey.

I hope that this memoir will enhance the enjoyment of this wonderful area by the many tourists, walkers and climbers that visit it and the people who are fortunate to live within it.

Peter J Cook, CBE, DSc, CGeol, FGS Director, British Geological Survey Kingsley Dunham Centre Keyworth

Nottingham. NG12 5GG

Grug y mynydd yn eu blodau

Edrych arnynt hiraeth ddug

Am gael- aros ar y bryniau

Yn yr awel efo’r grug

(Ceiriog, Nant y mynydd)

Chapter 1 Introduction

Most of the district (Figure 1) and (Figure 2) lies within the Snowdonia National Park and is renowned for its spectacular scenery. In the north it is dominated by the high ridges of the Snowdon massif, termed the Snowdon Horseshoe, but throughout it comprises deeply dissected mountainous terrain. Nowhere is this more clearly displayed than from the estuarine flats close to Porthmadog where the Glaslyn, Nantmor, Croesor and Ffestiniog valleys converge in an arc from north to east, and the intervening ridges dominate the skyline. The sparse population is spread between the main towns at Porthmadog, Penrhyndeudraeth and Blaenau Ffestiniog and the many small villages across the district.

The peaks of the Snowdon Horseshoe are Snowdon, 1085 m, Crib y Ddysgl, 1065 m, and Lliwedd, 898 m; other principal peaks of the district are Moel Hebog, 782 m and Moelwyn Mawr, 770 m. However the ground rises sharply from sea level, at Porthmadog, and much of the area lies more than 500 m above OD. In the east of the district the terrain is less rugged and here, to the south-west of Blaenau Ffestiniog lies the great peat covered swathe of the Migneint, the source of Afon Conwy. The main watershed lies along a line, trending approximately north-north-west, to the east of Blaenau Ffestiniog; to the west of the line, Mon Gwynant, Mon Nantmor, Mon Croesor and Mon Dwyfor flow south-westwards to Mon Glaslyn and its estuary. To the east of the watershed Mon Gwyryd, Mon Lledr and Mon Penmachno flow eastwards to join the Mon Conwy near Betws y Coed.

Hill farming, forestry, tourism and quarrying are the main industries. In the past, Blaenau Ffestiniog was one of the main centres of slate extraction in North Wales, but for many years the industry has been in decline and now the activity is much restricted and, in some instances, linked with tourism.

Previous research

Systematic geological work in North Wales began in the early 19th century. Both William Smith's map, of 1815, and Greenhough's map, of 1820, include observations made in the district. Sedgwick and Murchison commenced work in the area in 1831, and in the following years Sedgwick gathered much information which he incorporated into his geological map of North Wales, published in 1845. A year later Daniel Sharpe published his map which delineated the main structures of the area.

The Geological Survey began their survey of the district in 1846 and the map of the district in the vicinity of Snowdon (Old Series one-inch Sheet 75 NE) was published in 1851. These maps provided the basis for the first comprehensive account of the geology in the 1st edition of the memoir, The geology of North Wales (Ramsay, 1866). As a result of the maps and the memoir, Harker (1889), in his essay on the Bala Volcanic Series of Caernarvonshire, was able to extrapolate the relationships of the extrusive volcanic rocks in the stratigraphy and to relate them to their possible eruptive centres. During this century, significant contributions to the geology of the area have been made by Williams (1927), with his map of the Snowdon massif and detailed petrographical descriptions of the volcanic rocks, Fearnsides (1910) and Fearnsides and Davies (1944), on the stratigraphical relationships of the Tremadoc district, and Matley and Wilson (1946) on the Cambrian sequence. More recently, Shackleton (1959), Beavon (1963), Roberts (1969), Rast (1969), Bromley (1969) and Wilkinson (1988) have all elucidated aspects of the geology and influenced many of our interpretations.

Geological history

The oldest Cambrian rocks in the district lie in the upper part of the Harlech Grits Group. They comprise coarse-grained turbidites interbedded with silty mudstones and siltstones which are a segment of a thick basinal sequence (Allen and Jackson, 1985). They are overlain by dark grey carbonaceous mudstone of the Clogau Formation, at the base of the Mawddach Group, which marks a hiatus in turbidite deposition. Resumption of turbidity current activity occurred in the overlying Maentwrog Formation, but the influxes of detritus were much finer grained and the deposits more thinly bedded than previously. Towards the top of the formation there is a progressive increase in mudstone deposition which has been interpreted (Allen and Jackson, 1985) to mark the change from basinal to open-shelf deposition.

The Ffestiniog Flags Formation and the broadly equivalent Marchlyn Formation comprise interbedded fine-grained sandstones, siltstones and silty mudstones which reflect nearshore, shallow-marine environments and temporary influence of wave activity. Local shallowing, to littoral or deltaic settings, is indicated by the coarse-grained, channelised Carnedd y Filiast Grit member.

The Dolgellau Formation, black, parallel-laminated, carbonaceous mudstone, marked a sudden change to quiescent, dysaerobic conditions and was probably the result of a eustatic rise in sea level. The change is emphasised by the association of graptolitic mudstones with shelf sandstones in the overlying Dol-cyn-afon Formation, across the Merioneth–Tremadoc boundary. This association suggests that the anoxic interface periodically rose above the shelf edge and encroached the sublittoral areas. The Merioneth–Tremadoc boundary is marked by a renewed phase of transgression and is related to a global sea-level rise (Fortey, 1984; Landing, 1988). Within the district, thickness changes and facies variations in the Dol-cyn-afon Formation have been related to contemporaneous tectonic activity. Within the formation there is no indication of the Rhobell Volcanic Group which was emplaced at this time on the south-east side of the Harlech Dome.

Unlike other areas in North Wales, the Tremadoc–Arenig boundary is only marked by a slight disconformity even though there is a profound lithological change. The coarse conglomerates and sandstones of the Allt Lŵyd Formation represent the shallow-marine reworking of a fan delta, possibly alluvial, and subsequent deepening.

The overlying Nant Ffrancon Group and Cwm Eigiau Formation are dominated by blue-grey silty mudstones, siltstones and fine-grained sandstones which reflect open-marine conditions and poorly oxygenated bottom waters. These strata are of Llanvirn to Caradoc age, although the dearth of diagnostic faunas inhibits detailed biostratigraphy. Throughout this sequence local sedimentological changes are recognised in restricted formations and similarly three significant volcanic episodes are distinguished.

The Rhiw Bach Volcanic Formation, represented by a comparatively local accumulation of acidic ash-flow tuffs and much fine-grained acidic tuff, reflects mainly explosive rhyolitic activity which was largely contained within the marine environment. The Moelwyn Volcanic Formation is largely represented by reworked, volcanic-clastrich, debris-flow deposits, with minor tuffs and intercalated silty mudstones. Their stratigraphical association with intrusive to extrusive rhyolites indicate that these represent the source areas, and the local incursion of reworked, conglomeratic debris suggests temporary shallowing of these areas to within wave base.

Near the top of the sequence, the influxes of sand represented by the Prenteg Sandstone and Moel Hebog Sandstone members indicate local temporary shallowing into nearshore and deltaic environments. More widespread events are reflected in the Penamnen Volcanic and Capel Curig Volcanic formations although neither are as significant, within the district, as the subsequent volcanic activity. Both are represented by acidic ash-flow tuffs which were emplaced in a shallow-marine environment and locally reworked. The eruptive centres of both events lie to the north of the district.

Within this sequence there is evidence of uplift, transgression and local discontinuities. The most problematic and profound is that which occurs below the base of the Caradoc; it has been referred to as the sub-gracilis unconformity and, in the Tremadog area, it oversteps Arenig and Tremadoc strata. Local olistostromes and disturbed strata developed in response to the uplift.

The main episode of volcanic activity within the district is reflected in the Snowdon Volcanic Group. The Pitts Head Tuff Formation comprises two major, primary, acidic ash-flow tuffs which were erupted from a centre in the vicinity of Llwyd Mawr. The ash flows transgressed from a subaerial environment, as distinguished in the Moel Hebog Sandstone Member, into a marine environment to the north. The Lower Rhyolitic Tuff Formation represents a complex eruptive cycle which was centred along a major north-east-trending fracture zone. During the activity, a major asymmetric downsag caldera developed in a shallow-marine environment and following the main eruptive phase, the top of the intracaldera tuffs were locally reworked. A period of basaltic activity, represented by the Bedded Pyroclastic Formation, occurred later. The sequence is dominated by volcanoclastic sedimentary rocks and the complex patterns of sedimentation developed from the reworking of small basaltic island volcanoes in a shallow sea. The final expression of eruptive volcanism was renewed acidic activity, the Upper Rhyolitic Tuff Formation, and the emplacement of a major acidic ash-flow tuff.

The highest beds within the district are black graptolitic mudstones in the core of the Dolwyddelan Syncline and the absence of shallow-water reworking at the top of the underlying volcanic sequence indicates rapid post-emplacement subsidence.

The copper, lead and zinc mineralisation shows a close relationship with the volcanic centres. The deposits were derived by late-stage, hydrothermal activity during which circulating fluids leached metals from the volcanic rocks and deposited them in fractures which had developed during the construction of the centres.

Throughout the district, there is evidence of intrusive activity at all stratigraphical levels. Within the Cambrian, intrusions of intermediate composition are related to a complex of intrusions in the Harlech district to the south. These are probably genetically related to dolerite intrusion and effusive basaltic activity which occurred during Tremadoc times. Within the Ordovician, the intrusive rocks are both rhyolitic and basaltic in composition with minor intermediate compositions. The coincidence of the main concentrations of rhyolite and dolerite/basalt intrusions close to the eruptive centres is evidence against the intrusions being a separate, and possibly much later, event.

The main structural features developed during the climactic end-Silurian to early Devonian deformation. However, there is much evidence throughout the district that many of the faults indicate repeated activity of longstanding, deep-seated, basement-controlled fractures which influenced sedimentation and the siting of the volcanic activity. The main folds are broad and open periclinal structures, trending north-east-south-west, with a well-developed, steeply inclined, axial-plane cleavage. Locally, tighter folds, trending east-west, and areas of more gently dipping cleavage can be attributed to result from interference by subsurface intrusions.

Mesozoic and Tertiary rocks are nowhere exposed in the district. However, the effects of the Quaternary glaciations are dramatically displayed in the sculpted, mountainous landscape while the resultant deposits, mainly locally derived boulder clay, are in the lower ground.

Chapter 2 Cambrian

The Cambrian System was distinguished and named by Sedgwick (Sedgwick and Murchison, 1835) and, although subsequent work has restricted and refined the use of the term Cambrian (Stubblefield, 1956), the Harlech Dome, to the south of the district, has consistently been regarded as the historical type area for the Cambrian System. Salter, working mainly in the Ynyscynhaiarn area and in the Vale of Ffestiniog, palaeontologically characterised the Cambrian succession of North Wales (summarised in Ramsay, 1866). The succession was later lithologically subdivided by Belt (1867, 1868), and by Fearnsides (1905, 1910) who recognised and mapped a number of subdivisions around the northern fringes of the Harlech Dome. Matley and Wilson (1946) subdivided the Cambrian succession in the Harlech Dome area, and this work was revised by Allen and Jackson (1985).

The Cambrian succession (Figure 3) is dominated by sedimentary clastic rocks, including well-bedded siltstones and sandstones with subordinate black mudstones and silty mudstones. Within the district these rocks crop out in two areas. To the south of the Vale of Ffestiniog, the stratigraphy of the Cambrian rocks is essentially that of the Harlech district (1:50 000 Series Sheet 135) and can be divided into the Harlech Grits Group and the overlying Mawddach Group (Allen and Jackson, 1985).

The Harlech Grits Group is represented only by its upper divisions, exposed in the core of the Dolwen Anticline, east of Llyn Trawsfynydd, but a complete sequence of the Mawddach Group crops out about its periphery, and is repeated in the Ynyscynhaiarn Anticline in the south-west of the district. In the north-west of the district, the small Cambrian outcrop is part of the sequence more representative of the Bangor district (Howells et al., 1985).

A total thickness of about 1400 m is estimated for the Cambrian succession in the north-west of the district but, because of the deformation and restriction of outcrop this must be regarded as a minimum. In the south-west of the district a total thickness of about 2100 m is estimated.

The chrono- and lithostratigraphical subdivisions of the groups are shown in (Table 1).

Harlech Grits Group

This group was formally defined by Allen and Jackson (1985) in the Harlech district where it was divided into the Rhinog Formation, Hafotty Formation, Barmouth Formation and the Gamlan Formation, in upward succession (Figure 3). Their distribution is generally as that mapped by Matley and Wilson (1946) and detailed descriptions are provided by Allen and Jackson (1985).

Rhinog Formation

The Rhinog Formation, 300–400 m thick, consists of medium- to coarse-grained, normally graded, sandstone turbidites, commonly occurring in beds about 1 m thick, with interbedded thin siltstones and mudstones. Locally, the sandstones are conglomeratic. Within the district the formation is poorly exposed around the core of the Dolwen Anticline, east of Llyn Trawsfynydd. The strata are mainly interpreted as proximal turbidites (Allen and Jackson, 1985) with sole marks and groove clasts indicating derivation mainly from a northerly source (Crimes, 1970).

Hafotty Formation

The Haffoty Formation, 150 m thick, was described in detail by Woodland (1938, 1939) and it outcrops along the north-west shore of Llyn Trawsfynydd where it is mainly of sandstones with cross- and parallel-laminations. In the Harlech district, to the south, it comprises grey, thinly bedded, fine-grained sandstones and siltstones with a horizon of manganese ore and a thin bed of pyritous mudstone near the base; the manganese ore has not been observed as far north as Trawsfynydd.

Barmouth Formation

The Barmouth Formation, 60 m thick, crops out in low ridges around the closure of the Dolwen Anticline, north-west of Llyn Trawsfynydd. It comprises mainly coarse-grained, pebbly turbiditic sandstones which to the south of the district display four upward-fining cycles. The proportion of sandstone to mudstone is high. Flute-and groove-cast structures show a general north-northwest alignment and suggest derivation from the south (Crimes, 1970).

Gamlan Formation

The Gamlan Formation, 150–240 m thick, is well exposed in the Ceunant Llennyrch below Llyn Trawsfynydd dam [SH 672 381]. It consists of grey, green and purple mudstones, locally pyritous, with few beds of turbiditic sandstone. In places the upper beds are manganiferous and contain tuffitic mudstones (Allen and Jackson, 1985). Flute and groove casts indicate deposition from northerly directed currents (Crimes, 1970).

Sandstones of the Fachwen Formation (Arlon Group), (1:50 000 Geological Sheet 106; Reedman et al., 1984) crop out in quarries in the extreme north-west of the district. These are overlain by a thinned and faulted succession of silty mudstones, the Llanberis Slates Formation, and coarse turbiditic sandstones, the Bronllywd Grit Formation.

Biostratigraphy of the Harlech Grits Group

Within the district, faunal evidence from the Cambrian formations is sparse and there remains much uncertainty about precise correlations. In the Bangor district, the Llanberis Slates Formation has yielded trilobites (Howell and Stubblefield, 1950) which Cowie et al. (1972) assigned to the upper part of the Olenellid Biozone of the mid-Comley Series; the age of the overlying Bronllwyd Grits Formation remains uncertain.

The Rhinog Formation is equated with the Hell's Mouth Grits Formation of the St Tudwal's succession, which is correlated with the upper Comley Series (Bassett et al., 1976). The ages of the Hafotty and Barmouth formations remain unknown, but the overlying Gamlan Formation has yielded Paradoxidid trilobites (St David's Series) in the Harlech district (Allen and Jackson, 1985).

Mawddach Group

The Mawddach Group is divided, in upward succession, into the Clogau Formation, the Maentwrog Formation, Ffestiniog Flags Formation and Dolgellau Formation (the Dolgellau Member of the Harlech district).

Clogau Formation

This formation, 80–100 m thick, comprises dark grey to black banded pyritous mudstones and silty mudstones, with minor sandstones (Allen et al., 1981). It crops out around the north-west limb of the Dolwen Anticline, north of Llyn Trawsfynydd, and is faulted out on the eastern limb by the Trawsfynydd Fault. It is well exposed in Ceunant Llennyrch [SH 667 385] where up to 80 m of dark grey, cleaved mudstones overlie sandstones of the underlying Gamlan Formation. The pyritous black mudstones suggest deposition in a predominantly low-energy environment with periodic, storm-induced currents introducing coarser detritus from an adjacent landmass (Allen et al., 1981).

Maentwrog Formation

The Maentwrog Formation, 500–700 m thick, was originally established by Belt (1867) in the area around Maentwrog and was referred to the lower part of the 'Lingula Flags'. However, Allen et al. (1981) rationalised the subdivisions of the formation in the Harlech district and defined a type section at the southern edge, with the base exposed in Afon Hirgwm near Bontddu.

The Maentwrog Formation consists of interbedded dark grey, finely laminated silty mudstones, grey siltstones with some thin interbeds of quartzose and feldspathic sandstone (Plate 1.1). Within the district, the lower beds crop out at Rhaeadr-ddu in Ceunant Llennyrch [SH 667 388] but the base is obscured by faulting and an intrusion. Higher parts of the formation are well exposed in road cuttings on the south side of the Vale of Ffestiniog (Hawkins and Jones, 1981), south-west of Portmeirion [SH 5883 3680] just at the edge of the district, and on the foreshore at Graig Ddu [SH 5227 3745]. East of the Trawsfynydd Fault, up to 500 m of silty mudstones, with few sandstones, are exposed in numerous outcrops as far south as Cwm Prysor.

Sedimentary structures, including ripple-drift lamination, mud-drapes and cross-bedding, are common within the siltstone beds. Convolute lamination, loading, sedimentary dykes (Plate 1.2) and liquefaction structures are common in the sandstone beds (Smith, 1988). Just to the south of the district, at Ynys Cyngar [SH 5541 3653], Crimes (1970) recorded bioturbation and trace fossils (Rusophycus), and abundant directional sole-markings indicating deposition by southerly derived currents. The petrography of the beds was described by Allen et al. (1981).

Compared with the Clogau formation, deposition of the Maentwrog Formation was rapid, and occurred in a deepening basin (Crimes, 1970; Allen et al., 1981). The uppermost finer-grained beds represent more distal turbidite deposits than those below which were possibly reworked by deep sea currents. The presence of Olenid trilobites in the dark grey mudstones suggests periods of reduced oxygenation.

Ffestiniog Flags Formation

The Ffestiniog Flags Formation, 475–650 m thick, is equivalent to the 'Lingula Flags' or 'Ffestiniog Group' of the early surveyors (Ramsay, 1866; Belt, 1867) and is characterised by the presence of the brachiopod Lingulella clayish. The informal five-fold subdivision of Fearnsides (1910) has not been proved by this survey and here, as in the Harlech district (Allen and Jackson, 1985), the formation is undivided.

The Ffestiniog Flags Formation consist of regular alternations of dark grey, massive, poorly laminated mudstone and paler grey, quartzose siltstone and sandstone (Plate 1.3). The latter occur in beds, up to 2 m thick, which, with the intervening mudstones, give rise to a distinctive scarp and slack topography, as about the Ynyscynhaiarn Anticline and along the Vale of Ffestiniog. The formation is well exposed along the foreshore south of Borth y Gest and around Portmeirion [SH 5861 3674], at the southern edge of the district, where the transition from the underlying Maentwrog Formation is well exposed. Road sections in the Vale of Ffestiniog and south of Blaenau Ffestiniog have been described by Hawkins and Jones (1981) and Bromley (1963). The Ffestiniog Flags Formation ranges in thickness from about 475 m at Ynyscynhaiarn to more than 650 m in the Migneint area.

The mudstones consist of fine sericite, chlorite and muscovite with scattered chlorite-mica stacks and disseminated pyrite. The sandstones are mainly quartzose, well sorted and fine to medium grained. Their top and bottom contacts are sharp. Cross-lamination, ripple-drift lamination and scour-and-fill structures, indicative of high-energy environments, are common throughout, and at Borth y Gest [SH 5633 3721] intraformational slumping, convolute lamination and load structures indicate penecontemporaneous disruption of the sequences.

In the area between Criccieth and Ffestiniog, flute-and groove-casts indicate currents flowing from the south-south-west (Crimes, 1970). The lithologies are favourable for the preservation of trace fossils and various types have been recorded, including Cruziana, Dimorphichnus, Rusophycus, Skolithos, and subhorizontal burrows (Crimes, 1970; Briggs and Rushton, 1980).

The Ffestiniog Flags Formation is closely comparable with the Maentwrog Formation, but the absence of clearly determinable turbidites, the presence of shallow-water trace fossils (e.g. Skolithos) and occurrence of Lingulella, suggest the sequence was deposited in a shallow littoral or subtidal environment. Mudcracks, reported by Fearnsides (1910), have not been observed but they would imply temporary emergence. The formation is thought to reflect a period during which sedimentation rates were greater than the rate of subsidence (Crimes, 1970).

Marchlyn Formation

The Marchlyn Formation, the lateral equivalent of the Ffestiniog Flags Formation and possibly part of the Maentwrog Formation, was described in the Bangor area (Howells et al., 1985, British Geological Survey 1985 a, b). Within the district the formation, 240–430 m thick, crops out in the cores of a series of faulted periclines along Cwm Pennant to Mynydd Tal y Mignedd and, farther north, about the Mynydd Mawr intrusion. The Marchlyn Formation consists of grey, flaggy silty mudstones, with paler interbeds of laminated sandstone. In the upper part of the sequence, a group of coarser sandstones and grits is distinguished as the Carnedd y Filiast Grit Member. Across most of the district only the upper 200 m of the formation is exposed, of which the Carnedd y Filiast Grit Member represents about 30–60 m. The base of the formation is nowhere exposed and the top is poorly exposed and obscured by quartz veining and faulting.

The formation is overlain by the Dolgellau Formation in the faulted fold cores in northern Cwm Pennant thus indicating its lateral equivalence to the Ffestiniog Flags Formation. However, the lithologies of the Marchlyn Formation are coarser grained and quartzose, and locally channelled conglomerates have been determined. Slumping and local debris-flow deposits [SH 5295 4803] with rafts of coarse sandstone [SH 5267 4685] are locally well developed. Trace fossils, including Cruziana semiplicata (Crimes, 1970), are common in places and Lingulella davisii has been collected.

The Carnedd y Filiast Grit Member has a sharp base and consists of thick (about 0.5 m) beds of coarse quartz sandstone and conglomerate, composed predominantly of pebbles of hyaline and blue quartz, up to 20 mm in diameter, with poorly defined cross-lamination and grading. Some beds are irregularly channelled with erosional basal contacts. In the Bangor district, Crimes (1970) interpreted these sandstones to be shallow-water deposits, derived from a local source lying to the northwest.

The Marchlyn Formation is considered to be a coarser-grained equivalent of the Ffestiniog Flags Formation. The sedimentary structures and the upward-coarsening sequence with distinctive quartz clast assemblages indicate deposition close to a landmass which, from the evidence of the palaeocurrents, probably lay to the northwest (Crimes, 1970). In Cwm Pennant the formation reflects a transition between a near-shore, littoral environment, to the north, and an outer-shelf environment, to the south. In contrast, the Ffestiniog Flags Formation is considered to represent a thicker, more distal or shelf sequence which was deposited by southerly derived currents eroding a landmass of metamorphic and plutonic rocks.

Dolgellau Formation

The formation was originally described by Belt (1867) and Fearnsides (1905, 1910) and termed the Dolgellau Member in the Harlech district (Allen et al., 1981). Its persistence around the Harlech Dome, in the Porthmadog area and northwards into the Bangor district confirms its importance as a marker horizon throughout North Wales.

The Dolgellau Formation, 35–60 m thick, consists of dark grey to black, carbonaceous and pyritous mudstones, generally thinly laminated, which mark a major change in sedimentation. Thin sandstone beds are rare; large carbonate nodules and small phosphate nodules occur locally. At outcrop the formation forms a pronounced topographic depression which can be traced around the Ynyscynhaiarn Anticline and along the southerly facing slopes of the Vale of Ffestiniog. Good coastal exposures occur around the Deudraeth peninsula, and parts of the sequence are well exposed north of Pont Afon Gam [SH 747 421] and in Nant Derbyniad Quarry [SH 776 415]. At Ogof Dhû, east of Criccieth [SH 5137 3794], the complete sequence, 54 m thick, is exposed to just outside the district.

The mudstones of the Dolgellau Formation comprise very fine chlorite and sericite with phosphate nodules, opaque carbonaceous grains and framboidal pyrite. The beds are finely laminated with up to 25 to 30 laminae per centimetre; bioturbation is generally absent. The section at Ogof Dhû (Figure 4) was compiled from Ponsford (1955 and an unpublished report) who showed that the level of radioactivity was higher than that in the adjacent formations. The base of the formation is marked by dark grey to black mudstones with pale siltstone laminae and it weathers distinctively ('fingerstall bed'). The source of the radioactivity was shown to be uranium in the phosphate nodules and it is enriched in the dark mudstone bands. Chemical analyses showed uranium contents of 0.0017 per cent in the black mudstones and up to 0.006 per cent in the basal beds.

The Dolgellau Formation represents a prolonged period of quiescent deposition. The high sulphide content, the fine laminations, undisturbed by bioturbation, and the faunas all indicate conditions of low oxygenation, comparable with the facies of contemporaneous beds on the east of the Harlech Dome (Allen et al., 1981), the Welsh Borders and central England, and farther afield in eastern Canada and the Baltic area.

Biostratigraphy of the Mawddach Group

The Clogau Formation in Ceunant Lennyrch and Tafarn Helyg, near [SH 689 397] has yielded trilobites of the mid-Cambrian St David's Series. Although a detailed biostratigraphy has not been determined, the faunas recorded by Lake (1906–1946) and Matley and Wilson (1946, p.14) include the trilobites Anopolenus henrici, Meneviella venulosa, Paradoxides davidis, Plutonides [Paradoxides] hicksii, Ptychagnostus barrandei and Pt. ciceroides. These indicate the presence of the Biozones Tomagnostus fissus, Hypagnostus parvifrons and Ptychagnostus punctuosus.

In the eastern part of the Harlech Dome, the basal part of the Maentwrog Formation is partly referrable to the St David's Series (Allen et al., 1981). Within the district, only the Olenus Biozone of the lower part of the Merioneth Series is proved. Poorly preserved Olenus and Homagnostus abound in the cleaved mudstones of Coed Cae'n-y-coed (Salter, 1866, p1.4, fig.11). Olenus cataractes (indicating the cataractesSubzone) has been collected from Cae'n-y-coed quarries and at Treflys, near [SH 532 374] (Lake, 1908, p.58); Homagnostus obesus occurs at the latter locality. Olenus cf. solitarius was collected from the highest beds of the formation just east of the intrusion at Graig Ddu (Rushton, 1983, p1.18, fig.3).

The Ffestiniog Flags Formation generally lacks fossils of proved stratigraphical value but Lingulella davisii occurs throughout the formation and is abundant at many localities, commonly occurring in winnowed layers. The lectotype specimen (Cocks, 1978, p.14) is from 'south of Penmorfa', about [SH 540 403]. The crustacean Hymenocaris vermicauda has been collected from various localities in the Tremadog area (Jones and Woodward, 1892). The age of the formation remains poorly constrained but must lie between the upper part of the Olenus Biozone and the middle part of the overlying Parabolina spinulosa Biozone (Allen et al., 1981).

The Marchlyn Formation underlies beds referred to the Parabolina spinulosa Biozone, and may thus be referable to that zone or to the underlying Olenus Biozone, or partly to each. On the ridge between Cwm dwyfor and Cwm y Ffynnon [SH 5395 5140], siltstone beds at the top of the Marchlyn Formation yielded Lingulella davisii (Plate 2). Acritarchs from Craig Trum y Ddysgl, around [SH 544 519] include Stelliferidium cortinulum, Timofeevia phosphoritica and Vulcanisphaera turbata in typical Merioneth Series microfloras.

The faunal succession in the Dolgellau Formation represents the greater part of the standard zonal sequence recognised in the Merioneth Series of Scandinavia (the zonal numbers follow Henningsmoen, 1957):

(Figure 5) shows the faunal distribution and inferred biozones for the measured section at Ogof Dhû where parts of zones III, IV, V(a), V(b) and V(c) are represented. Zone VI may be present but is not positively proved. Zone III is characterised chiefly by Orusia lenticularis and Parabolina spinulosa, and has been recognised at Penmorfa Church, about [SH 542 402] and in the area of Garreg-wen, about [SH 558 375]. Farther east, Zone III is proved above the Vale of Ffestiniog at Dduallt [SH 6745 4183], west of Moel Llechwedd gwyn [SH 7526 4175] and in Nant Derbyniad Quarry [SH 776 415] (Lynas, 1973). The faulted slice of Dolgellau Formation in Cwm y Ffynnon [SH 5403 5141] (Shackleton, 1959) is referable to the upper part of Zone III; the fauna is like that at Ogof Dhû and includes Cermatops discoidalis, Maladioidella abdita, abundant Orusia lenticularis and other fossils (Hughes and Rushton, 1990 p.430). The presence of Eurycare? at Garreg-wen (Lake, 1919, p1.11, figs 6–8) suggests the presence of Zone IV there. Zone Vc, proved from Penmorfa Church and Garreg-wen, is characterised by Peltura scarabaeoides, together with Lotagnostus trisectus and other species, including Richardsonella? invita (Plate 2).

Chapter 3 Ordovician

Rocks of Ordovician age form the greatest part of the district. They comprise marine sedimentary rocks which in parts of the sequence are interbedded with thick accumulations of extrusive volcanic rocks (Figure 6). The latter include lavas and a wide variety of pyroclastic rocks and range from basic to acidic in composition. These volcanic rocks can be broadly divided into two groups, pre-Caradoc and Caradoc. The pre-Caradoc volcanic rocks outcrop about the Manods, to the east of Blaenau Ffestiniog, and on the Moelwyns, on the north side of the Vale of Ffestiniog. They occur at the northern edge of the main outcrop of Arenig–Llanvirn volcanic rocks which can be traced about the Harlech Dome (Allen and Jackson, 1985) to the south. To the north of the Moelwyns there is no indication of this volcanic activity but here the Caradoc volcanic rocks crop out in a large area about the Snowdon massif in the north of the district (Howells et al., 1991).

These two episodes of volcanic activity broke the pattern of sediment accumulation, which had developed throughout the Welsh Basin in Cambrian times. Within the district, there is no indication of the Tremadoc volcanism recognised at Rhobell Fawr on the south-east side of the Harlech Dome (Kokelaar, 1979, Allen and Jackson, 1985). The volcanic activity was caused by the convergence of oceanic and continental crust along a southeasterly dipping subduction zone on the south side of the Iapetus Ocean.

The subdivisions of the Ordovician sequence are shown on the generalised vertical sections of the 1:50 000 map, in (Table 2) and on (Figure 6). The current stratigraphical nomenclature has been derived, as far as possible, from that established by earlier workers, in particular, Fearnsides (1910), Williams (1927), Williams and Bulman (1931), Fearnsides and Davies (1944), Shackleton (1959), Bromley (1963), Roberts (1967) and Lynas (1973).

The Dol-cyn-afon Formation, of Tremadoc age, is the uppermost formation of the Mawddach Group (Allen and Jackson, 1985). The Nant Ffrancon Group, of Arenig to early Caradoc age, is broadly the Nant Ffrancon 'Formation' (Howells et al., 1991) upgraded to include the constituent formations. Within the district, the Arenig strata are represented by the Allt Lŵyd Formation. The Arenig–Caradoc sedimentation was mainly of silty mud, silt and fine-grained sand with restricted, local accumulations of coarse sand. The subdivision of the Llanvirn–Caradoc sequence is complicated by the wedging-out of the volcanic sequences, the presence of a locally profound mid-Ordovician unconformity below the Nemagraptus gracilis Zone and, in places, large-scale lateral disruption.

In the Bangor district, to the north, the Caradoc sequence is dominated by the products of two cycles of intense volcanic activity represented by the lower, Llewelyn Volcanic Group, and the upper, Snowdon Volcanic Group. In this district, the former is represented only by restricted outcrops of the feather edges of the Capel Curig Volcanic Formation, to the east of the Nantmor Fault but the latter is the major feature of the geology. The sequence below the Snowdon Volcanic Group sequence is divided into the Nant Ffrancon Group and the Cwm Eigiau Formation, respectively below and above the Capel Curig Volcanic Formation. To the west of the Nantmor Fault, where there is no evidence of the Capel Curig Volcanic Formation, the Cwm Eigiau Formation directly overlies the Nant Ffrancon Group.

Biostratigraphical framework

The strong variations of lithofacies shown by the Ordovician rocks of the district reflect changes in the marine depositional environments and the contained fossils vary accordingly. The biostratigraphy is interpreted with reference to a composite scheme of biozones and stages, outlined in (Table 3). Correlation between the stages based on shelly faunas and the graptolite zones is in many cases not precise and it remains one of the most interesting biostratigraphical problems within the district. Generally, the graptolites provide reliable correlation over wide areas, but because of their sparse occurrence the zones normally recognised are relatively crude and require refinement (Rushton, 1990); in particular, zones of the Llanvirn Series and the teretiusculus Biozone remain difficult to characterise. The shelly stages can provide a local refined correlation, especially where they are subdivided into zones.

During this survey new collections were made at several localities and the valuable old collections made during the primary survey, and by such workers as McKenny Hughes and Fearnsides, were re-examined where available. Reliance has, to some extent, been placed on more recent published records by Shackleton (1959) and Lynas (1973) and unpublished reports by Bromley (1963) and Zalasiewicz (1992), although parts of their collections were also re-examined.

All the studies of acritarch floras in the district were made in support of the present survey. Acritarchs have proved of value in the Tremadoc and Arenig, and provided indispensible evidence in areas where other fossil control is poor. However, there is, as yet, inadequate stratigraphical control in the Llanvirn to Caradoc strata to provide fine resolution at these levels (Molyneux, 1990). Chitinozoa recorded from the Nant Ffrancon Group have also proved to be of stratigraphical value. These microfossils were first recorded from the Ordovician rocks of the district by Atkinson and Moy (1971), and offer the possibility of correlation with zonation schemes erected in south-west Europe (Paris, 1981). In addition to their biostratigraphical application, derived acritarchs have provided evidence for sources of eroded material.

Mawddach Group

The Mawddach Group extends into the base of the Ordovician and the uppermost part, the Dol-cyn-afon Formation, is entirely of Tremadoc age.

Dol-cyn-afon Formation

The Dol-cyn-afon Formation, 410–900 m thick, comprises grey mudstones and siltstones with intercalated sandstones. It is equivalent to the Tremadoc Slates or Beds of Fearnsides (1905, 1910) and the strata exposed within the district probably represent the most complete Tremadoc sequence exposed in North Wales. The formation has been upgraded from the Dol-cyn-afon Member, the lower part of the incomplete Tremadoc Series of the Harlech district (Allen and Jackson, 1985), and it differs markedly from that sequence in that it includes significant thicknesses of medium- to coarse-grained sandstone.

The base of the Dol-cyn-afon Formation is recognised at Ogof Dhû [SH 5137 3794] where dark grey mudstones of the Dolgellau Formation are overlain by beds of quartzose, phosphate-rich sandstone; the graptolite Rhabdinopora [formerly Dictyonema] is found 8.5 m above this contact (Figure 7).

Bromley (1963) and Lynas (1973) attempted to subdivide the formation and Smith (1988) distinguished four informal members in the outcrop along the Vale of Ffestiniog (Figure 8) and (Figure 9), from the Ynyscynhaiarn Anticline in the west, to south of the Migneint in the east. However, the members are locally difficult to distinguish because of poor exposure, faulting, lack of faunal control and thermal metamorphism. In addition, to the east of the Ffynnon Eidda Fault the sequence is overstepped completely by the mid-Ordovician unconformity. Only the main sandstones are distinguished on the map (British Geological Survey, 1995) but the informal members are described below and summarised in (Figure 9).

At the southern end of Cwm Pennant, at Bryncir [SH 5226 4340] and Golen Mill [SH 5280 4230], siltstones have yielded Tremadoc faunas (Shackleton, 1959). Farther north, siltstones overlying the Marchlyn Formation that were previously considered to be of Arenig age have yielded acritarch assemblages confirming a Tremadoc age.

The Lower Sandstone member is equivalent to the Tynllan, or Niobe Beds of Fearnsides (1910). It is well exposed in the west of the district and eastwards crops out across the Deudraeth peninsula and into the aureole of the Tan y Grisiau granite. It comprises up to 100 m of dark grey silty mudstone and siltstone overlain by 35 m of coarse, quartzofeldspathic, volcaniclastic sandstone. The silty mudstones contain thin, planar- and cross-bedded siltstone beds, less than 10 mm thick, and secondary phosphate nodules are common. The uppermost sandstones weather pale-grey and form beds, 0.02–1.5 m thick which coarsen upwards and locally show normal grading. South-west of Llyn Mair [SH 6471 4081], these sandstones are well exposed and contain mudstone rip-up clasts. Beds of similar volcaniclastic sandstones occur locally at the base of the Dol-cyn-afon Member in the Harlech district (Allen et al., 1981).

At the southern end of Cwm Pennant, around Bryncir and south of Craig Isallt, fault-bounded slivers of silty mudstones have been assigned to the Lower Sandstone member.

The Lower Mudstone member, 150–300 m thick, comprises a monotonous sequence of silty mudstones with minor sandstone beds. Fearnsides (1910) first distinguished the member at Moel y Gest, west of Porthmadog. The most complete section is exposed south of Hafod y Llyn [SH 6458 4118] where the member is up to 300 m thick. Westwards it thins to less than 150 m on the western limb of the Ynyscynhaiarn Anticline. Its base and top are sharply defined and conformable. Sedimentary structures are rare and consist mainly of planar laminations in the few fine-grained sandstone beds. Phosphate nodules and traces are common and large cone-in-cone carbonate and sideritic nodules are abundant along bedding planes in the upper part.

The Upper Sandstone member includes the Portmadoc Flags and Penmorfa Beds of Fearnsides (1910) and at its type section, west of Tan y Grisiau Reservoir [SH 6712 4317], it is more than 200 m thick. It comprises alternations of coarse- and medium-grained sandstones with interbeds of silty mudstone (Plate 1.4). The alternations form a distinctive scarp-and-slack topography which is particularly well displayed on the flanks of Moelwyn Bach. The member can be traced across the Deudraeth peninsula to Porthmadog and north-westwards to Penmorfa. The sandstones are well bedded, 0.5–2 m thick, with sharp bases which are commonly loaded. Sedimentary structures are common and include grading, cross-lamination, ripples and channelling. The sandstones are dominantly quartzofeldspathic and contain a distinctive fraction, up to 40 per cent, of chlorite-mica stacks; locally bioturbation is intense. To the east of the type section, the grain size generally diminishes and in the vicinity of Blaenau Ffestiniog volcaniclastic debris forms a significant component of the sandstones (Bromley, 1969). In the Migneint area, the member comprises some 70 m of siltstones and mudstones with a few sandstone beds.

In Cwm Pennant, at Dol Ifan Githin Quarry [SH 5378 4952] and Cwm Dwyfor [SH 5404 5055], grey, well-bedded siltstones, interbedded with thin laminated sandstones, have been assigned to the member. They are considered to be equivalent to the feldspathic sandstones exposed at Golen Mill [SH 5280 4230].

The Upper Mudstone member is equivalent to the Garth Hill Beds of Fearnsides (1910). It is absent in places due to pre-Arenig erosion and, in the Ynyscynhaiarn area, to strike-faulting. The thickest and most complete sections occur in the Porthmadog area where up to 60 m of finely laminated silty mudstones are preserved in sections at Bwlch Bryn [SH 6025 3925] and at Ynys Towyn [SH 572 386]. Locally, as in the vicinity of Rhiw Goch farm [SH 6220 4026] and east of Blaenau Ffestiniog, the member passes laterally into the Upper Sandstone member, which suggests it is but a facies of the latter. Locally, as at Y Garth [SH 593 392], the silty mudstones are intensely bioturbated. At Croes y Ddwy Mon Quarry [SH 7527 4238] the bedding is well preserved with laminae of dark and light grey mudstone that show evidence of synsedimentary disruption, and small lenticles of phosphate.

Environmental interpretation

The Dol-cyn-afon Formation reflects a more active marine environment than that established during the deposition of the Dolgellau Formation. A more vigorous circulation ventilated the sea floor and led to more rapid sedimentation as displayed in the coarsening-upwards sequence of the Lower Sandstone. Wave and current activity affected deposition of much of the Upper Sandstone member, and only as these waned did the outer-shelf fauna of the S. pusilla Biozone become established. Around Ynyscynhaiarn, the Upper Mudstone member reflects well-oxygenated, marine conditions which to the east of Ffestiniog pass into a restricted, possibly lagoonal, environment. Between the Rhyd and Cwm Bowydd faults, marked thickness and facies variations and slump folds in the Upper Sandstone member suggest synsedimentary fault activity with the development of a small north-south-trending, fault-bounded trough in end-Tremadoc times.

Biostratigraphy

Apart from the basal few metres, which may include part of the Acerocare Biozone (Figure 5), the Dol-cyn-afon Formation lies wholly within the Tremadoc Series (Figure 7). The Tremadoc Series was based on the individuality of the faunas from the Ynyscynhaiarn (Tremadoc) Anticline which had been described by Salter (1866). He recognised a 'lower' and an 'upper' fauna and these have generally been correlated with the faunal subdivisions of the Shineton Shales of Shropshire (Stubblefield and Bulman, 1927). However, several problems of correlation remain, which will only be resolved with study of more detailed, stratigraphically controlled collections.

The table illustrating the faunal distribution in the Ynyscynhaiarn district (Figure 7) owes much to the unpublished work of Mr S Jusypiw. The lower part of the Tremadoc sequence is characterised especially by Rhabdinopora flabelliformis (Plate 6)i. Specimens with a close mesh referred (with some doubt) to the subspecies socialis occur 8.5 m above the base of the Lower Sandstone member and range up to the base of the Lower Mudstone member. The wider mesh of subspecies flabelliformis appears to occur at the latter horizon, and subspecies anglica with an even wider-spaced mesh, seems to occur, with Adelograptus [formerly Clonograptus] tenellus, a little higher. These last-named species indicate an horizon equivalent to the Transition Beds of the Shineton Shales (Stubblefield and Bulman, 1927). Rhabdinopora (Plate 6)i has been found at various localities in the Vale of Ffestiniog, for example around Llyn Mair [SH 6473 4084] and, in the Migneint area, at Llechwedd Deiliog [SH 784 399]. The trilobites most typical of the Lower Sandstone member are Niobella homfrayi (Plate 6)f, Beltella depressa and Psilocephalinella innotata.

Trilobites from the Lower Mudstone member include Asaphellus homfrayi, a large undescribed Niobella and species of Platypeltoides and Proteuloma (information from S Jusypiw). The higher parts of the Lower Mudstone member and most of the Upper Sandstone member cannot be assigned to a zone because of the poverty of the faunas, the commonest fossil being the long-ranging Asaphellus homfrayi. Shackleton (1959) recorded a small fauna from these beds near Golen Mill [SH 525 426]. The mudstones and thinly bedded sandstones at the top of the Upper Sandstone member have yielded trilobites and other fossils comparable with those of the Shumardia pusilla Biozone of the Shineton Shales, notably Conophrys salopiensis [ =S. pusilla],Orometopus praenuntius and Parabolinella triarthra. This fauna, detailed by Fearnsides (1910, p.161) from Penmorfa and Porthmadog, is encountered farther east at Amnodd, in the Arenig area (Fearnsides, 1905), but not in the intervening ground.

The fauna of the Upper Mudstone member is dominated by Angelina sedgwickii (Plate 3) in the Porthmadog area but this species is much less common farther east; Bromley (in unpublished manuscript) recorded Angelina at two slate workings near Ty Nant y Beddau [SH 7241 4277] and [SH 7246 4257] but these have not been confirmed. Angelina sedgwickii has recently been determined in the highest Tremadoc of the Welsh Borderland (Fortey and Owens, 1992) and two of its Welsh associates, Niobina davidis and Peltocare olenoides prove the sedgwickii Biozone in the Lake District (Rushton, 1988). At Croes y Ddwy Afon Quarry, beds ascribed to the Upper Mudstone member have yielded only small Lingulella and very sparse nondiagnostic acritarchs.

In the south of Cwm Pennant, the Dol-cyn-afon Formation is determined in outcrops adjacent to the Bryncir Fault, but restricted exposure inhibits the recognition of individual members. An acritarch assemblage which includes Acanthodiacrodium ubui?, collected 20 m west of Bryncir Tower [SH 5225 4349], suggests an early Tremadoc age since this species ranges from the Rhabdinopora flabelliformis Biozone to the base of the Brachiopod Beds in the Shineton Shales (Rasul, 1979). Shackleton (1959) suggested that slates in the vicinity [SH 523 424] might represent the high Tremadoc 'Tenmorfa Beds' but his fauna is not diagnostic and could represent horizons suggested by the acritarch floras. Tremadoc acritarch assemblages have been obtained also from other localities in the vicinity (e.g. at [SH 5358 4436] and [SH 5246 4391] including Golen Mill [SH 5280 4230], but they comprise poorly preserved examples of long-ranging forms such as Cymatiogalea cristata, C. velifera?, Vulcanisphaera africana ? and V. cirrita?. Near Golen Mill, Shackleton (1959) recorded Asaphellus homfrayi, Micragnostus calvus, 'Cyclopyge' (?= Prospectatrix) and other fossils typical of the Upper Tremadoc pusilla Biozone as developed at Penmorfa.

Farther north in Cwm Pennant, siltstones previously assigned to the Arenig have yielded later Tremadoc acritarchs and are placed in the undifferentiated Dol-cynafon Formation. Typical is an assemblage collected 280 m north-north-east of Beudy Newydd [SH 5359 4896] which includes Acanthodiacrodium ovatum, Cymatiogalea cristata?, Stelliferidium fimbrium? and Vulcanisphaera britannica, suggesting correlation with the Shumardia pusilla Biozone in the Shineton Shales (Rasul, 1979). A sparse and poorly preserved assemblage from Dol-Ifan Githin Quarry [SH 5373 4953], north-north-east of Beudy Newydd, includes Cymatiogalea cristata, indicating a Tremadoc age. In Cwm Dwyfor [SH 5404 5055], [SH 5425 5065], [SH 5425 5066], Tremadoc acritarch assemblages include forms provisionally assigned to A. ovatum, suggesting a possible correlation with the S. pusilla Biozone for strata previously assigned to the Arenig.

Tremadoc - Arenig boundary

Within the district, and throughout North Wales, this boundary is marked by a major transgression which has led many British geologists to regard it as the base of the Ordovician System. In the Snowdon district the boundary lies between the Dol-cyn-afon Formation (Tremadoc) and the Allt Lŵyd Formation (Arenig). Lithological and thickness variations in the strata on either side of the boundary have been variously interpreted to represent continuous deposition (Jennings and Williams, 1891; Lynas, 1973) or repetition in thrust slices within the 'Tremadoc Thrust Zone', (Fearnsides and Davies, 1944; Shackleton, 1959). Along the Vale of Ffestiniog, bedding attitudes in Tremadoc and Arenig strata indicate little evidence of a significant angular unconformity to support end-Tremadoc tectonism prior to the Arenig marine transgression (Figure 10) (Plate 4.1) and (Plate 4.2). In addition at many localities, as at Moel y Gest Quarry, Rhyd [SH 6404 4188], Croes yr Ddwy Mon Quarry [SH 7527 4238] and locally around Blaenau Ffestiniog, where the basal Arenig conglomerate does not occur, the sandstones of the Dol-cyn-afon Formation appear to grade up into the Allt Lŵyd Formation without any appreciable break in sedimentation. However, at Croes yr Ddwy Mon Quarry, large, isolated, phosphate-rimmed pebbles and rip-up clasts of mudstone supported in a mudstone matrix with wisps of sandstone suggest reworking of the basal conglomerate by strong currents or wave activity and such action could explain its sporadic development. Farther east, the boundary is marked by an abrupt lithological change and the Allt Lŵyd Formation oversteps the Dol-cyn-afon Formation. At Llechwedd Deiliog [SH 7810 4015] it overlies lower Tremadoc mudstones and at Nant Derbyniad Quarry (Jennings and Williams, 1891, p. 383; Lynas, 1973) it rests directly on the Dolgellau Formation. At Arenig Fawr, the Allt Lŵyd Formation overlies beds of the Shumardia pusilla Biozone (Zalasiewicz, 1984).

There is evidence of a biostratigraphical break across the Tremadoc–Arenig boundary of at least one graptolite zone, and possibly as many as four (Lindholm, 1991, p.285). A distinctive acritarch assemblage which occurs in beds of latest Tremadoc and earliest Arenig age in the English Lake District (Molyneux and Rushton, 1988), South Wales (Molyneux and Doming, 1989) and Spain (Mette, 1989) has not been recorded in the Snowdon district, and this would support a stratigraphical hiatus occurring between the Tremadoc and Arenig series.

Nant Ffrancon Group

The Nant Ffrancon Group includes strata from the base of the Arenig to the base of the Capel Curig Volcanic Formation, or its equivalent position, of Soudleyan (early Caradoc) age. It has yielded Arenig, Llanvirn and Caradoc faunas but none of proved early Llandeilo age (teretiusculus Biozone). It can be broadly correlated with the Nant Hir Formation of the Bala district (Lynas, 1973; Campbell, 1984; Zalasiewicz, 1984). The group comprises mainly silty mudstones, siltstones and fine-grained sandstones in which a basal sandstone sequence, the Allt Lŵyd Formation, and the Rhiw Bach, Llyn Conwy and Moelwyn Volcanic formations have been distinguished; all of these formations are laterally impersistent.

The top of the group lies in the upper Llandeilo or lowermost Caradoc, and within the district the presence of a stratigraphical break at this level is emphasised by the lateral stratigraphical changes. In a sequence east of the Trawsfynydd Fault (Figure 23), about Blaenau Ffestiniog, the Manods and farther east to the Migneint, there is evidence of a conformable sequence, with possible minor breaks, from Tremadoc through to Lower Caradoc. Here the base of the Nant Ffrancon Group is of early Llanvirn age, upper Llanvirn strata are doubtful, Llandeilo strata are unproved and the lower Caradoc strata have been determined by both graptolitic and shelly faunas. In contrast, to the west of the Trawsfynnydd Fault in a road cutting near Penrhyndeudraeth [SH 6151 4000], Smith (1988) proved that strata, possibly as young as Caradoc, rest directly on Arenig strata with no indication of disruption. However, farther west, in Cwm Pennant, Arenig strata are overlain by an estimated 700 m of Llanvirn silty mudstones. These contrasting relationships support the existence of a mid-Ordovician unconformity, at a level near or below the base of the Caradoc (Smith et al., 1995), which can be traced for some 18 km between the Trawsfynydd Fault, in the east, and the Cwm Pennant Fault in the west. Locally, as at Tremadog and Rhyd, the recognition of this unconformity is complicated by pre-tectonic gravity sliding, with the development of sedimentary mélange (olistostrome) deposits within the Nant Ffrancon Group. These gravity slides are interpreted to have been initiated by uplift associated with the unconformity and the deposits locally cut down on to Cambrian strata. For the purposes of description, the post-Arenig sequence is subdivided into strata above and strata below the unconformity; the olistostrome and disturbed strata are described separately.

Allt Lŵyd Formation

The Allt Lŵyd Formation, established in the Harlech district (Allen and Jackson, 1985) to the south, includes all the sandstone strata of possible Arenig age; it distinguishes the base of the Nant Ffrancon Group. The formation is broadly equivalent to the Carnedd Iago Formation (Lynas, 1973 and Zalasiewicz, 1984) and, in the Bangor district, the Graianog Sandstone (Howells et al., 1985).

The formation is well exposed along the Vale of Ffestiniog where it is broadly subdivided, in upward succession, into a basal conglomeratic sandstone (the Garth Grit Member), flaggy sandstones interbedded with siltstones, and volcaniclastic sandstones. However, along strike there are considerable thickness and facies variations. The type section is at Allt Lŵyd in the upper Mawddach valley (Allen and Jackson, 1985, p.33). The best reference section within the district occurs in a series of exposures north of Tan y Grisiau [SH 6800 4528]. In northern and central Cwm Pennant, Arenig strata have been proved forming the envelope to a series of tight folds. The base of the formation is exposed at several localities, notably Ynys Towyn [SH 5716 3852], Moel y Gest Quarry, Rhyd [SH 6404 4188] and at Llechwedd Deiliog [SH 7810 4015].

Garth Grit Member

The Garth Grit Member forms a prominent feature between Y Garth and Tan y Grisiau (Plate 4.3). At Y Garth, the member is about 20 m thick but, to the east, it reaches its maximum thickness of 130 m at Fron Goch [SH 6383 4181] and thins north-eastwards to 2 m about Blaenau Ffestiniog. Farther east, across the Cwm Bowydd Fault (Figure 23), it thickens markedly and is up to 70 m thick at Bryn Glas Quarry [SH 7324 4214]. To the west of Y Garth, the member thins and is locally absent. The stratotype is taken north of Tan y Grisiau dam [SH 6800 4528].

Typically, the member comprises massive bedded, quartz-pebble conglomerates which grade laterally into, and intercalate with, medium- to coarse-grained, quartzose sandstones. Weathered surfaces are cream to green-grey, but are dark red-brown in places where they are associated with hematite enrichment. Beds vary between 0.01 m and 2 m thick with subplanar, locally erosive contacts.

In places, as at Cwm Teigl [SH 7240 4338] and around Tan y Grisiau, a basal, polymict conglomerate, up to 1.5 m thick, has been recognised. Rounded and sub-rounded pebbles are set in a variable sandstone and mudstone matrix. The pebbles are mainly of quartz and rhyolite with a few pebbles of altered volcanic rock and mudstone. Phosphate nodules and phosphate encrustations on pebbles are common. Elsewhere, the basal beds comprise massive bedded, channelised, grain-supported microconglomerates, as at Clogwyn Pwrsiog [SH 6030 3946], and interbedded thin flaggy sandstones and siltstones, as at Croes y Ddwy Mon Quarry [SH 7527 4238].

Above these basal beds the main body of the Garth Grit Member comprises channelised, lenticular beds of microconglomerate. Sedimentary structures are common and include large-scale trough cross-bedding with mud-draped foresets, decimetre-scale ripples, scours and crude normal grading. Limited palaeocurrent data suggest deposition by east- and south-east-directed currents. Local intercalations of finer-grained sandstones are bioturbated with small subvertical burrows (Skolithos) and less commonly, larger 'Thycodes type' burrows. The only recorded 'body-fossil' is Bolopora undosa, a phosphatic chemogenic deposit of possible algal (stromatolite) origin (Hofmann, 1975; Niedermeyer and Langbein, 1989).

Interbedded flaggy sandstones and siltstones

These beds overlie the Garth Grit Member and correspond to the Garth Flags of Fearnsides (1910), Fearnsides and Davies (1944) and to the Llyfnant Flags, of the Arenig district to the south-east. They are up to 100 m thick and comprise thinly bedded sandstones and siltstones in a progressively upward-fining sequence. In places, such as the flank of Moelwyn Bach, the sequence is well exposed. The only complete sequence occurs in the east of the district, where it is up to 94 m thick near Tan y Grisiau.

Thin and laterally impersistent bands of microconglomerate are restricted to the lowermost beds, as exposed at Rhiw Goch [SH 6224 4048]. Above, the sequence is of sandstone and silty mudstone cycles, 0.04–0.1 m thick, with interbedded sandstones 0.1 to 1.3 m thick. The sandstones are planar bedded with sharp basal contacts and graded tops. In thin section, the sandstones are seen to comprise approximately 40 per cent angular quartz grains in a matrix of finer quartz, sericite, carbonate, zircon and iron oxides. Clastic chlorite-mica stacks are common. The silty mudstones are finely laminated and include isolated large quartz grains. Sedimentary structures are well preserved and include ripple-drift laminations, cross-laminations, symmetric and asymmetric ripples and normal grading. In places pervasive internal slumping is common and, locally, bioturbation has caused complete homogenisation of the beds. Palaeocurrent data (Traynor, 1990) indicate derivation from a source area to the north and north-west.

Volcaniclastic sandstones

Volcaniclastic sandstones, with interbedded siltstones and mudstones, 60–135 m thick, lie at the top of the Arenig sequence across most of the district. To the west of Rhyd, the member is progressively cut out by the mid-Ordovician unconformity. Near the base of the member are well-bedded, flaggy, tuffaceous sandstones, 0.5–1 m thick with abundant sedimentary structures which include herringbone cross-bedding, flaser bedding, and wave ripples. In the east of the district, near Llyn Morwynion, these beds are represented by massive bedded, tuffaceous sandstones, 90 m thick, which have been correlated (Lynas, 1973) with the Henllan Ash in the Arenig district. The higher beds comprise tuffaceous mudstones, siltstones and tuffites with well-developed cross-bedding, grading, and internal slumping (Plate 4.4). Locally, as to the south-west of Moelwyn Bach and to the south of Manod Mawr, the volcanic component of the member is less well defined. At a fault-bounded section [SH 6151 4002], near Penrhyndeudraeth, the member is represented by 65 m of grey sandstones and siltstones which directly overlie the Dol-cyn-afon Formation.

In the cores of folds in Cwm Pennant, some 120 m of cleaved, laminated silty sandstones and silty mudstones overlie sandstones of Cambrian age. The age of these beds is problematic as they have yielded no macrofaunas, but recent microfloral determinations indicate an

Arenig–Llanvirn age and thus they are probably the lateral equivalent of the uppermost volcaniclastic sandstones of the Allt Lŵyd Formation. The silty mudstones contain abundant chlorite-mica stacks and isolated quartz grains. The microconglomerates are well exposed, 5–10 m thick, around Y Foel [SH 5233 4555] and Hendre Ddu [SH 5190 4461] and here they are intensely deformed. South of Hendre Ddu, Crimes (1970) recorded the trace fossil Phycodes circinatum. Phosphate fragments are common and locally the matrix is completely phosphatised. At the top, these strata grade up into mudstones of Llanvirn age.

Environmental interpretation

The lateral thickness and facies variations within the Allt Lŵyd Formation are considered (Smith, 1988; Traynor, 1990) to reflect a turbulent, nearshore, marine environment with rapid sedimentation rates and extensive reworking. The Garth Grit Member is considered to represent a complex of channels which evolved on a fan delta, with the interbedded sandstones and siltstones being overbank and floodplain deposits. In the Rhyd and Cwm Bowydd areas there are indications that the distribution of the channel systems was to some extent influenced by contemporaneous faulting. Generally such deltas develop where alluvial fans prograde into standing water on steep continental or island slopes (Stow, 1985).

Within the interbedded flaggy sandstones and siltstones the bimodal current activity, bioturbation and clay-draped ripples support an interpretation as subtidal deposits (Zalasiewicz, 1984; Beckly, 1987; Traynor, 1990). The volcaniclastic sandstones, with its benthic brachiopod-trilobite fauna and abundant sedimentary structures, suggest a more turbulent, shallow-marine environment.

Biostratigraphy

In South Wales, the Arenig Series is relatively complete and it is divided into three stages, in ascending order, Moridunian, Whitlandian and Fennian (Fortey and Owens, 1987). However, in the area of Arenig, just to the south-east of the district, the series is less complete (Zalasiewicz, 1984); the lower part, including the Garth Grit and the Henllan Ash, are referred to the Moridunian, the Whitlandian is absent, and the Fennian is represented only by a relatively thin mudstone beneath the Llanvirn Series.

Throughout most of the district the Arenig strata lack detailed faunal control. The volcaniclastic sandstones yield shelly fossils of the nearshore Neseuretus biofacies at several places but these are commonly fragmented; however, in the east of the district, for example around Carnedd Iago, Lynas (1973, p.486) collected Moridunian faunas, including Neseuretus parvifrons and Paralenorthis [Orthambonites] proava, typical of the Henllan Ash. South of the Manods, old collections from Cae Clyd, about [SH 709 444] contain fragmentary material of a similar fauna and these may also be Moridunian. At other localities in Cwm Teigl and north of Penrhyndeudraeth only brachiopod fragments were found. Fearnsides and Davies (1944, p.253) cited a record of Didymograptus extensus from siltstones immediately above the Garth Grit Member at Garth Barn [SH 6010 3939] but the material has not been located and the record was not confirmed in this survey.

Whitlandian strata are not known in the district but, as in the Arenig area, locally there are thin representatives of Fennian strata below the Llanvirn. In Cwm Teigl, a collection made by Fearnsides from a 'slate trial' on the east flank of Manod Mawr, includes brachiopods and bellerophontids with several graptolites indicative of the Fennian Didymograptus hirundo Biozone ((Figure 11), i-m): Aulograptus sp., Didymograptus acutidens, Eoglyptograptus dentatus (group), Pseudotrigonograptus ensiformis, Tetragraptus sp. and Undulograptus austrodentatus (group). North of Penrhyndeudraeth, spoil from a level at Pant y wrach, about [SH 617 402] has yielded Azygograptus lapworthi (Figure 11)n and phyllograptids (Fearnsides and Davies, 1944, p.253) indicating probable Fennian strata.

The laminated silty sandstones and silty mudstones in Cwm Pennant have yielded acritarch assemblages at two localities. From a disused quarry [SH 5208 4518], west of Pont Gyfyng, Arkonia?, Coryphidium cf. bohemicum, ?Frankea hamata and Veryhachium trispinosum s.l. have been determined, suggesting a late Arenig to Llanvirn age. The presence of Acanthodiacrodium spp., Cymatiogalea? and Trichosphaeridium? suggests reworking of Tremadoc acritarchs into this assemblage. South-south-west of Y Foel [SH 5214 4533] an acritarch assemblage with Arkonia tenuata, Coryphidium sp., Frankea sartbernardensis, Orthosphaeridium bispinosum, 0. ternatum and Striatotheca quieta indicates beds no older than Llanvirn.

At Croes y Ddwy Mon Quarry a sparse assemblage of poorly preserved acritarchs suggests an early Arenig age. This is dominated by simple spined acanthomorph types, referred to the genus Polygonium and associated with Veryhachium trispinosum s.l. The assemblage most closely resembles those of the lower Arenig Didymograptus dejlexus Biozone in the Lake District (e.g. Molyneux and Rushton, 1988; Molyneux, 1990)

Post-Arenig strata below the mid-Ordovician unconformity

In Cwm Pennant, the sandstones of Arenig age are overlain by an estimated 700 m of micaceous mudstones and silty mudstones of Llanvirn age. These strata are broadly equivalent to the Maesgwm Slates of Shackleton (1959).

Between Criccieth and Cwm Pennant, the strata overlying the sandstones are mainly finely laminated, well-cleaved, dark-grey to blue-black silty mudstones with thin siltstone ribs and laminae which locally, as at Llyn y Betws [SH 5192 4574], are well exposed. Bedding planes are commonly marked by concentrations of muscovite flakes, pyrite cubes and framboids. The siltstone component increases progressively upwards, bioturbation occurs throughout the sequence and a few chamosite oolite beds, recorded by Shackleton (1959) near the base at Dolbenmaen and Tyddyn Madyn, were confirmed during this survey.

East of Blaenau Ffestiniog and in Cwm Teigl, the sandstones of Arenig age are overlain by silty mudstone and siltstones which form the background sediments of the Rhiw Bach Volcanic Formation and have been extensively quarried and mined for slate around the Manods. East of the Ffynnon Eidda Fault, silty mudstones which crop out in the vicinity of Llyn Serw [SH 779 428] are laterally equivalent.

Rhiw Bach Volcanic Formation

This formation lies to the east of the Cwm Bowydd Fault. It forms a prominent element of the sequence between the type area at Rhiw Bach Quarry [SH 7410 4600] in the north and Foel Gron Quarry [SH 7450 4290] in the south. It is a modification of the Rhiw Bach Formation which Lynas (1973) defined on the west side of the Ffynnon Eidda Fault and considered to be the equivalent of the Serw and Llyn Conwy formations on the east of the fault. Here, it is considered that the Rhiw Bach and Serw formations are lateral equivalents and the influence of the Ffynnon Eidda Fault on their emplacement was not as profound as Lynas (1973) envisaged.

The formation, up to 640 m thick, comprises high-level intrusions and extrusions of both dioritic (quartz latite) and rhyolite composition, acidic ash-flow tuffs, fine-grained dust tuffs and tuffites with intercalated sedimentary rocks, mainly silty mudstones but locally conglomerates (Plate 5.1).

At Carreg y Foel Gron [SH 7441 4275], near the base of the formation, an autoclastic, quartz latite breccia is capped by, and faulted against, crystal lithic tuffs of similar composition. Clasts in the breccia, are generally less than 0.3 m across, equant and angular, matrix-supported and indented on weathered surfaces. In thin section (KB 1280), clasts are seen to be more chloritised, with a greater proportion of albite phenocrysts than the matrix. A more cohesive quartz latite crops out in a similar position at Craig y Garreg-Kvyd [SH 7298 4275] with sericitised feldspar phenocrysts, less than 3.5 mm across, set in a highly altered matrix. To the west of Llynnau Gamallt, autoclastic brecciated quartz-latite outcrops in the fault scarp [SH 742 441] and is overlain by bedded, lithic crystal tuffs, probably grain-flow deposits, with some epiclastic debris (KB 1291) locally included.

The formation is dominated by acidic rocks, particularly the rhyolites which form prominent crags as at Carreg y Fran [SH 735 450] and Y Garnedd [SH 742 431]. The form of these bodies is mainly sill-like but they are locally autobrecciated and intimately associated with ash-flow tuffs, interbedded with dust tuffs, and tuff-turbidites of similar composition. In places, the relationships are extremely difficult to unravel. In the scarp face on the west side of Y Garnedd [SH 740 432], flow-banded rhyolite intrudes autoclastic brecciated quartz latite at the base of the feature. Above, flaggy bedded, crystal, lithic tuffs with plane-parallel-laminations and low-angle, cross-laminations, interbedded with silty mudstones, underlie a massive acidic ash-flow tuff, 5–6 m thick, with faint internal foliation. The tuff forms a prominent feature to the south-east and is well exposed in Foel Gron quarry [SH 7445 4289] where the massive internal beds suggest separate pulses, rather than a single flow, and a composite accumulation. In thin section (KB 1279), a well-defined, non-welded shardic fabric is distinguished, with whole and fragmented feldspar crystals, a few wispy pumice clasts and a faint foliation.

Northwards, on either side of Mon Gamallt valley, acidic tuffs, interbedded with silty mudstones and thin tuffaceous sandstones form well-developed features, for example west of Sam Helen [SH 735 440]. East and north of Llynnau Gamallt, the formation is dominated by intrusive rhyolites. West of Rhiw Bach the formation is well exposed in the vicinity of the extensive slate workings about Manod Mawr where acidic ash-flow tuffs, thinly bedded, fine-grained silicified dust tuffs and rhyolites are interbedded with locally thick sequences of silty mudstone.

In the Manod quarry, on the north-west side of Llyn Pysgod [SH 726 456], a white weathered, crystal-rich, ash-flow tuff, 5–15 m thick forms a prominent feature against the blue-grey slates in the quarry face (Plate 5.2). The vitroclastic tuff contains clearly defined shards (KB 1269) and is foliated, with numerous potassic and plagioclase feldspar crystals showing local concentration in zones within the fabric. The tuff is normally graded and its top is overlain by silty mudstone with thin beds of crystal-rich, tuffaceous debris (probably tuff turbidites) (Plate 5.3) reworked from pyroclastic debris previously emplaced in shallower areas of the basin. In the slope above the quarry, bleached, silicified, fine-grained tuffs indicate emplacement by small ash-flows and the plane-parallel bedded tops possibly represent dust elutriated from the top of the flows during transport and subsequently settled out of the water column. The non-welded tuffs are composed of a devitrified, recrystallised quartzose aggregate with well-preserved shards, and complete vesicles, accentuated by chlorite concentration about their peripheries (KB 1270).

The formation can be traced around the north side of the Manod intrusion where the various elements are discernible in the quarry complex between Blaenau Ffestiniog and Cwt y Bugail. Three beds of green-grey dust tuffs form a prominent feature through the Llechwedd and Diphwys Casson quarries. The beds, up to 4 m thick, are finely banded with silty mudstone layers and probably represent turbiditic reworking of fine ash previously emplaced in shallower areas of the basin.

A distinctive conglomerate, 5–8 m thick, occurs close to the top of the bedded tuff sequence to the west of Llynau Gamallt. It overlies silty mudstone and comprises well-rounded boulders, less than 0.6 m in diameter, of rhyolite, quartz latite and tuff, both clast- and matrix-supported, in a matrix of silty mudstone with patches of tuffaceous debris. The roundness and size of the clasts, considered together with the mudstone matrix, suggest that the boulders were reworked from shallower parts of the basin and incorporated into a debris flow. The preponderance of rhyolite and quartz latite boulders indicates contemporaneous erosion of the high-level intrusive to extrusive rocks within the formation.

In the vicinity of Carreg y Foel-gron [SH 7440 4271] and to the east of the Ffynnon Eidda Fault, in the poorly exposed ground at the edge of Migneint, the relationships in the lower part of the formation (referred to the Serw Formation by Lynas, 1973) suggest that the sequence was laterally disrupted on a scale similar to that of the Rhyd Melange, farther west. The deposits are described below (p.34).

Biostratigraphy of the post Arenig strata below the mid-Ordovician unconformity

To the west of the Bryncir Fault, where the succession consists mainly of mudstones and siltstones, the Llanvirn Series is proved by graptolite faunas at many localities, and upper Llandeilo graptolites are recorded locally.

Within the district, most of the Llanvirn faunas cannot be assigned to a zone. The lower Llanvirn artus Biozone is suggested by isolated records of Didymograptus artus and D. cf. spinulosus (e.g. at [SH 5421 4957] and [SH 5599 5240]). The upper Llanvirn murchisoni Biozone, characterised by D. murchisoni itself and other large species, appears to be present at several localities (e.g. [SH 5420 4924] and [SH 5244 4803]) and at other localities recorded by Shackleton (1959). In the north of Cwm Pennant, Shackleton (1959, p.232) recorded graptolites indicative of the gracilis Biozone (upper Llandeilo Series or basal Caradoc Series).

East of the Trawsfynydd Fault, graptolitic Llanvirn strata succeed beds that have yielded hirundo Biozone graptolites, so the succession may be continuous. The presence of Didymograptus artus with Eoglyptograptus dentatus at Nant Rhos ddu [SH 812 398], just to the west of the district, and D. artus with Amplexograptus confertus at Nant Rhydau Gloewon [SH 797 400] both indicate the artus Biozone. In mudstones associated with the Rhiw Bach Volcanic Formation, west of the Ffynnon Eidda Fault, Lynas (1973) recorded Llanvirn faunas suggestive of the artus Biozone but nothing definitive of the murchisoni Biozone. Material from the vicinity of Manod Mawr, in the Sedgwick Museum collections, Cambridge, includes D. artus ((Figure 11) g, h) from the 'Rhiw Bach Tramway' [SH 737 460], and Ogygiocaris cf. seavilli (Plate 6)h with D. artus from 'Llynbywydd' (Llyn Bowydd, c. [SH 729 466]. These also indicate the artus Biozone.

Several localities in Cwm Pennant have yielded micro-fossils, both acritarchs and chitinozoa (Plate 7), but their diversity and abundance vary greatly. Assemblages from near Pont Gyfyng [SH 5224 4518], west of Y Foel [SH 5197 4557], Cwm Llefrith [SH 5491 4727] and [SH 5494 4696], Cwm Dwyfor [SH 5402 5062] and near Isallt Fawr [SH 5335 4469] contain some or all of the artitarchs Frankea longiuscula, Stellechinatum celestum, Striatotheca frequens amd S. quieta, indicating that they are not older than Llanvirn. However, acritarch biostratigraphy does not preclude a younger age at present, and they could be as young as Caradoc. An assemblage from Cwm Llefrith [SH 5458 4662] also contains S. celestum and S. quieta together with Quadruilobus sp. and ?Veryhachium sp. A of Turner (1985). These last two taxa compare with assemblages from ironstones at Pen y Gaer, on St Tudwal's Peninsula, and Llandegai, and both have been designated an early Llandeilo age (Trythall et al., 1987); Llandeilo age for the Pen y Gaer locality has been questioned by Young (1991) who considers a Llanvirn age to be more probable.

The mid-Ordovician unconformity

The possibility of a stratigraphical break at the base of the Caradoc sequence across north-west Wales was first proposed by Jones (1938, 1956) and was inferred from isopach maps (Shackleton, 1954). However, the evidence to support the unconformity is not everywhere forthcoming and is particularly difficult to demonstrate within the mudstone-dominated sequence of Snowdonia. Within the district, a mid-Ordovician break (pre-Nemagraptus gracilis Biozone) has been locally determined between Cwm Pennant and Blaenau Ffestiniog (Smith et al., 1995). It is possible that a thin, attenuated sequence of Llanvirn strata may be present in places but it has not been palaeontologically distinguished.

At Ty Obry [SH 6048 3960] and Penrhyndeudraeth [SH 6157 4009] (Figure 12), the lowest Llandeilo–Caradoc strata comprise thinly bedded, medium-grained, quartzose sandstones with oolites and thin bands and concretions of phosphatic ore, interbedded with fine-grained, structure-less debris-flow deposits and dark grey, pyritic, graptolitic mudstones. These strata conformably overlie massive sandstones and siltstones of the Ant Bvsyd Formation and the contact represents a break in clastic sedimentation.

Eastwards, in the environs of Tan y Grisiau, the post-Arenig sequence is complicated by the rhyolite intrusions and the thermal effects of the microgranite has destroyed any fossil evidence. Bromley (1963) recorded a thin microconglomerate bed, with quartz pebbles, between a rhyolite sill and Arenig mudstones near Wrysgan incline [SH 6786 4541], and this has been interpreted to represent a local lag deposit at the base of the mid-Ordovician sequence (Smith et al., 1995). Farther east, at the north end of Cwm Hafod y Rhedrwydd [SH 767 475], the Llyn Conwy Volcanic Formation oversteps the underlying strata to rest on Tremadoc strata. In the extreme southeast of the district, and eastwards in the Arenig area (Zalasiewicz, 1992), the mudstones underlying the Llyn Conwy Volcanic Formation are of the multidens Biozone and rest directly on lower Llanvirn strata.

On this evidence, it is considered that there is a strati-graphical break, of variable magnitude, at the base of the mid-Ordovician sequence throughout the district.

The possibility as to whether parts of the sequence had been tectonically removed led Fearnsides (1910, fig. 2) to excavate two trenches across the Tremadoc–Caradoc boundary at Penmorfa; their positions [SH 5421 4112] can still be distinguished. He concluded that the boundary was a low-angle thrust fault; later, Shackleton (1959) considered it to be an unconformity subsequently modified by end-Caledonian thrusting. More recently, Smith (1987, 1988) has demonstrated that at Tremadog, and at various other localities along the Vale of Ffestiniog and in the Migneint, the mid-Ordovician break is profoundly affected, and locally accentuated by, pre-tectonic gravity sliding with associated sedimentary mélange deposits.

Melange and disturbed beds

The recognition (Smith, 1987, 1988) that the local unconformable relationships at the base of the mid Ordovician sequence are, in part, the result of pre-tectonic, gravity sliding and mass flow, has necessitated a reinterpretation of the underlying sequence. The disrupted sequences are most clearly exposed between Rhyd, in the east, and Penmorfa, in the west. It was here that Smith (1988) demonstrated that the 'crush belts' and 'slaty thrust breccias' of Fearnsides (1910) and Fearnsides and Davies (1944) were not the result of later tectonism but represent different facies of a sedimentary mélange and the deformation of partly lithified sediment.

The melange deposits, 50–150 m thick, are most easily recognised where there is a marked lithological contrast in the strata involved. This is best demonstrated in the irregular topography at the base of the Moel y Llys scarp, near Rhyd, which reflects allochthonous rafts, up to 300 m, in length, by 70 m wide, of sandstones and grits of the Allt Lŵyd Formation within sheared and disrupted mudstones and siltstones of the Nant Ffrancon Group (Figure 13). The contacts of the sandstone rafts with the adjacent mudstones are generally well defined, e.g. [SH 6340 4191] (Figure 14) (Plate 8.1). Locally, small flames of mudstone intrude the sandstones and elsewhere, small quartz pebbles and sandstone patches enclosed in the mudstone close to the contact indicate partial lithification at the time of incorporation. There is no evidence of tectonic discontinuity at the contacts of the rafts.

The mélange at Rhyd also contains rafts of Ffestiniog Flags Formation?, fossiliferous Dolgellau Formation and Dol-cyn-afon Formation. The rafts are broadly concordant with the regional dip. The base of the Rhyd Melange is not exposed but is interpreted to be a high-angle slide plane, probably keyed into a fault scarp. In contrast, the top grades up into disturbed and folded mudstones and is conformably overlain by the Moelwyn Volcanic Formation.

To the west of Rhyd, the mélange includes the ironstone deposits at Penmorfa [SH 5445 4100], Tremadog [SH 5561 4025] and Pensyflog [SH 5619 3958]. Large rafts and pods of oolitic grainstone are enclosed in a matrix of disrupted mudstones with smaller clasts of phosphate nodule breccia and quartzose sandstones probably of the Allt Lŵyd Formation. Subsequent working and backfilling of the mine at Tremadog has obscured much of the outcrop and extraction of the ironstone pods has left a series of irregular hollows. The smooth, curved surfaces of mudstone, which enclosed the pods, can still be seen.

Within the Vale of Ffestiniog, to the east of the Rhyd fault complex, there is no indication of the mélange deposits. However at Carreg y Foel Gron [SH 7440 4271] the large rafts, up to 10 m across, of pebbly grit, probably Allt Lŵyd Formation, and the volcanic breccias, indicate similar lateral disruption. Farther east, near Mon Serw [SH 768 424], small isolated hummocks, up to 25 m diameter, above the peat are composed of microconglomerates of the Garth Grit Member of the Allt Lŵyd Formation which Lynas (1973) referred to as thick localised lenses. However in this vicinity similar isolated outcrops of quartz latites, tuffs, grits and silty mudstones also occur, and although the contacts are largely obscured a similar relationship to that of the Rhyd Mélange is envisaged. Here, the sole of the mélange locally rests on the Ffestiniog Flags Formation and the magnitude of the break is accentuated by its coincidence with the Tremadoc–Arenig boundary.

The mélange deposits represent extreme disruption of strata. Less clearly distinguished are the widespread, laterally impersistent zones of intensely disturbed strata which are common throughout the lower part of the Nant Ffrancon Group between Rhyd and Cwm Pennant (Figure 15). The zones range up to 5 km long and 300 m thick. Typical are those in upper Llandeilo–Caradoc strata in the vicinity of Tremadog, which Fearnsides (1910) described as 'gnarled, shivered and kneaded into a regular augen schist' and 'repeatedly cut by small overthrust faults' and 'shattered by jointing'. The zones include folds, low-angle slides, thrusts, normal and reverse faults, and a gradation is commonly recognised from regularly bedded strata into coherent folds, semi coherent folds and total disruption of bedding (Plate 8.2). Such features are also well exposed in the mudstones at Ty Obry [SH 6023 3985], Llanfrothen [SH 6247 4178], Y Bengam [SH 5555 4130] and north of Pen y Clogwyn [SH 5362 4344].

The mélange and the disturbed zones are spatially and intimately related, and are clearly derived by the same process. Smith (1988) considered that the disturbed zones were not formed at a free surface but were the result of interstratal slip on a series of planes or weaknesses which probably developed initially from bedding-plane slip. The propagation of such structures to the surface, through variably lithified strata, would result in wholesale disruption and lateral collapse as reflected in the melange. Palaeoslope data (Smith, 1988) suggest movement down north-west-facing slopes in the vicinity of the Glyn Valley Fault on the northern edge of the Harlech Dome. Movement on minor faults, such as Rhyd or Ffynnon Eidda, probably influenced the lateral distribution of the mélange.

The timing of these events is moderately well constrained. Intrusions and associated contact metamorphism at Rhyd and Tremadog indicates that this disruption occurred in early Caradoc times (pre-Longvillian), prior to the main phase of Caradoc volcanism and emplacement of the Tan y Grisiau granite. The mélange at Rhyd and Tremadog is overlain by the Moelwyn Volcanic Formation with faunas indicative of the N. gracilis Biozone.

Strata above the mid-Ordovician unconformity

The strata above the mid-Ordovician unconformity are dominated by monotonous silty mudstones and siltstones with impersistent beds of cross-laminated, silty sandstones. In the Tremadog area, slumped, thinly bedded, locally disturbed, silty mudstones are interbedded with the Moelwyn Volcanic Formation. East of the Nantmor fault, the silty mudstones, up to 1400 m thick, form a broad swathe of country from the Glaslyn valley, in the south-west, to the Lledr valley and the south side of the Gwryd valley, in the north-east. Locally, bioturbation is intense and bedding structures have been obliterated. In the north-east of the district sandstones are prominent in the sequence underlying the Capel Curig Volcanic Formation. At the south-west closure of the Capel Curig Anticline large wave ripples and hummocky cross stratification have been described by Fritz and Axelrod (1987) and Orton (1988) and interpreted to reflect high-energy shallow-water conditions, possibly on a marine tidal flat.

Moelwyn Volcanic Formation

The Moelwyn Volcanic Formation crops out from the scarp slopes of the Moelwyn hills, in the east, to the vicinity of Tremadog, in the west. It can be divided informally into three members, the upper and lower volcanic members being separated by siltstones and sandstones which wedge out towards the east.

The Lower member is equivalent to the Y Glog Volcanics of Shackleton (1959) and the Lower Moelwyn Volcanic Series of Bromley (1963). It is up to 100 m thick and exhibits marked variations in thickness and lithologies. The member comprises coarse volcaniclastic debris flow deposits, thin acid tuffs and tuffites, local concentrations of accretionary lapilli (Bromley, 1963) and fine tuffaceous mudstones and siltstones. Its base is marked locally by a conglomeratic bed, with well-rounded cobbles of rhyolite and quartz in a sparse silty mudstone matrix; in the Tremadog area the conglomerate is loaded into the underlying mudstones. Elsewhere, for example north of Penmorfa [SH 5492 4116], thin crystal tuff beds occur at the base. These tuffs comprise whole and fragmented euhedral crystals of albite in a finely recrystallised matrix. The debris-flow deposits are irregularly bedded, up to 5 m thick, with angular clasts, 0.10–0.15 m across, of flow-foliated rhyolite, basalt and mudstone in a matrix of finer fragments with a considerable mudstone component. Some beds fine upwards into planar-bedded, crystal, lithic, acidic tuffites. At its top, the member grades up into mudstones with stringers and impersistent thin beds of volcanic detritus. In the western outcrops, as exposed near Glan y Morfa [SH 5557 4052] and north-west of Cwm Mawr Farm, the member is represented by discontinuous, lenticular, poorly, sorted beds containing a similar range of volcaniclastic debris.

Near Tremadog, the upper and lower members are separated by up to 90 m of planar-bedded siltstones and thin sandstones of the Middle member; these beds are intruded by the Tremadog dolerite. The sandstone beds, up to 0.05 m thick, have sharp bases and parallel laminated tops. The sequence is lithologically similar to strata of approximately basal Caradoc age in Cwm Pennant. In the most easterly outcrops, in the scarp slopes of Moelwyn Mawr, this member wedges out and the lower and upper members merge.

The Upper member is lithologically similar to the lower member, but it is more persistent laterally. It is equivalent to the Middle and Upper Volcanic series of Bromley (1963). In the vicinity of Rhyd, it comprises a sequence, up to 100 m thick, of bedded acid tuffites and volcaniclastic debris-flow deposits which are locally slumped and disaggregated. In the Tremadog area, near Cefn y Fedw [SH 5572 4144], the member is represented by coarse, crudely stratified, conglomeratic debris-flow deposits, up to 10 m thick, which contain large clasts, up to 0.25 m, of mudstone and rhyolite supported in a fine-grained silty matrix with patchy concentrations of chlorite. The conglomerates are overlain by feldspar crystal-rich tuffites with bedding accentuated by recrystallised siliceous layers and chlorite smears. The tuffites grade up into dark grey mudstones with isolated, fine-grained, acid volcanic clasts with irregular flame-like outlines which suggest that they were unlithified when they were incorporated into the mudstones.

Environmental interpretation

The volcaniclastic, debris-flow deposits of the Moelwyn Volcanic Formation indicate reworking of a predominantly volcanic source of rhyolitic composition. The deposits are spatially related to generally concordant rhyolite bodies, with an irregular sill-like shape suggesting high-level intrusion and possible extrusion. It is suggested that the debris flows resulted from the local uplift of the rhyolites to within wave base, to cause their reworking, and subsequent collapse of these deposits by the tectonic instability. The few primary tuffs in the sequence reflect infrequent and restricted explosive activity at the centres.

Llyn Conwy Formation

The formation, established by Lynas (1973), is moderately well exposed around Llyn Conwy [SH 780 463] close to the northern edge of the Migneint. It is confined entirely to the east of the Ffynnon Eidda Fault and mainly comprises acidic ash-flow tuffs which can be traced from the vicinity of Cefn Garw [SH 792 421] in the south, to above Cwm Hafod y Rhedrwydd [SH 770 460] in the north. In this direction the formation progressively oversteps the underlying strata down to the Tremadoc.

The formation was subdivided into the Clogwyn Hir Tuffs and the overlying Pen y Bedw Tuff (Lynas, 1973); the Clogwyn Hir Tuffs, 120–480 m thick, were further subdivided into two members but, because of the poor exposure, this subdivision is difficult to sustain. The sequence indicates a composite accumulation of white-weathered, ash-flow tuffs which are variably massive, locally crudely bedded, foliated, brecciated and, in places, flow folded. Although devitrification and recrystallization is locally intense the original shardic fabric and phenocrysts can commonly be discerned. The tuffs are mainly non-welded but the flow-folded foliation reported by Lynas (1973) suggests rheomorphism following emplacement. At Llyn Conwy, the pyroclastic fabrics are almost entirely obscured (KB 1367) by devitrification and recrystallisation and, where the potassic and plagioclase feldspar phenocrysts are the only discernible element of the original fabric (KB 1369), it is difficult to distinguish from possible rhyolite. Four specimens in this vicinity contained 83–85 per cent SiO2 and clearly indicate the problems of recrystallisation and secondary quartz veins.

North of Mon Conwy, the Pen y Bedw Tuff [SH 780 472], up to 75 m thick, overlies the Clogwyn Hir tuffs. It comprises fine-grained, cream-coloured tuffs interbedded with blue-grey, silty mudstone. The two lithologies grade, imperceptibly through tuffites, into each other thus producing a distinctively striped sequence. The laminations are generally plane-parallel and suggest pelagic settling of distal ash but it is possible that parts of the sequence represent reworked tuff-turbidites. Near the top of the Llyn Conwy Formation, a few fossiliferous mudstone intercalations were shown (Lynas, 1973) to be of Costonian age and interpreted to be lateral equivalents of the 'Derfel Limestone' (Fearnsides, 1905). At one locality in the River Conwy [SH 7815 4445] the mudstones overlie well-foliated tuffs of the Llyn Conwy Formation and at another locality [SH 7785 4694], the mudstones grade up into the Pen y Bedw Tuff.

Environmental interpretation

The acidic ash-flow tuffs of the Llyn Conwy Volcanic Formation reflect a period of intense explosive activity. The close association of these tuffs with high level rhyolite intrusions suggest that they represent restricted near-source accumulations. However, the formation is part of an extensive outcrop of acidic ash-flow tuffs in southern Snowdonia which probably developed from a number of contemporaneous centres. The few intercalated sedimentary rocks are entirely of silty mudstone and there is little evidence to suggest local shallowing.

Penamnen Tuff Formation

These tuffs can be traced from north-east of Llyn Llagi in the west, to the vicinity of Penmachno, in the east. They comprise up to 10 m of white weathered, acid tuffs with local siltstone intercalations. At the type section, at the head of Cwm Penamnen [SH 7340 5001], two tuff sequences, approximately 9 m thick, are separated by green-grey siltstones (Howells et al., 1978). Typically the tuffs are massive bedded with plane parallel-bedded tops suggesting emplacement as grain flows. They are composed of a tightly packed fabric of devitrified shards, whole and fragmented albite crystals and a few quartz crystals in a fine-grained aggregate of sericite and chlorite after vitric dust. Small, rounded clasts of iron-rich siltstone are common.

The tuffs are possible correlatives of the Gwern Gof Tuff in the core of the Tryfan Anticline to the north (British Geological Survey, 1985a; Howells et al., 1991) although their eruptive centre has not been distinguished.

Biostratigraphy of the Lower Caradoc strata above the mid-Ordovician unconformity.

Along the western side of Cwm Pennant, in the strata below the Pitts Head Tuff Formation, there is no evidence of the lower Caradoc, either of shelly faunas of the Costonian to Soudleyan stages or of graptolites of the multidens Biozone. As the lower outflow tuff is considered to be of late Soudleyan age then a break, possibly of volcano-tectonic origin, is inferred.

To the east of the Bryncir Fault, at Tyddyn Deucwm [SH 5421 4112], the gracilis Biozone is distinguished by a large fauna, including Climacograptus bicornis, Didymograptus superstes, Hallograptus mucronatus, Nemagraptus gracilis and species of Dicellograptus, Dicranograptus and Orthograptus ((Figure 11), a-e). The gracilis Biozone directly overlies Tremadoc strata, so that the Arenig and Llanvirn strata, so well developed in Cwm Pennant, are missing. The gracilis Biozone has also been determined at Pensyflog.

Between the Trawsfynydd and the Nantmor faults, the lowest fossiliferous mudstones, about 5 m above the unconformity, are exposed in the roadside north of Penrhyndeudraeth [SH 6151 4000] (Smith et al., 1995). A poorly preserved fauna has been collected: Amplexo graptus sp., Climacograptus sp., Dicranograptus rectus (of Elles and Wood, 1904), Glossograptus sp. and Orthograptus sp. (possibly O.calcaratus acutus), together with numerous specimens of the brachiopod Paterula. This collection represents either the gracilis or multidens Biozone; Elles and Wood (1913, p.519) reported that D. rectus is commoner in the multidens Biozone than below it; although it occurs in the gracilis Biozone at Tyddyn Deucwm, there is no positive indication at this roadside locality of the gracilis Biozone.

An important fossil locality at Ty Obry [SH 6049 3946], at a similar stratigraphical level, has long been known. Salter (in Ramsay, 1866, p.256) recorded a considerable fauna and subsequently the trilobites were re-examined by Stubblefield and the graptolites by Bulman (in Fearnsides and Davies, 1944, pp.255, 273), and a Llandeilo–Caradoc age was determined. The fauna comprises sponge spicules, 'Palaearca' socialis, 'Orthoceras', 'Conularia' corium and 'C. ' margaritifera, with the trilobites Dionide atra, Microparia caliginosa (Plate 6)a-e and Platycalymene cf. duplicata, and the graptolites Amplexograptus (including A. perexcavatus?), Climacograptus spp. (including C. cf. bekkeri, C. cf. putillus), Cryptograptus tricornis?, Dendrograptus furcatula, Glossograptus cf. ciliatus, Glyptograptus' cf. teretiusculus, Hallograptus mucronatus (var.) and Orthograptus cf. calcaratus acutus. The Dicranograptus recorded by Salter (in Ramsay, 1866) is so badly preserved that even its generic determination is doubtful.

During this survey, a similar fauna, together with Pseudoclimacograptus and a possible example of Lasiograptus harknessi, was collected from a locality 600 m north-east of Y Garth [SH 5995 3951]. The graptolites indicate the gracilis or multidens Biozone and if the presence of L. harknessi is confirmed it favours the latter. The overlying strata [SH 5997 3986] and [SH 6236 4097] yielded only undeterminable climacograptids.

Strata below the lower member of the Moelwyn Volcanic Formation have yielded non-diagnostic, poorly preserved shelly faunas including dalmanellid and sowerbyellid brachiopods, e.g. at [SH 5562 4047]. At Bodawen Gate [SH 567 397], Shackleton (1959, p.231) recorded diplograptids possibly, but not wholly, diagnostic of the multidens Biozone. Above the upper member are Caradoc shelly faunas among which Harper (in Shackleton, 1959, p.249) identified Broeggerolithus harnagensis, suggestive of the Harnagian Stage.

At Ceseiliau Duon [SH 6571 4450], strata above the Moelwyn Volcanic Formation yielded a shelly fauna (Bromley, 1965) and during this survey the following were collected: Dolerorthis tenuicostata (D, L, S?), Harknessella cf. subquadrata, Horderleyella?, Leptellina?, Nicolella humilis (D, L), Onniella cf. avelinei, Platystrophia major (D, L, S ?), Sowerbyellids, Sinuites?, trinucleid and crinoid fragments ((Plate 9) j, k, n-q). In addition, Bromley (1965) recorded Howellites sp., Kullervo?, Leptestiina derfelensis (D), Salopia?, Ceraurinella sp. and Platylichas?. Several of these species, marked D above, are known from the Derfel Limestone member, which is referrable to the multidens Biozone (Zalasiewicz, 1992); a few, marked L, occur high in the gracilis Biozone at Llanbabo on Anglesey; two, marked S, are similar to forms from the multidens Biozone of the Shelve area (Williams, 1974); P. major occurs in the Llandeilo of the Berwyn Hills (Macgregor, 1961). H. subquadrata occurs in the Costonian (uppermost gracilis Biozone) in the Caradoc area and O. avelinei in the Harnagian. This fauna is therefore considered to be approximately Costonian to Harnagian in age and to represent a horizon near the gracilis-multidens biozonal boundary.

Mudstones interbedded with the Moelwyn Volcanic Formation, around [SH 672 451] have yielded acritarch assemblages with Stellechinatum celestum and Striatotheca frequens, indicating that these beds are not older than Llanvirn. The evidence is insufficient to suggest a more precise age. However, these assemblages also contain Acanthodiacrodium angustum, A. ignoratum, Cymatiogalea cristata and Vulcanisphaera sp., which strongly suggests that Tremadoc strata were being eroded contemporaneously.

Localities between Bwlch Cwmorthin and Bwlch y Rhosydd [SH 6666 4622], [SH 6662 4622], [SH 6606 4617] and [SH 6613 4625], above the Moelwyn Volcanic Formation, yielded relatively few acritarch species, but these include Stellechinatum celestum, Striatotheca frequens and S. quieta, which indicate a Llanvirn or younger age, accompanied by the chitinozoan Spinachitina bulmani. In the type section of the Caradoc Series the latter is restricted to the base of the Glenburrell Formation, of late Harnagian age (Jenkins, 1967).

In the area east of the Ffynnon Eidda Fault, Zalasiewicz (1992) has shown that Fearnsides' (1905) record of the murchisoni Biozone is incorrect; the graptolite faunas below the Llyn Conwy Formation include Amplexograptus? molestus, Corynoides curtus, Dicellograptus salopiensis and Lasiograptus harknessi, and are referrable to the multidens Biozone. Above the Llyn Conwy Formation the calcareous mudstone referred to the Derfel Limestone (Lynas, 1973, p.490) yielded a diverse shelly fauna (Whittington and Williams, 1955) considered to be of early Caradoc (Costonian–Harnagian) age. Zalasiewicz (1992) has shown that the associated graptolitic fauna, which includes Dicranograptus nicholsoni minor, D. spinifer, Diplograptus foliaceus and Normalograptus brevis, is also referrable to the multidens Biozone.

Shelly faunas are sparse in most of the strata above the Penamnen Tuff, up to the base of the Capel Curig Volcanic Formation. They included Howellites cf. ultima [SH 7082 4960] and Broeggerolithus broeggeri [SH 6922 4948] and are Soudleyan in age. A rich and varied assemblage of acritarchs and chitinozoans from strata on the northeastern slopes of Moel Dyrnogydd, between [SH 6989 4909] and [SH 6974 4979] includes the acritarchs Fractoricoronula trirhetica and Veryhachium triangulatum and the chitinozoans Desmochitina minor and Spinachitina bulmani. These taxa suggest an early Caradoc, possibly Harnagian age, based on their documented occurrences in the Welsh Borderland (Jenkins, 1967; Turner, 1984).

Llewelyn Volcanic Group

The Llewelyn Volcanic Group reflects the first major eruptive cycle recognised in the Caradoc sequence of central and north Snowdonia (Howells et al., 1991). Within the district it is represented solely by the southern edge of the outcrop of the Capel Curig Volcanic Formation.

Capel Curig Volcanic Formation

The formation represents the first major period of ash-flow tuff volcanism in Caradoc times. It comprises four members (Howells and Leveridge, 1980) three of which crop out in the Capel Curig Anticline at the northern edge of the district. Here the formation, up to 220 m thick, contains the Garth, Racks and Dyffryn Mymbyr tuffs, in upward succession, with intercalated silty mudstones and fine-grained sandstones.

The Garth Tuff, up to 40 m thick, is a white weathered, primary, acidic ash-flow tuff. Its base is remarkably discordant and flames of the underlying sediment deeply penetrate the tuff (Francis and Howells, 1973). In places large tuff bodies, up to 100 by 250 m in plan, are detached and surrounded by the underlying sedimentary rocks. The tuff is welded in its lower and middle parts and at its top it is evenly bedded, reworked and is concordantly overlain by sandstones. On the south limb of the anticline, the reworked top contains isolated accretionary lapilli. The tuff comprises devitrified and recrystallised shards, isolated whole and fragmented albite phenocrysts in a fine-grained matrix of sericite, chlorite, quartz and feldspar after original vitric dust. Lithic fragments include devitrified perlitic glass and welded tuff. In places large siliceous nodules, up to 0.20 m in diameter, are well developed and chloritic fiamme are common.

The Racks Tuff, up to 30 m thick, is lithologically closely comparable with the underlying Garth Tuff. Within the district it forms a fairly regular outcrop on the south-east limb of the Capel Curig Anticline. However, just to the north, its base is extremely irregular and locally the tuff is discontinuous and represented by isolated pods. Adjacent to the contacts, the mudstones are intruded by apophyses of tuff and the tuff, welded to the contact, contain blebs of mudstone (Francis and Howells, 1973, p1.2).

The Dyffryn Mymbyr Tuff only just encroaches the district, on the north limb of the Capel Curig Anticline. It is most clearly recognised when it contains well-developed accretionary lapilli but laterally the frequency of these diminish and the rock grades into a tuffaceous mudstone.

To the south, the Capel Curig Volcanic Formation crops out about a tight anticline in the Lledr valley [SH 770 540] although here only the Garth Tuff is represented (Francis and Howells, 1973) and the outcrop is discontinuous. Isolated welded tuff bodies, up to 150 m diameter, are enclosed by siltstones and silty mudstones and the tuff adjacent to the contacts is crowded with siliceous nodules. From the discontinuity of the tuff outcrop on the north limb of the anticline it is possible that most of the tuff in this area is represented by similar isolated bodies; farther south, they have been distinguished about Pigyn Esgob and along the southern limb of the Dolwyddelan Syncline, as at Carreg Alitrem [SH 740 507], and they have been referred to as the pod facies of the Garth Tuff (Howells et al., 1985).

To the west of Dolwyddelan, the Capel Curig Volcanic Formation has been correlated with the Clogwyn Gottal Tuff which is exposed on both limbs of the syncline north-north-east of Yr Arddu. The tuff lies close to the base of a sandstone sequence and approximates to the stratigraphical position of the Capel Curig Volcanic Formation. On the western limb of the syncline the acid tuff, up to 15 m thick, is interbedded with volcaniclastic sandstones and siltstones. The tuff is poorly sorted and heterolithic with subrounded clasts, up to 0.10 m across, of rhyolite, acid tuff and sedimentary rocks.

Environmental interpretation

The Capel Curig Volcanic Formation within the district is but a small part of its total outcrop which lies mainly to the north in the Bangor district (British Geological Survey, 1985a). The formation has been the subject of detailed study (Howells et al., 1979, Howells and Leveridge, 1980; Howells et al., 1991) and the tuffs have been related to three eruptive centres. The lowest two members, the Garth and Racks tuffs, are considered to have been erupted from a subaerial centre in the north, they were transported southwards and transgressed a shoreline into a submarine environment just to the north of Capel Curig. It is argued that the ash flows crossed the shoreface without disintegration and continued to be transported in the marine environment, and that they maintained sufficient heat to weld on their emplacement. The remarkably transgressive bases of the ash-flow tuffs within the district are the result of the emplacement of hot pyroclastic debris on to the unlithified, water-saturated substrate; the bases collapsed by unequal loading and flames of sediment intruded the body of the tuff. These processes were probably facilitated by fluidisation of the sediment at the basal contact (Kokelaar, 1982).

The isolated pods at the edge of the outcrop within the district are interpreted as a distal facies and were probably caused by disruption of the ash flow across a fault scarp.

The accretionary lapilli tuffs have been related to a centre sited within a submarine setting within the Glyders, just to the north of the district. Closer to the centre they are closely associated with water saturated, pyroclastic debris-flow deposits. However, the accretionary lapilli indicate periodic subaerial eruptions of fine ash.

Cwm Eigiau Formation

On the east side of the Nantmor Fault, the Cwm Eigiau Formation overlies the Capel Curig Volcanic Formation; where the latter is solely represented by the distal pod facies, as between Dolwyddelan and Yr Arddu, its base is difficult to define. On the west side of the Nantmor Fault, the base of the Cwm Eigiau Formation is placed at the base of the Prenteg Sandstone Member. Throughout the district, the top is defined by the base of the Snowdon Volcanic Group.

The formation is distinctive because it records a major influx of coarse detritus into the district and, locally, temporary shallowing of the marine environment. It comprises siltstones and sandstones with subordinate silty mudstones and shows marked lateral variations in lithology along its outcrop. Sandstones generally dominate the sequence in the south-west, but to the north-east they progressively diminish, interdigitating with and passing laterally into siltstones. Thin beds of fine-grained acid tuff are common, especially in the sequence about the synclines on the south slopes of Moel Siabod and at Dolwyddelan.

On the south-east side of the Nantmor valley [SH 640 491], Orton (1988) described a 350 m-thick section with two sandstone-dominated sequences, within siltstones with thin sandstone beds. The sandstones, medium to coarse grained and locally pebbly, vary from thin flaggy beds, 20–30 mm thick, to massive beds, up to 3 m thick. They display a wide range of sedimentary structures including plane-parallel bedding, hummocky cross-stratification, ripple-drift lamination and normal grading. The sequence has been interpreted (Orton, 1988) to represent an upward transition from a delta front overwhelmed by a channel-fill complex to a storm-dominated shelf, a shoreface and inner shelf.

On the southern limb of the Dolwyddelan Syncline, a distinctive, 40 m thick sequence of sandstones occurs some 180 m above a welded ash-flow tuff pod of the Capel Curig Volcanic Formation. The sandstones show planar laminations, low-angle and hummocky cross-stratification, and were interpreted (Orton, 1988) to represent inner shelf deposits within reach of fair-weather currents. The sequence includes two acid tuffites, up to 2 m thick, comprising a fine aggregate of quartz, feldspar, sericite and chlorite, after devitrified vitric dust, which is locally mixed with siltstone.

Towards the east, on the southern limb of the Dolwyddelan Syncline the sandstones in the upper part of the Cwm Eigiau Formation pass laterally into a silty mudstone sequence and this transition is reflected in the incidence of slate quarries close to the eastern closure. Similarly, to the north of the Lledr valley, the formation is almost entirely of silty mudstone which locally, as at Hafod Las [SH 780 560], have been extensively worked for slates. However, on the south-east slopes of Moel Siabod, sandstones distinguished in the central part of the formation in the vicinity of Capel Curig (Howells et al., 1978), form a prominent outcrop about the north-east-trending synclines. Here they are up to 150 m thick and locally contain thick siltstone intercalations and fine-grained, bedded acid tuffs, up to 10 m thick. The finely recrystallised, siliceous tuffs have been quarried for honestones and are interpreted as distal air-fall deposits which settled into the marine environment and were subsequently reworked into turbidity flows.

On the west side of the Nantmor Fault, the formation comprises two thick sandstone sequences, the Prenteg Sandstone and Moel Hebog Sandstone members, with intervening siltstones, although considerable lateral disruption causes complex outcrop patterns. In the west of the district, between Beddgelert and the Nantmor valley, the sequence forms a broad periclinal anticline and was much disordered by bedding-plane disruption and sliding while semi-lithified. The structures have been interpreted (Howells et al., 1991) to be the response to magma movement at depth, near the site of the main-phase volcanic centre of the Snowdon Volcanic Group.

Prenteg Sandstone Member

The basal stratotype is taken north of Hendre-selar [SH 5684 4180]. The member forms a prominent scarp feature between Prenteg village and Llyn Cwmystradllyn. To the east it is exposed in small scarps, as at Coed Trwyn Bryniau [SH 6006 4211], within the alluvial flat of the Glaslyn valley. To the north, in the area about the Nantmor Valley, it cannot be separated from the Moel Hebog Sandstone Member.

The Prenteg Sandstone Member is 100–200 m thick and comprises fine- to coarse-grained sandstones which are, in places, magnetite-rich. Its base is marked by planar-bedded and cross-laminated sandstones and intercalated beds, less than 0.5 m thick, of silty sandstones and mudstones occur throughout and locally the bedding is disrupted. At its top the member is sharply overlain by thinly interbedded mudstones and sandstones. The bases of the sandstone beds are commonly loaded into the underlying mudstones. Sedimentary structures are common and include normal grading, planar- and cross-lamination with the laminations accentuated by magnetite concentrations. Locally, mud-draped foresets and asymmetric ripples occur. Palaeocurrent data indicate derivation from the south-south-west.

The sandstones comprise well-sorted, angular to sub-rounded quartz grains, approximately 50 per cent, with chlorite-mica stacks and isolated albite crystals in a fine-grained matrix of quartz, sericite and chlorite with accessory zircon, magnetite and pyrite.

Moel Hebog Sandstone Member

The member is restricted to the area around Moel Hebog and Moel Ddu. It is equivalent to the Gorllwyn Grits of Shackleton (1959) and has recently been described by Reedman et al. (1987). The stratotype is taken east of Moel Hebog [SH 570 466]. Around the Moel Hebog Syncline it forms a prominent scarp above the lower Ordovician mudstones. The complicated outcrop patterns, for example about Mynydd Gorllwyn [SH 5800 4250] and Moel Ddu (Howells et al., 1986, fig. 5), are the result of contemporaneous slumping and mass transport. Consequently, accurate estimates of thickness are difficult to make, but around Moel Hebog the member is about 250 m thick.

The member comprises well-bedded, pebbly micro-conglomerates, coarse-grained sandstones and a few thinner silty sandstones. On the western limb of the Moel Hebog Syncline the base is gradational and the top is defined by the base of the Pitts Head Tuff Formation. The sandstone beds are typically 1–2 m thick, with erosive, channelled bases and planar tops. In places, large pebbles of acid tuff, up to 0.20 m in diameter, occur in discrete bands. Large scale cross-bedding, ripple laminations, asymmetric ripples (Plate 8.3), grading cycles and channels are well-developed and syndepositional convolute laminations occur locally. The section at Moel Hebog [SH 5705 4680] has been interpreted (Orton, 1988; Howells et al., 1991) (Figure 16) to represent a progressively shallowing sequence, consisting of a prodelta overlain by distributary mouth bar sediments, and an alluvial plain and fan sequence forming the substrate to the Pitts Head Tuff Formation.

Environmental interpretation

The Cwm Eigiau Formation predominantly reflects a period of shallow and offshore, marine siliciclastic sedimentation between two major periods of volcanism. The main change of basin configuration was the change in the direction of the palaeoslope from south facing, during the emplacement of the Capel Curig Volcanic Formation, to north facing, during Pitts Head Tuff Formation times (Howells et al., 1991). The coarse clastic sandstones of the Prenteg Sandstone and Moel Hebog Sandstone members indicate the establishment of shallow-deltaic and near-shore conditions, and the restricted alluvial fans probably indicate local emergence. The widespread disruption of the sequence probably resulted from instability caused by differential uplift in fault zones. Such activity is most clearly defined within the Beddgelert Fault Zone (Howells et al., 1991) which subsequently exerted a profound influence on the siting of the vents in the evolution of the Lower Rhyolitic Tuff Formation.

Biostratigraphy

The biostratigraphy of the Cwm Eigiau Formation is complicated by lateral variations in biofacies which are shown by the various faunal associations. The faunas that are dominated by Dinorthis, and especially those with Plaesiomys multifida (Plate 9)i, are considered to represent inhabitants of shallow-water and high-energy environments (Pickerill and Brenchley, 1979); dalmanellid–Sowerbyella associations favoured deeper-water and lower-energy environments; diverse shelly assemblages, in places with mollusca and trinucleid trilobites, occupied even more distal and slightly deeper sites. The associations may be diachronous and correlation is more reliably based on successive species of prevalent genera, such as the trinucleid trilobite Broeggerolithus and the brachiopods Dinorthis, Howellites, Macrocoelia and Sowerbyella.

Faunas at the base of the Prenteg Sandstone Formation are suggestive of the Soudleyan Stage, but one locality [SH 5755 4223] yielded Flexicalymene planimarginata which is more typical of the Longvillian. North-east of Moel Hebog [SH 571 475] and west of Hafod Owen [SH 6193 4882], Shackleton (1959) collected Broeggerolithus broeggeri, which is typical of the Soudleyan Stage, from localities below the Snowdon Volcanic Group.

The lowest fauna in the Cwm Eigiau Formation is associated with pods of the Capel Curig Volcanic Formation above Cwm Penamnen [SH 7353 5117] where Salopia globosa (Plate 9)l & m, is common. A succession of three Soudleyan brachiopod faunas, based largely on evidence from north of the district (e.g. Cwm Idwal [SH 64 59]), has been determined (Howells et al., 1991). The lowest of these assemblages, characterised by Dinorthis berwynensis berwynensis and Sowerbyella sericea permixta was observed in a sequence east of Yr Arddu [SH 6384 4678]. Examples of the middle assemblage, with Bicuspina spin]: eroides, D. berwynensis angusta and Sowerbyella musculosa were collected on a traverse west of Llyn Edno [SH 6597 4980] to [SH 6590 5000] and also south-east of Haffodydd Brithion [SH 6428 4894]. The highest Soudleyan is characterised by an assemblage, generally of low diversity, containing Plaesiomys multifida, which locally forms shell beds; this is interpreted as uppermost Soudleyan. To the north of the district, it is present at about the horizon of the Pitts Head Tuff Formation and extends into the district, north of Cefn y Cerrig, e.g. [SH 6742 4904]. The same assemblage occurs south-east of Hafodydd Brithion, around [SH 6452 4904] and south of Yr Arddu [SH 6197 4524]. Fragments of Soudleyan Broeggerolithus occur rarely, for example B. broeggen? [SH 6354 4687] and B. soudleyensis? [SH 6450 4885]. In general, these faunas suggest shallow-water conditions, particularly at the close of Soudleyan times.

Where the Pitts Head Tuff is absent, as in the eastern part of the district, the top of the Cwm Eigiau Formation is of Longvillian age and is characterised by the trilobites Broeggerolithus nicholsoni and Kloucekia apiculata. Flexicalymene planimarginata (Plate 9)d is widely found at this level but it is also known from Soudleyan strata. B. nicholsoni and K. apiculata have been collected below the Yr Arddu Tuff Formation, north-north-east of Yr Arddu [SH 6360 4701]. The assemblages suggest a more distal, possibly deeper facies than in the Soudleyan.

Around the Dolwyddelan Syncline the faunal succession differs; the Soudleyan faunas that have been found in strata above the pod facies of the Capel Curig Volcanic Formation are relatively diverse and, compared with those farther north and west, are less dominated by Dinorthis. They appear to represent a more distal facies and, like the pods, suggest a possible change, from north to south from shallow marine to outer shelf. Near Roman Bridge railway halt [SH 7163 5121] a diverse Soudleyan fauna with Broeggerolithus broeggeri (Plate 9)h contains examples of molluscan and brachiopod genera that are more commonly found in Longvillian faunas to the north of the district. To the north-west of this locality, and possibly slightly higher in the sequence, Diggens and Romano (1968) and Wright (1979) recorded Longvillian faunas with B. nicholsoni and Kloucekia apiculate, e.g. [SH 7146 5117]. As yet neither of these trilobites has been found at or below the horizon of the Capel Curig Volcanic Formation, but because the respective localities are so close together, a local break or condensed succession is suspected. To the east, 1 km south-east of Dolwyddelan (e.g. [SH 7441 5165] and [SH 7467 5182], Longvillian faunas with B. nicholsoni occur well above the Capel Curig Volcanic Formation. Longvillian faunas were also collected from the northern limb of the Dolwyddelan Syncline [SH 6949 5222] to [SH 6970 5212], but the P. multifida sandstones have not been detected on either limb.

Snowdon Volcanic Group

The Snowdon Volcanic Group was defined (see Howells et al., 1983 for details), and revised (Howells et al., 1991), to modify the Snowdon Volcanic 'Suite' (Williams, 1927) and the broadly equivalent Crafnant Volcanic 'Series' (Davies, 1936) to modern stratigraphical nomenclature. The group is the main representative of the 2nd Eruptive Cycle of Caradoc volcanism in central Snowdonia (Howells et al., 1991) which comprises acidic ash-flow tuffs, intrusive and extrusive rhyolites, basalts, hyaloclastites and basic tuffs, which are variably associated with marine sedimentary rocks. The sequence reflects a complex development of eruptive centres where the activity shifted temporally and spatially. Three main centres of activity have been defined, two of which, at Llwyd Mawr and Snowdon, occur within the district, and the other, the Crafnant centre, lies to the north, in the Bangor district.

The siting of the Llwyd Mawr and Snowdon centres was controlled by north-south- and north-east-southwest-trending fracture zones which are considered to have been the upward propagation of basement faults into the cover sequence. The central, Beddgelert Fracture Zone had the most profound influence on the development of the Snowdon centre. Its earliest expression was probably in the disruption of the unlithified substrate to the Snowdon Volcanic Group in the area between Beddgelert and the Nantmor valley (Figure 17). Here the base of the volcanic sequence lies discordantly upon a periclinal anticline within the substrate, and the contrast between this and the syncline in the overlying tuffs indicates that the anticline developed before the initiation of the volcanic activity.

Within the district, the group can be divided, in upward succession, into the Pitts Head Tuff Formation, the Lower Rhyolitic Tuff Formation (including the Yr Arddu Tuffs), the Bedded Pyroclastic Formation and the Upper Rhyolitic Tuff Formation.

Geochemically, the group comprises three main magmatic compositions: subalkaline basalt, rhyolite and a transitional rhyolite-comendite/pantellerite. A near- continuum of compositions exists between them although intermediate compositions are comparatively rare. The geochemical details of the group, and interpretations, have been comprehensively covered elsewhere (Howells et al., 1991; Thorpe et al., 1993 and references therein) and are not repeated in this account.

Pitts Head Tuff Formation

The formation has two distinct expressions which have been interpreted (Roberts, 1969; Reedman et al., 1987; Howells et al., 1991) to represent an intracaldera facies and an outflow facies (Figure 18). The intracaldera tuffs are a restricted accumulation of welded acidic ash-flow tuff, up to 700 m thick, in the vicinity of Llwyd Mawr to the west of the district, but are exposed south of Cwm Silyn to the west of Cwm Pennant within the district. The tuff forms a synclinal outlier trending broadly north-south and is partly discordant and partly concordant with siltstones and silty mudstones of the Nant Ffrancon Group. Roberts and Siddans (1971) interpreted, from compactional variations, two eruptive pulses but geochemical analyses of serially collected samples do not indicate any related compositional change (Howells et al., 1991). The tuff is intruded by large, high-level rhyolite domes and is considered to have been entrapped in a volcano-tectonic depression, a graben or caldera.

The outflow tuffs are represented by two primary acidic ash-flow tuffs, up to 100 m thick, which are exposed on the flanks of the adjacent Moel Hebog Syncline to the east (Figure 19). The tuffs are lithologically identical (see below) to the Llwyd Mawr tuff and the evidence from the associated sedimentary rocks suggest they were emplaced subaerially (Reedman et al., 1987). Locally, e.g. [SH 5684 4694], the lower outflow tuff overlies up to 1 m of bedded, non-welded vitroclastic tuffs. The outflow tuff has a non-welded, crystal-rich basal zone, up to 1 m thick, which sharply grades up into welded tuff with chloritic fiamme, below a prominent zone of siliceous nodules. The nodular zone, 1–3 m thick lies 3–5 m above the base of the tuff and the nodules are up to 0.40 m in diameter. The nodules developed from the entrapment of volatiles below the zone of most intense welding. Above, the main body of the tuff is extremely uniform, with a pronounced parataxitic fabric accentuated by thin, 1–3 mm, siliceous segregations (Plate 10.1). The top of the tuff is eroded. In places on the eastern flanks of Moel Hebog the tuff is distinctively brecciated. The breccias comprise rotated, angular clasts of welded tuff (Plate 10.2) occurring in either discontinuous zones, commonly aligned along joints, or as major zones which, in extreme cases, occupy the entire thickness of the tuff [SH 568 467]. The breccias have been described in detail by Reedman et al. (1987) and the process has been ascribed to post-emplacement, mass gravity flow, due to contemporaneous tectonism, and volatile streaming. The disruption is most extreme in the area about Beddgelert [SH 592 482] and Moel Ddu [SH 586 440] where there are large allochthonous tuff rafts, both isolated and stacked.

On the south-east side of Moel Hebog, the lower tuff is directly overlain by the upper tuff. However on the west and south-west sides, the two tuffs are separated by a sequence of medium- to coarse-grained sandstones overlain by a boulder conglomerate, which thickens south-westwards to 40 m. The conglomerate is massive with well-rounded clasts, up to 0.30 m in diameter, and a sparse siltstone and sandstone matrix. It is interpreted as a proximal alluvial fan deposit. On the north side of Moel Hebog, the upper tuff is absent and the lower tuff is directly overlain by the Lower Rhyolitic Tuff Formation.

The upper outflow tuff is restricted to the vicinity of Moel Hebog. However the lower outflow tuff can be traced northwards, into the north side of Cwm Tregalan [SH 606 535] but on the south side it is represented by a coarse-grained sandstone almost entirely composed of clasts of the tuff. It is suggested that this reworking was caused by localised uplift, in the vicinity of the Beddgelert Fault Zone. Farther north, the tuff can be traced into Clogwyn du'r Arddu at the northern edge of the district. Along this outcrop the tuff, 70–100 m thick, is parataxitically welded throughout and the basal non-welded tuff and the nodular zone are generally absent.

On the north side of the Snowdon Massif the lower outflow tuff is similarly restricted to the western side of the Beddgelert Fault Zone. Within the district it is exposed to the west of Llyn Cwm y Ffynnon [SH 644 563] at the northern edge of the sheet, but to the east it passes laterally into a sequence of sandstones and thin acid tuffs and tuffites. This change occurs across the projected western edge of the Beddgelert Fault Zone.

Petrography

The devitrified tuffs (KB 553), (KB 556), (TL 034), (TL 061) comprise a fine aggregate of quartz, feldspar, sericite and chlorite in varying proportions. In places, impersistent ribs of coarser recrystallised quartz accentuate the welding fabric. Shards are locally well defined although more typically the original fabric is overprinted by recrystallisation. Phenocrysts are predominantly of euhedral, subhedral and fragmented crystals of albite-oligoclase feldspar (less than 2 mm) with very few quartz; they are particularly concentrated in the basal zones. Lithic fragments are mainly of siltstone or fine sandstone and acid tuff. Perlitic fractures in the matrix are locally well developed, especially in the basal zones. Siliceous nodules, as small as 0.4 mm, comprise a quartz mosaic and the welding foliation can be distinguished both passing into and deflected around them, indicating that they developed during compactional welding.

Environmental interpretation

The eruptive centre of the Pitts Head Tuff Formation lay in the vicinity of Llwyd Mawr, just to the west of the district. The absence of early Caradoc strata below the intracaldera tuffs, compared with the thick development (more than 1 km) below the proximal outflow tuffs, implies local uplift and erosion in the vicinity of the centre prior to caldera collapse. The subaerial setting of the proximal outflow tuffs on Moel Hebog suggests that the caldera also had a subaerial expression. Transport of the ash flows was to the north and east of the centre. The lower tuff can be traced for some 25 km, and its sedimentary associations indicate that the ash flow transgressed a shoreline, just to the north of Moel Hebog and continued its transport and emplacement in a marine environment (Figure 19). In both environments the tuff was hot enough to weld on emplacement.

The brecciation of the lower ash-flow tuff and the large autochthonous rafts on the eastern side of Moel Hebog, Moel Ddu and near Beddgelert, and the restricted outcrop of the upper tuff, were caused by volcano-tectonic activity related to the Lower Rhyolitic Tuff Formation caldera.

Strata between the Pitts Head Tuff and Lower Rhyolitic Tuff formations

About Moel Hebog, the Pitts Head Tuff Formation is directly overlain by the Lower Rhyolitic Tuff Formation. To the north, near Cwm Caregog, the lower outflow tuff is overlain by medium- to coarse-grained pebbly sandstones which are intruded by a dolerite sill. In Cwm Tregalan [SH 660 535] the dolerite is interdigitated with pebbly sandstones, and on the north and east sides of the cwm up to 100 m of pillowed basalt and pillow breccias, with lenses of basaltic sandstone, overlie sandstones derived from the reworking of the lower outflow tuff.

To the north, the strata between the Pitts Head Tuff and Lower Rhyolitic Tuff formations thicken progressively. These strata, up to 250 m thick, crop out at the northern edge of the district, across the Pass of Llanberis between Dinas Mot and Llyn Cwm y Ffynon. They comprise a fining-upward, sandstone-dominated sequence overlain by siltstones. The sandstones contain brachiopods of the Dinorthis–Macrocoelia community (see below). In the lower part, the sedimentary structures include trough cross-stratification, swaley cross-stratification and low-angle cross-lamination to parallel-lamination. Above, the strata are characterised by planar bedding, winnowed fossiliferous beds about 0.20 m thick, coquina-filled scours and low to moderately inclined cross-bedding with shells along the foresets. The sequence is interpreted to represent a transgressive, non-barred, wave-influenced shoreline (Orton, 1988; Reedman et al., 1987).

On the north side of the Pass of Llanberis [SH 636 565], the strata are intruded by a basalt sill (Howells et al., 1991) which was previously interpreted (British Geological Survey, 1985b) as an extrusion. The sill wedges out to the east. The top of the sequence below Dinas Mot is marked by about 9 m of well-bedded, fine-grained, acidic tuffs which are interpreted to reflect distal air-fall, water-settled ash. The tuffs have been locally worked for hone-stones. Similar tuffs are common within the sequence to the north of the district.

Lower Rhyolitic Tuff Formation

The Lower Rhyolitic Tuff Formation represents a major period of acidic ash-flow tuff eruption and caldera collapse in central Snowdonia (Howells et al., 1986, 1991). Within the district, the formation is well exposed about the Snowdon massif; its correlatives have been distinguished about the synclines at Dolwyddelan and the south side of Moel Siabod, and in small restricted out-crops to the west of Betws y Coed (Figure 20) (Howells et al.; 1973, 1978).

The formation is thickest, up to 600 m, at Lliwedd [SH 625 532] (Plate 11.1) and on the south side of the Pass of Llanberis, to the west of Pen y Gwryd. Here the primary ash-flow tuffs form an estimated volume of 30 km3 within the 200 m isopach and, outside it, a minimum of 20 km3 occurs, partly within the Snowdon district and partly within the Bangor district to the north. From the evidence of thickness variations of the primary ash-flow tuffs, the character of the basal contacts, the internal facies variations and the distribution of associated rhyolites, a caldera has been distinguished about the Snowdon massif.

Yr Arddu Tuffs

The Yr Arddu Tuffs (Howells et al., 1987) crop out in a synclinal outlier about Yr Arddu [SH 628 463] on the east side of the Nantmor Fracture on the south side of Snowdon. They represent the earliest activity at the Snowdon centre (Howells et al., 1991) and comprise a sequence, approximately 200 m thick, of welded, acidic, ash-flow tuffs which are locally discordant on a substrate of shallow-marine sandstones and siltstones. The tuffs are intruded by two large boss-like intrusions of intensely flow-folded rhyolite (Plate 10.4).

The tuffs, up to 40 m thick, are lithologically distinctive in their block and pumice content, and locally grade into block and ash-flow tuff and pyroclastic breccias. The blocks, up to 1.5 m across, are dominantly of acid tuff and rhyolite of subangular to subrounded equant shape and fewer, smaller clasts of siltstone, sandstone, basalt and dolerite. Pumice fragments are typically elongate, up to 0.35 m long, with splayed and frayed terminations. Siliceous nodules up to 0.40 m diameter are commonly concentrated in zones near the base or top of flows (Plate 10.3). Although individual flow units are well defined they can, despite excellent exposure, only be traced for limited distances. Locally, thinner, reworked, cross-laminated and flaggy bedded tuffs occur. Disarticulated brachiopods and echinoderm debris in reworked tuffs at the top of the lowest ash-flow tuff [SH 6219 4524] indicate its shallow-marine reworking.

To the north-west of Yr Arddu, close to Moel y Dyniewyd, a non-welded acidic ash-flow tuff, about 40 m thick, at the base of the Lower Rhyolitic Tuff Formation, has been correlated with the Yr Arddu Tuffs on its geochemical composition (Howells et al., 1991).

Petrography

The Yr Arddu Tuffs (KB 546), (KB 695) comprise albite-oligoclase feldspar crystals and rare quartz with lithic clasts in a matrix of devitrified, recrystallised shards and vitric dust. The proportions of these components is highly variable. Feldspar crystals are either euhedral, slightly rounded or fragmented; locally they form up to 70 per cent of the rock but elsewhere they are sparsely distributed. Clasts of acid tuff are closely comparable with the host tuff, and common rhyolite clasts are characterised by spherulitic recrystallisation of silica crowded with iron oxide grains and sericite flakes. Pumice clasts are distinctive, both as collapsed elongate fragments and as irregular chloritised vesicular fragments. The tuff matrix is variably recrystallised, although only rarely is the original fabric obliterated. Shards are typically well preserved with eutaxitic to parataxitic textures, and locally non-welded textures. The devitrification of the original glass mainly resulted in a quartzose aggregate, but the matrix includes some sericite and chlorite.

Environmental interpretation

The Yr Arddu Tuffs represent the earliest activity at the Snowdon eruptive centre. The siting of the activity was controlled by the north-east-trending Yr Arddu Fracture, one of a suite of extensional structures in the vicinity of Snowdon which controlled the evolution of the Snowdon centre (Figure 20) (Howells et al., 1991). The fracture facilitated the emplacement of the rhyolite intrusions both at the Yr Arddu centre (Plate 10.4) and at Castell [SH 638 480] to the north-east (Howells et al., 1987).

The heterogeneity of the Yr Arddu Tuffs, with patchy concentration of blocks, suggest that they accumulated close to their eruptive centre. With increasing transport from the vent, the block component would be concentrated at the base of the flow and individual flows would be more clearly defined than the impersistent units which occur in the outlier. It is suggested that the eruptions were characterised by low, or suppressed, eruption columns.

The restricted development of reworked tuffs, and the absence of deep erosion and intercalated sedimentary rocks within the sequence suggest that the eruptions were fairly continuous. The geochemical similarity of the tuffs to the associated rhyolite intrusions suggest that the latter represent late stage intrusion into the vent. The rhyolites occur close to the axis of the syncline and although this structure has been accentuated by later tectonism it is interpreted as being, in part, primary, with the local discordance at the base of the tuffs being volcano-tectonic; the tuffs ponded in a downsag about the vent area.

The character of the underlying sedimentary rocks and the marine reworking of the lower part of the tuff sequence, indicates that the centre developed in a shallow-marine environment.

Basal Welded Tuff

On the south-west of the outcrop, the base of the formation is represented by a welded tuff which is well exposed about Moel y Dyniewyd [SH 613 478], Moel Ddu [SH 580 442] and between Cwm Tregalan and Moel Hebog. The tuff is typically white weathered and intensely jointed with, in places, well-developed columnar joints. At Moel y Dyniewyd, its basal zone, up to 8 m thick, is crowded with siliceous nodules. The overlying tuff, up to 150 m thick, is fine grained and silicified with a distinctive even, planar foliation. Between Beddgelert and Moel Ddu, in the southern end of the Beddgelert Fault Zone, the welded tuff overlies and encloses large slide blocks of the Pitts Head Tuff (Howells et al., 1986, 1991; Reedman et al., 1987).

In the syncline between the west side of Cwm Llan and Moel Hebog, the basal welded tuff forms a prominent feature, up to 30 m thick, on the south-east limb and thins to less than 10 m on the north-west limb. The main body of the tuff is finely recrystallised, well foliated and locally columnar-jointed, with siliceous nodules mainly concentrated at its base. In places [SH 586 519], the top is not clearly defined as the silicification extends into the overlying non-welded, ash-flow tuffs.

On Moel Hebog [SH 565 465], the welded tuff is generally less than 20 m thick, it wedges out to the south and is overlain by non-welded, lithic-rich tuff locally with a 1 m clast-supported breccia at its base. Welding to the upper contact suggests that the flow unit was eroded prior to deposition of the overlying breccia.

South of Cwm Tregalan, on the east side of Cwm Llan, the contiguous basal welded tuff passes laterally into impersistently welded tuff with extensive pods, up to 500 m across [SH 618 528], of silicified welded tuff with an even foliation. The relationship of these pods to the contiguous sheet is unclear although they are geochemically similar (Howells et al., 1991). Their occurrence is restricted to the vicinity of the Beddgelert Fault Zone, and the possible fissure vent, which suggests they are the result of localised hydrothermal activity.

Petrography

The basal welded tuffs comprise a very fine-grained aggregate of quartz, sericite and chlorite with a foliation defined by segregations of coarser quartz recrystallisation. Generally, the recrystallisation totally overprints the original fabric with the only remnant being isolated sericitised and chloritised feldspar phenocrysts. However, in rare instances a shardic fabric has been determined (MH 1275) which shows that the foliation is the result of welding.

Environmental interpretation

The basal welded tuff, in its distribution, lithology, geochemistry (Howells et al., 1991) and locally its clearly defined upper contact with the overlying tuffs suggests that it represents a primary, welded, ash-flow tuff and is not the basal welded zone of a much thicker deposit. Its distribution about the Beddgelert Fault Zone on the south-west side of Snowdon suggests that its vent lay within the zone. The locally well-developed nodular base and its even foliation suggests that the flow was emplaced in a subaerial environment. However, in Cwm Tregalan, its relationship with the underlying pillowed basalts indicates that here it extended into a marine environment.

Sedimentary and volcanic megabreccias

To the east of the Beddgelert Fault Zone, between Hafod Owen [SH 627 490] and Hafod Tan y Graig [SH 633 503], a thick zone of volcanic and sedimentary megabreccias (Llyn Dinas Breccias of Beavon, 1963) lie at the base of the formation (Howells et al., 1986). At the southern end, the breccias abut against the basal welded tuff on the north-east side of Mynydd Llyndy and in the north they abut against the non-welded tuffs (see below). The sedimentary breccias lie at the northern end of the outcrop and contain rafts, up to 50 m long, of local sedimentary rocks (Cwm Eigiau Formation) in a mixed sandstone and mudstone matrix. To the south, they interdigitate with, and are partly overlain by the volcanic megabreccias containing large rafts and blocks of acid tuff and sedimentary rocks in a tuffaceous matrix. Because of the size of the blocks and the limited exposure the exact relationships of the breccias are difficult to distinguish.

The breccias probably developed by collapse associated with movement of the Beddgelert Fault Zone, caused by uplift and instability prior to the onset of volcanism (Beavon, 1963; Howells et al., 1986), and again during the eruption of the basal welded tuff. The movements along the fault zone and the initiation of volcanism in the vicinity were a continuation of the movements which had earlier caused the local Beddgelert pericline and the disruption of Nant Ffrancon Group strata on its flanks.

Non-welded tuffs

Within the district, the main expression of the Lower Rhyolitic Tuff Formation is of non-welded, acidic ash-flow tuffs, up to 500 m thick. Nowhere is this more graphically displayed than on the north face of Lliwedd (Plate 11.1), where the cliff face consists entirely of massive, acidic, non-welded ash-flow tuff although the base of the sequence is not exposed. To the north, in the Pass of Llanberis, the base is exposed to the east of Dinas Mot [SH 629 563] where a distinctive, coarse, pyroclastic breccia, up to 40 m thick, lies conformably on the bedded acidic air-fall dust tuffs at the top of the Cwm Eigiau Formation. The breccia is clast-supported at its base with blocks, up to 0.70 m across, of basalt, acid tuff, rhyolite, and rare sandstone and siltstone, in a tuffaceous matrix. On Dinas Mot, the breccia is intruded by a thick dolerite sill, but, in the vicinity, it can be seen that the blocks in the breccia decrease in frequency upwards, to become matrix-supported, and that the breccia grades into ash-flow tuff.

Just to the north-west of Dinas Mot, in the Bangor district, the basal breccia sharply downcuts the underlying strata, down to the level of the Pitts Head Tuff. This relationship is interpreted to reflect the infilling of a series of fault scarps which developed close to the northern edge of the caldera along the Pass of Llanberis (Howells et al., 1986, 1990). The breccias are interpreted as co-ignimbrite lag breccias (compare with Druitt and Sparks, 1982) emplaced close to the vent.

South of the Pass of Llanberis, on the south-east limb of the Snowdon Syncline, the overlying non-welded, ash-flow tuffs, are up to 500 m thick; they are massive, generally well cleaved and local indications of a bedding foliation are weak and impersistent. In this area they are intruded by high level, flow-banded, rhyolite intrusions which are graphically displayed in the face of Gym Las [SH 614 558]. To the east of Crib Goch, small, wedge-like rhyolite intrusions within the tuffs [SH 633 556] are commonly aligned and associated with thick quartz veins. The massive, non-welded tuffs maintain their relatively uniform character and thickness through most of the outcrop about Snowdon. However to the south-west, about Moel Hebog there is a marked diminution in thickness, to 20 m, and reworked tuffs (see below) are the dominant component.

To the south-east of Snowdon, the Lower Rhyolitic Tuff Formation crops out around the synclines on the south flank of Moel Siabod and at Dolwyddelan (Figure 21). At Dolwyddelan it is represented by acidic tuffs (Unit B, Howells et al., 1973) which thin from 150 m in the west to 33 m in the east. The tuffs are massive, well bedded, generally 1 to 2 m thick, but up to 10 m thick, and the clast and crystal content decreases upwards. Clasts are mainly of mudstone and some have extremely irregular outlines suggesting that they were unlithified when incorporated into the tuffs. Planar-bedding laminations and crystal concentrations are locally well developed.

On Moel Siabod [SH 7168 5375], the formation is represented by similar bedded tuffs (Upper Unit, Howells et al., 1973), up to 40 m thick, but there is a distinctive epiclastic component in the tuffs towards the top of the sequence. The base is locally erosional with patchy development of clast-rich tuffs. Towards the top of the sequence, the beds are less than 0.20 m thick with load casts at their bases and delayed grading at the top. Locally, these beds are deeply eroded and reworked (see below).

Petrography

The non-welded tuffs in the thick sequence about Snowdon, comprise a variable admixture of shards and feldspar crystals in a matrix of sericite and chlorite (MH 1244). Locally the shards are tightly packed; elsewhere the matrix component is greater, and here the tuffs are commonly well cleaved and the shards are tectonically distorted. The shards are typically less than 0.2 mm occurring mainly as rods and spikes although multicuspate shards and complete bubble forms are not uncommon (MH 1243). In places, a distinct bimodal shard population can be distinguished with larger, cuspate tabular fragments set in a matrix of finer shards. The devitrified and recrystallised shards comprise a fine quartz mosaic and contrast sharply with the sericite and chlorite after vitric dust in the matrix.

Phenocrysts are of albite-oligoclase feldspar and quartz, although locally both are of rare occurrence. Above the basal zone, clasts are almost entirely of small, up to 4 mm, tubular pumice with ragged terminations.

The bedded tuffs at Dolwyddelan and Moel Siabod show much variation in the proportions of shards, crystals, lithic clasts and matrix (Howells et al., 1973). The characters of these components are similar to those in the thicker, unbedded sequences about Snowdon, but it is apparent that the variation is due to the processes of reworking and transportation. Commonly the shards are replaced by a fine micaceous aggregate, especially at the bases and where there is a distinct epiclastic muddy fraction in the matrix.

Environmental interpretation

Within the district the non-welded tuffs are mainly the infill of the asymmetric, downsag caldera. The thickest sequence in the central part of the Snowdon massif represents the deepest collapse which was caused, to a large extent, by the explosive eruption of these tuffs. The outflow tuffs are only represented in the lower parts of the sequence at the eastern end of the Dolwyddelan Syncline.

Reworked tuffs

Within the district, reworked tuffs overlie primary ash-flow tuffs both within and outside the Lower Rhyolitic Tuff Formation caldera (Figure 21). The bedforms of these tuffs and, locally, their contained faunas indicate reworking in a marine environment. The thickest sequence of primary, intracaldera tuffs, about the Snowdon massif, is overlain by only 10–20 m of shallow-marine, reworked tuffs. However, at the south-west margin of the caldera the reworked tuffs extend down locally to the top of the basal welded tuff.

South of the Pass of Llanberis, the reworked top of the Lower Rhyolitic Tuff Formation is well exposed in Cwm Glas [SH 623 555]. The top of the unbedded tuffs grades into massive bedded tuffs with ill-defined, internal laminations. The tuffs contain little evidence of epiclastic debris, they are pale green coloured, and weathered surfaces are distinctive with many cavities after carbonate nodules, less than 50 mm across. Up sequence, over 10 to 20 m, bedding becomes progressively better developed, and more flaggy. The tuffs grade into volcaniclastic sandstones and lenses and impersistent layers of well-rounded quartz and rhyolite pebbles are common. Locally, these reworked, cross-laminated tuffs surround angular clasts of rhyolite, up to 3 m across.

A similar intracaldera section on the north side of Llyn Gwynant [SH 643 522] has been examined by Fritz et al. (1990) (Figure 21). This 38 m section comprises volcaniclastic sandstones and siltstones with interbedded tuffs. Sedimentary structures in the sandstones include dune, trough cross- stratification, wave oscillation ripples, plane beds and hummocky cross-stratification. Convolute lamination, flame structures and small clastic dykes indicate soft-sediment deformation caused by rapid deposition. The lithologies and the bed forms reflect deposition in fluctuating water depths (Fritz et al., 1990), with a general upward change from low-energy deposits at the base to higher-energy deposits at the top of the sequence.

At Moel Hebog, close to the south-west edge of the caldera, about 90 m of reworked tuffs overlie the eroded top of the basal welded tuff (Fritz et al., 1990) (Figure 21). Near the base the tuffs include large rafts of the welded outflow tuffs of the Pitts Head Tuff Formation which were displaced from the caldera rim. The reworked sequence comprises volcaniclastic siltstones and sandstones with pebble and cobble conglomerates; fine-grained tuff beds, 0.20–0.50 m thick, possibly represent small ash-flows or remobilised, tuff turbidites. The bedforms are variably horizontal laminated, hummocky cross-stratified, herring bone cross-stratified and small-scale, trough cross-stratified. An exposed bedding plane, about 60 m above the base (Figure 21), displays straight-crested, symmetrical wave oscillation ripples. Contorted soft sediment structures are common.

Within the district, the reworked tuffs are the dominant element of the formation outside the defined caldera, and are exposed in the synclinal outliers at Dolwyddelan and on the south flank of Moel Siabod. In both areas, the primary ash-flow tuffs near the base of the sequence contain much epiclastic debris, and pass upwards into beds in which sedimentary features, rather than volcanic features, dominate (Howells et al., 1973). At Moel Siabod a 20 m section [SH 718 537], described by Fritz at al. (1990) comprises coarse-grained, volcaniclastic sandstones interbedded with fine-grained, silicified tuff; the bases of the sandstone beds locally downcut into the underlying tuffs. The sandstones are characterised by plane-parallel bedding, low-angle hummocky cross-stratification and small-scale, wave-ripple, cross-lamination.

Environmental interpretation

The primary ash-flow tuffs of the Lower Rhyolitic Tuff Formation were reworked in a shallow-marine environment. The sedimentary structures in the reworked facies indicate depositional environments from below storm wave base to high-energy conditions above fair-weather wave base. Within the caldera, in Cwm Glas, the large rhyolite blocks within the bedded reworked tuffs have been interpreted (Kokelaar, 1992) to represent collapsed stacks of contemporaneous extrusions of rhyolite. The extrusions were probably associated with resurgence which caused temporary emergence in their vicinity and the establishment of small beaches.

Outside the caldera, pyroclastic debris was shed from the caldera rim and large pyroclastic aprons, which extended into deeper water, to below storm wave base, were developed. Elsewhere shallow-water reworking, in foreshore to shoreface water depth, was the dominant process (Fritz et al., 1990).

Bedded Pyroclastic Formation

The Bedded Pyroclastic Formation forms extensive outcrops about the Snowdon massif and in the cores of the Dolwyddelan and Moel Hebog synclines (Willams, 1927; Williams and Bulman, 1931; Shackleton 1959; Howells et al., 1991; Kokelaar, 1992). The formation represents a period of basaltic activity and is a distinctive element of the geology of the district.

The formation is dominated by tuffaceous sedimentary rocks of basaltic derivation so its name is somewhat of a misnomer. However, high-level intrusive and extrusive basalts, hyaloclastites and basic tuffites are locally abundant. The sequence developed from a number of small eruptive centres, of which only a few are either preserved or recognised. Locally, thin beds of acidic tuff occur which probably represent small expressions of explosive volcanism associated with synchronous effusion of rhyolite related to the Lower Rhyolitic Tuff Formation caldera.

The formation, exposed between Snowdon summit and Cwm Glas [SH 618 556], has been the subject of detailed study (Kokelaar, 1992) (Figure 22). In this area, the contact with the underlying, reworked top of the Lower Rhyolitic Tuff Formation forms a very clear feature. In Cwm Glas, rhyolitic pebble and cobble conglomerates, interpreted as a beach deposit near the base of the sequence, are overlain by a rhyolite lava, up to 6m thick. Locally, the conglomerates unconformably overlie a syncline in the underlying tuffs which was probably caused by uplift associated with the high-level emplacement of the rhyolite magma.

In this area on the north side of Snowdon, Kokelaar (1992) has distinguished nine stages in the development of the formation. The basal conglomerates in Cwm Glas represent Stage 3 deposits which are related to uplift and erosion. The earliest, Stage 1, deposits are preserved in 95 m of strata exposed on the north-east face of Snowdon [SH 610 546], and comprise mainly basaltic tuffs, pillow lavas and hyaloclastites, with few sandstone turbidites, reflecting submarine eruption and emplacement. The two agglomerate-filled vents, Britannia and Glaslyn, within this face are interpreted as remnants of subaerial structures, Stage 2, which were truncated by a later, Stage 5, unconformity (Figure 22).

The main sedimentary development of the formation occurred through Stages 4, 5 and 6. There was complex interplay of volcanism, uplift and subsidence with the development of small basaltic island volcanoes, characterised both by Strombolian and quiet basaltic effusive activity, and their subsequent reworking with littoral sandstones and conglomerates and a complex of turbidite fan deposits. This sequence is dramatically exposed about Cwm Glas, with a wide variety of sedimentary structures including planar and trough cross-lamination, hummocky cross-stratification, current and wave-rippled surfaces. The rocks are dominated by basaltic- derived material, locally enriched with brachiopod and crinoid debris and in places intense carbonate alteration is interpreted to reflect sea-floor weathering.

The late stages of the formation's development are characterised by renewed magmatic activity. Explosive and effusive activity (Stage 6) is preserved in the 85 m of basaltic tuffs, hyaloclastites and lavas in the vicinity of Glaslyn Vent and high in the sequence on Crib y Ddysgl [SH 6175 5519]. This expression can be directly related to a basaltic sill complex within the underlying littoral deposits, Stage 5. Subsequently these deposits subsided and were overlain by turbiditic sandstones and siltstones (Stage 8). These sandstones contain an abundant derived shelly fauna, including brachiopods of a Dinorthis assemblage, indicating derivation from a shallow sublittoral environment, 0–10 m deep (compare with Pickerill and Brenchley, 1979).

On the south side of the Snowdon massif, the Bedded Pyroclastic Formation is extensively exposed to the east of Lliwedd, into the Gwynant valley. To the east of Lliwedd, the contact with the underlying Lower Rhyolitic Tuff Formation [SH 625 532] is gradational with the reworked acid tuffs gradually being overwhelmed by the influx of marine reworked basic tuffites, tuffaceous sandstones and siltstones, up to 200 m thick. These basic volcaniclastic beds are characterised by thin flaggy bedding, cross-lamination, wave-rippled bedding planes and locally contain marine shelly faunas. A few thicker beds, up to 1 m, of poorly sorted admixtures of pyroclastic and epiclastic debris, suggest emplacement by debris flows. The source of this basaltic debris has not been determined but they indicate reworking of a basaltic centre; despite the well-exposed sequence only a few thin, altered basalt flows are preserved.

The bedded sequence is overlain by a thick accumulation of massive, locally pillowed basalt. Estimates of its thickness are difficult to make because of folding and the general dip-slope feature, but it is thought to exceed 150 m. The basalts are highly altered, locally distinctively vesicular with albitised feldspar phenocrysts up to 9 mm across. Individual flows are difficult to determine and there is little evidence of extensive reworking.

To the south-west along the Gwynant valley, the formation is exposed within the apical graben of the Lower Rhyolitic Tuff Formation caldera and in this direction bedding becomes progressively less well defined. Characteristic are massive, poorly sorted beds of fine- to coarse-grained pyroclastic and epiclastic debris with blocks of basalt, rhyolite and acid tuff. In the vicinity of Hafod y Porth [SH 610 497] it is evident that the deposition of these beds was profoundly affected by penecontemporaeous fault activity and the emplacement of rhyolite domes; the faults were subsequently mineralised. Basaltic eruptive centres have not been distinguished within this outcrop and it is suggested that the sequence was emplaced by debris flows transported into the asymmetric graben (Howells et al., 1991).

In the Dolwyddelan Syncline the formation varies from approximately 180 m thick at the western end to less than 50 m, in the east. The sequence comprises basaltic tuffites, volcaniclastic sandstones and siltstones which are dominantly deposits of turbidity currents and debris flows. There is a general fining upwards and towards the top of the sequence black mudstone dominates. Shelly fossils, particularly in the lower part of the sequence, include dinorthid and dalmanellid brachiopods and few trilobites, most notably Estoniops alifrons.

In the core of the Moel Hebog Syncline, the lower part of the formation is mainly of extrusive basalts, variably pillowed, up to 120 m thick, with pillow breccias and hyaloclastites. The basalts are massive, columnar jointed and pillowed; they pass laterally and vertically into pillow breccias, hyaloclastites and well-bedded, fine- to medium-grained tuffites. The basalts are succeeded by up to 110 m of bedded basic tuffites and hyaloclastites with two relatively massive basalt flows, 45 and 10 m thick. The basaltic tuffites are flaggy bedded, 0.1–0.10 m thick, with plane-parallel- and cross-lamination, and in a few places more massive beds, up to 0.40 m thick occur. General distribution of facies suggest (Howells et al., 1991) that a basaltic centre lay in the vicinity of Moel yr Ogof [SH 558 478] and this is supported by a concentration of thin discordant basalt sills in the vicinity.

Environmental interpretation

The basaltic volcanism of the Bedded Pyroclastic Formation was profoundly influenced, both in the siting and distribution, by the tectonism within the Snowdon graben. During the accumulation of the sequence between Cwm Glas and the main face of Snowdon, Kokelaar (1992) has calculated that total uplift exceeded 336 m and subsidence was greater than 500 m; marked subsidence preceded episodes of copious magma movement to the surface. Small basaltic volcanoes, generally the result of a single eruptive cycle of short duration, were the main source of the volcaniclastic debris and, at times, the rate of supply was catastrophic. The pillow basalts and hyaloclastites probably lie close to their source vents, but the complex sequence of volcanogenic sedimentary rocks indicates that the volcanoes were comprehensively reworked into the marine environment.

Upper Rhyolitic Tuff Formation

The Upper Rhyolitic Tuff Formation forms restricted outcrops, generally small outliers in the cores of synclines, on Moel Hebog, on the high ridges of Snowdon, and about Dolwyddelan. It comprises acidic ash-flow tuffs, bedded tuffs and tuffites with a few tuffaceous siltstones, and for most of its outcrop it lies with slight angular unconformity on the Bedded Pyroclastic Formation.

On the north ridge of Snowdon the formation is spectacularly exposed about the high ridges and cliffs of Crib y Ddysgl and Clogwyn y Person. A basal basaltic and rhyolitic pebbly sandstone locally overlies a slightly undulose angular unconformity and indicates limited emergence and littoral erosion. The overlying acidic ash-flow tuff, up to 35 m thick, forms the prominent scarp of Clogwyn y Person and, in places, oversteps the basal sandstone to lie directly on basalt and hyaloclastite (Stage 7, Bedded Pyroclastic Formation). The blue-grey tuff has bleached weathered surfaces which display impersistent bands of small lithic clasts and carbonate nodules. It comprises a microcrystalline aggregate of quartz, feldspar, sericite and chlorite with dispersed shards, few fragmental quartz and albite crystals and lithic clasts. The proportions of these constituents varies markedly. As the tuffs overlie the basaltic volcaniclastic rocks of the Bedded Pyroclastic Formation, the locally high proportion of sericite-chlorite matrix has been interpreted (Howells et al., 1991) to reflect devitrified dust, rather than incorporated silt or mud. The tuff grades up into the overlying fine-grained, bedded tuffs with plane-parallel and low-angle cross-laminations. On Crib y Ddysgl ridge, these tuffs include thin impersistent intercalations of basaltic tuffs.

At the north end of Clogwyn y Person [SH 6167 5555], a rhyolite dyke, which intrudes basic tuffs of the Bedded Pyroclastic Formation, can be traced upwards into a dome overlying the main ash-flow tuff scarp. The contacts of the dyke within the basic tuffs indicate that the latter were unlithified and water-saturated at the time of intrusion. The rhyolite is sparsely porphyritic, flow-folded and autobrecciated with well-developed columnar joints.

A small outlier of bedded acidic tuffs at the Snowdon summit is considered to be part of the formation as is a sequence, up to 25 m thick, in the small synclinal outliers on the ridge east of Lliwedd [SH 635 585] and [SH 640 533]. Here, thin flaggy-bedded acid tuffs and tuffites are intercalated with tuffaceous siltstones, sandstones and conglomerates. The last contains pebbles and cobbles of acid tuff and rhyolite in a siltstone matrix and locally the clasts are strongly deformed. In the largest outliers, the bedded tuffs are overlain by high-level, rhyolite intrusions which have hydrothermally altered the tuffs in the vicinity.

To the east of the Snowdon massif, the Upper Rhyolitic Tuff Formation is exposed in the core of the east-west-trending Dolwyddelan Syncline. The sequence, up to 75 m thick, comprises poorly bedded and massive acidic tuffs, tuffites and tuffaceous mudstones. Locally, beds, up to 3 m thick, of primary acid tuff can be distinguished but generally they are laterally impersistent and the sequence mainly consists of heterogeneous admixtures of pyroclastic and epiclastic debris. The pyroclastic component includes devitrified shards, blocky cusp-edged bubble walls, altered albite-oligoclase crystals and tubular pumice fragments. The epiclastic fraction is dominantly argillaceous, mud and silt grade, and this fraction becomes more dominant towards the eastern closure of the syncline. Bedding is generally poorly defined and the massive units are interpreted as high-density, turbidity flow deposits generated by remobilisation of previously deposited pyroclastic debris and unlithified mud. The prevailing mud facies in this general vicinity reflects a continuance of the low-energy environment established here immediately prior to the emplacement of the Snowdon Volcanic Group (Howells et al., 1991).

To the north of Moel Hebog, in the core of the syncline, the tuffs of the underlying Bedded Pyroclastic Formation grade upwards, with increasing fractions of acidic debris into the Upper Rhyolitic Tuff Formation. The acidic tuffs are typically compact and silicified, thick flaggy and massive bedded with locally well-developed cross-lamination.

Cadnant Shales

The cover to the Snowdon Volcanic Group is exposed only in the core of the Dolwyddelan Syncline where black graptolitic mudstones (Black Slates of Dolwyddelan, Williams and Bulman, 1931; Howells et al., 1978, 1991) up to 70 m thick, overlie the Upper Rhyolitic Tuff Formation. They are equivalent to the Llanrhychwyn Slates and Cadnant Shales (Howells et al., 1978, 1981) in north-east Snowdonia. The contained graptolites indicate a Dicranograptus clingani Biozone age. Pyrite in the mudstones is mainly developed along bedding and cleavage planes and the mudstones are unusual in that they contain no chlorite (Merriman and Roberts, 1985). Black mud deposition was widespread through North Wales at this time (Cave, 1965) and here, in spite of the close association with thick volcanic sequences, there is no indication of shallow-marine reworking. The evidence suggests extensive postvolcanic subsidence.

Biostratigraphy of the Snowdon Volcanic Group

The Snowdon Volcanic Group lies within the Longvillian Stage (extended), equivalent to the Longvillian plus Woolstonian stages of Hurst (1979). The older limit is set by the highest faunas in the Cwm Eigiau Formation: where the base of the group is marked by the Pitts Head

Tuff Formation it is correlated with a level near the Soudleyan–Longvillian boundary. The Yr Arddu Tuffs overlie strata with lower Longvillian fossils (Howells et al., 1987) and a similar fauna, but including some Soudleyan elements (Broeggerolithus broeggeri), has been collected from rafts in the sedimentary megabreccias south of Llyn Dinas. Where the Pitts Head and Yr Arddu tuffs are absent, the Lower Rhyolitic Tuff Formation overlies beds with lower Longvillian faunas. All the diagnostic faunas from within the Snowdon Volcanic Group are of Woolstonian age (equivalent to post-Cyrnerig Limestone of the Bala succession; Whittington, 1968).

In the Lower Rhyolitic Tuff Formation, faunas only occur in the reworked facies. At Moel Hebog two faunal associations are defined. The lower fauna, consisting of monospecific assemblages of a species of Dinorthis (Howells et al., 1991, pl. 4, figs. 8–10) is not very diagnostic of age, but it suggests a shallow-water, high-energy environment. At higher levels more diverse faunas, of low-energy, Nicolella community type (Pickerill and Brenchley, 1979), were collected [SH 5540 4610]. These are similar to faunas from the Bedded Pyroclastic Formation and the presence of Estoniops alifrons [SH 5656 4684] indicates a Woolstonian age. These observations suggest either very localised variations in environment or a relatively rapid deepening of the sea during the main emplacement of the Lower Rhyolitic Tuff Formation. In the Dolwyddelan Syncline, Wright (1979) collected a Nicolella-type fauna with E. alifrons at a level near the base of the Upper Rhyolitic Tuff Formation [SH 7194 5195], and a less diagnostic Nicolella-type community from the outlier on the south flank of Moel Siabod [SH 7135 5345].

The Bedded Pyroclastic Formation has yielded faunas at several localities. The diagnostic faunas are all of Woolstonian age and they include the same faunal associations as are present in the Lower Rhyolitic Tuff Formation. Thus the Bedded Pyroclastic Formation around Snowdon shows the same transition from a Dinorthis-rich, high-energy environment fauna to a varied, low-energy environment fauna as seen in the reworked Lower Rhyolitic Tuff Formation on Moel Hebog. The monospecific Dinorthis assemblage is seen at the summit of Snowdon [SH 6098 5446] and near Clogwyn Station [SH 6070 5487] on the Snowdon Mountain Railway. The diverse Nicolella community assemblages, as described by Dean (1965) in Cwm Idwal to the north of the district, are developed at the mine workings in Cwm Meirch [SH 6343 5301], north-west of Craig yr Hendre near Dolwyddelan [SH 7010 5119] and south of Crib y Ddysgl [SH 6134 5489]. At the first two localities Estoniops alifrons and other post-Cymerig Limestone species are present. E. alifrons has also been found south-east of Llyn Glas [SH 6191 5558].

The Black Slates of Dolwyddelan mark an abrupt change into a distinct graptolitic facies. The faunas from Ty'n y Ddol [SH 700 510], Pen y Rhiw [SH 712 518] and Chwarel Ddu [SH 721 521] include Dicranograptus clingani (rare), Diplograptus compactus?, Orthograptus calcaratus basilicas, 0. pauperatus and Pseudoclimacograptus scharenbergi and indicate the lower part of the D. clingani Biozone. Shelly faunas are absent and direct correlation with the stages of the Caradoc Series is not possible.

Intrusions

Intrusive igneous rocks form an important component of the geology of the Snowdon district. They occur both in Cambrian and Ordovician strata. Ramsay (1881) recognised 'feldspathic traps' and 'greenstones' and these were later found to include basic, intermediate and acidic compositions (Andrew, 1910; Wells, 1925; Williams, 1927; Rast, 1969; Bromley, 1969). More recently, Allen and Jackson (1985) have distinguished a suite of dolerite, diorite and a few dacite intrusions in the Harlech district and these encroach into the southern edge of the Snowdon district. On the eastern side of the Harlech Dome, Allen et al. (1976) distinguished a pre-Arenig intrusive phase of subalkaline, basic and intermediate composition from a later post-Arenig dolerite phase.

Within the district, intrusions of intermediate composition are mainly restricted, in the Ordovician outcrop, to the area of the Manods and, to the Cambrian outcrop in the south. Within the main Ordovician outcrop, to the north of the Vale of Ffestiniog, the intrusions are predominantly dolerites and rhyolites, and these are both intimately associated with the evolution of the Snowdon Volcanic Group (Caradoc) (Howells et al., 1991). The district also includes two of the major granitic plutons, Tan y Grisiau and Mynydd Mawr, in north-west Wales.

Dolerite and basalt intrusions

The dolerite and basalt intrusions in north-west Wales are largely restricted to the Ordovician outcrop and do not occur in strata younger than Caradoc in age (Campbell et al., 1988; Howells et al., 1991). However, north of Llyn Trawsfynydd, on the south side of the Vale of Ffestiniog, thin dykes, and possibly a few thin sills, intrude the Upper Cambrian sequence down to below the Clogau Formation. Dolerite sills, locally up to 200m thick, and extensive, are common within the Nant Ffrancon Group between the south side of Dolwyddelan, along the Croesor valley to Cwm Pennant. The most impressive outcrops are probably those of the three sills to the north of Tremadog village (Plate 12.1). In the vicinity of basaltic eruptive centres, as on Moel Hebog and Snowdon, intrusions of basalt grade down into dolerite at deeper levels.

All the intrusions are to some extent affected by cleavage and the broadly arcuate patterns reflect the regional structure. The distribution of intrusions throughout central and northern Snowdonia and their close association with the Caradoc sequence indicate a direct relationship with the volcanic activity (Campbell et al., 1988; Howells et al., 1991). The dolerites are typically blue-grey-green with a brown weathered surface. Columnar joints are commonly well developed. Away from thin chilled margins, they coarsen to a uniform, coarse to medium grade, although in places they are distinctively glomeroporphyritic. Well-defined internal layering and gabbroic textures are rarely present. Zones of alteration adjacent to the intrusions are generally less than 2 m, although the sills in the Tremadog area have unusually wide aureoles; this feature has been ascribed to intrusion into wet unlithified sediment (Smith, 1988). Typically the dolerites are medium grained with a poikilophitic texture of plagioclase and olivine (pseudomorphs) enclosed in large plates of clinopyroxene, up to 20 mm in diameter. Ilmenite, apatite and rare kaersutite are also present. Variable alteration has resulted in the development of secondary, generally hydrous minerals. Locally, as in the dolerites at Moel y Gest [SH 553 388] and Pant Ifan [SH 570 420], leucocratic veins are common. The veins, up to 0.20 m thick, have irregular diffuse margins and show no preferred structural alignment (Smith, 1988). The veins have a granophyric texture and comprise orthoclase, microperthite, albite and quartz with little intersertal plates of augite with actinolitic alteration.

Multiple intrusions

In the area of the Croesor valley, sills, from 2 to about 100 m thick, comprising an acidic core and basic margins, are common. The relationship between the two components is variable, in some instances the basic component occurs only on one edge of the intrusion. The contrasting components represent two phases of intrusion and are termed multiple (Howells et al., 1991) rather than composite (Beavon, 1963). The basic component varies from fine-grained basalt to coarse-grained dolerite and the younger core consists either of feldspar porphyry or rhyolite. Only locally is the acidic component chilled against the basic. In the vicinity several feldspar porphyry and microgranite sills occur with no basic component.

The feldspar porphyries comprise subhedral albite-oligoclase and alkali feldspar phenocrysts, up to 3.8 mm, and fewer, smaller, rounded, quartz phencrysts in a groundmass of variably recrystallised, fine-grained quartz-feldspar with sericite and chlorite flakes and segregations. Spherulitic, granophyric and platy feldspathic recrystallisation textures are commonly well developed. Accessories include apatite, epidote, carbonate, iron oxide, sphene, biotite, and zircon. Clots of basalt with diffuse margins and clasts of dolerite are common, particularly near the margins. The typical feldspar porphyries grade into microgranite which is more common in the thicker intrusions.

Intrusions of intermediate composition

Intrusions of generally intermediate composition are mainly restricted to the area of the Manods and the Cambrian sequence south of Cwm Cynfal. However the Llyn Teyrn intrusion within the Snowdon massif has recently been distinguished (Howells et al., 1991) to be of andesitic, icelandite, composition. The Manod intrusion is a stock-like body of quartz latite composition (Bromley, 1963, 1965). The smaller bodies at Craig y Garreg lwyd [SH 730 427] and Carreg y Foel Gron [SH 727 435] are petrographically similar to the main Manod stock. Much of the Manod intrusion is entirely autobrecciated with angular to subrounded blocks, up to 1 m, in a finely recrystallised quartzofeldspathic matrix. In places, close to the margin, blocks of hornfelsed mudstone and tuff are of common occurrence. The core of the intrusion and the breccia blocks contain isolated phenocrysts, and clusters, of plagioclase feldspar of oligoclase-andesine composition although they are generally much altered and replaced by aggregates of sericite, chlorite and calcite. The groundmass is a fine-grained aggregate of sodic and potassic feldspar and quartz. In places, spherulites are well developed and indicate the originally vitric character of the groundmass. Patches of coarser facies near the western margin [SH 7086 4507] have diffuse margins and have been interpreted as of pegmatitic origin (Bromley, 1965).

A concentration of sill-like bodies, some thick and of limited lateral extent, intrude the Maentwrog and Ffestiniog Flags formations to the south of Cwm Cynfal in the south-east of the district. These are part of the suite of microtonalite intrusions in the Harlech district (Allen and Jackson, 1985). The most impressive outcrop forms the large crag in Cwm Cynfal [SH 737 412] and the northern contact of this intrusion partly controlled the development of the waterfall, Rhaeadr y Cwm. The grey-green rock contains rare altered plagioclase phenocrysts and amphibole phenocrysts, pseudomorphed by chlorite, epidote and calcite. The groundmass is a fine-grained, equigranular mosaic of feldspar, quartz, chlorite and epidote (Allen and Jackson, 1985, p.47, and fig. 14).

Major acidic plutons

Two major acidic plutons, the Tan y Grisiau granite and Mynydd Mawr microgranite, crop out within the district. The Tan y Grisiau granite is well exposed in quarries and road sections to the south and south-west of Tan y Grisiau but overall does not form a positive feature. In contrast, the Mynydd Mawr microgranite forms the upstanding, steep-sided, boss-like intrusion to the northwest of Rhyd ddu.

Tan Y Grisiau Granite

The Tan y Grisiau granite crops out over an area of less than 4 km2 and it has been described in detail by Bromley (1963). The extent of its hornfels zone (Bromley, 1965, 1969) and associated gravity (Institute of Geological Sciences, 1978) and magnetic anomalies (Geological Survey of Great Britain, 1965) indicate a much larger body at depth. It has been interpreted (Cornwell et al., 1980) to form a steep-sided, subvertical body, with a north-north-west-dipping roof, which extends some 10 km to the north-east and 5 km to the south-west of its outcrop.

The granite is homogeneous, fine-grained, an equigranular mosaic of subhedral quartz, albite-oligoclase, potassic feldspar and perthite, with intersertal biotite and chlorite (KB 362), (KB 365). Granophyric intergrowth of quartz and alkali feldspar is common and sericitic alteration of the feldspars is locally intense. Accessories include iron oxide, apatite, zircon, anatase and carbonate. Bromley (1969) distinguished a pegmatitic facies marginal to the main intrusion and thin granophyric sills with brecciated and tourmalinised outer zones which cross-cut adjacent strata. Recently, Evans (1990) has determined an Rb/Sr isotopic age of 384 ± 10 Ma for the granite but has argued that this represents a reset age.

The intrusion of the granite produced an extensive thermal aureole affecting both Upper Cambrian and Lower Ordovician strata between Llanfrothen and Ffestiniog. The albite-epidote-chlorite-sericite-quartz assemblage indicate albite-epidote hornfels facies (Miyashiro, 1973). However, the presence of relict andalusite and cordierite, and minor amounts of biotite and hornblende indicate an original hornblende facies which subsequently retrogressed, possibly as the result of streaming volatiles and regional deformation (Bromley, 1963). The main zone of spotting occurs between 15 and 500 m of the contact and affects all lithologies. On the inner edge of this zone the strata are more intensely recrystallised and, on the outer edge, the strata show slight indications of baking over about 100 m (Bromley, 1963).

Mynydd Mawr Microgranite

At outcrop the Mynydd Mawr microgranite covers an area of about 3 km2 and intrudes Upper Cambrian to Lower Ordovician sedimentary rocks. Its aureole is generally well defined with indurated hornfelsed rocks adjacent to contacts and an outer zone of spotting up to 0.5 km wide. The adjacent sedimentary rocks are anomalously steeply dipping, locally overturned and broadly concordant with the contact, but bedding becomes consistent with the regional dip away from the contact.

The microgranite comprises a fine-grained, quartz-feldspar aggregate with tabular phenocrysts, up to 2 mm, of alkali feldspar with distinctive perthitic intergrowths (TL 105), (TL 107). Rare quartz phenocrysts typically occur as rounded corroded crystals with myrmekitic overgrowths. Riebeckite is a distinctive component, occurring as small, subhedral prismatic crystals and acicular aggregates. Accessories include iron oxide, muscovite, chlorite and acmite (Nockolds, 1938).

Evans (1990) has determined a Rb/Sr isotopic age of 438 ± 4 Ma for the microgranite and is one of the few intrusions whose Rb/Sr systematics has not been reset during early Devonian metamorphism.

Rhyolite intrusions

Within the district the main episode of rhyolite emplacement was related to the development of the Lower Rhyolitic Tuff Formation caldera. However, an earlier, restricted pre-Caradoc episode is distinguished in the Moelwyn Volcanic formations on the north side of the Vale of Ffestiniog. Both episodes are characterised by high-level intrusions and extrusions, and the pattern they create about the Lower Rhyolitic Tuff Formation caldera is extremely complex (Campbell et al., 1987; Howells et al., 1991). The rhyolites are massive, locally feldsparphyric, columnar jointed, flow banded and autobrecciated. They occur as dykes, sills, small stocks and large domes.

The relatively deep-seated dykes and sills are generally massive with sharp contacts and narrow thermal alteration zones. On the south-west side of the Snowdon massif, some of the sills in the Lower Rhyolitic Tuff Formation substrate form the cores to multiple intrusions. However shallower intrusions, within or close to the base of the Snowdon Volcanic Group, form dome-like bodies with brecciated margins, locally intruded by flames of sedimentary rock, for example at the north edge of Clogwyn y Person [SH 6167 5555], indicating intrusion into water-saturated sediment (Kokelaar, 1982). The near-surface intrusions also caused updoming and slumping of unlithified sea-floor sediments and some broke surface, creating exogenous domes, as at Carnedd y Cribau [SH 675 535] or flows with autobreccia carapaces. In the shallow-marine environment, some high-level intrusive domes, such as that at Cerrig Cochion [SH 660 515], were exhumed and eroded (Howells et al., 1991). The eroded rhyolite debris was incorporated into parallel-laminated and trough cross-laminated rhyolitic sandstones across the bevelled tops of the domes (Howells et al., 1991, fig. 61).

Three main phases of rhyolite extrusion, and by implication intrusion, have been determined (Campbell et al., 1987) during the evolution of the Lower Rhyolitic Tuff Formation caldera. The first preceded the main phase of caldera-forming eruptions. The second occurred during resurgence and reworking of the primary ash-flow tuffs and emplacement of the overlying Bedded Pyroclastic Formation. The third phase occurred during the emplacement of the Upper Rhyolitic Tuff Formation.

The rhyolites are sparsely porphyritic with phenocrysts of albite, alkali feldspar and some quartz. The feldspar phenocrysts, less than 3.5 mm across, are subhedral with rounded terminations and are slightly to completely sericitised. The rare quartz phenocrysts, less than 1 mm, occur as intensely corroded relicts. The groundmass is devitrified and variably recrystallised as a platy quartz-feldspar mosaic crowded with inclusions, as spherulitic quartz-feldspar and as a snowflake texture (MH 1175), (KB 250) (Howells et al., 1991). Perlitic textures with the arcuate fractures accentuated by chlorite segregation are common. Locally, flow-oriented feldspar microlites in a devitrified originally glassy mesostasis can be distinguished (KB 197). Chlorite and iron oxide pseudo-morphs of ferromagnesian minerals are rare, although apatite is a common constituent.

The rhyolites form five, geochemically defined groups and each of these can be related to discrete phases of emplacement. The distribution of the groups was controlled by the volcano-tectonic structures, particularly the arcuate caldera fractures and the apical graben (Campbell et al., 1987; Howells et al., 1991). The first phase of rhyolites (Group A1) are aligned along the Yr Arddu Fracture and typical are those on Yr Arddu [SH 627 461] and Castell [SH 638 480]. The second phase represents the major episode of rhyolite emplacement and show a range of compositions (groups A2, B1, B2, B2a); these are distributed around the caldera and in the apical graben. Typical of this phase are the large intracaldera intrusions (B2) in the Pass of Llanberis [SH 615 566] and on Moel Lefn (A2) on the south-west side of the caldera. The third phase (B3) intrusions are distributed about the caldera margin and include the Crib Goch intrusion [SH 625 552] on the north ridge of Snowdon.

Chapter 4 Structure, metamorphism and geophysical investigations

The structure of North Wales records the development of Caledonian deformation caused by the gradual closure of the Iapetus Ocean (Soper and Hutton, 1984; Soper et al., 1987) between early Ordovician times and early Devonian times. The closure resulted in oblique plate collision which initiated a regime of sinistral, transpressional shear on the northern edge of the southern plate, Avalonia. The Lower Palaeozoic rocks which had accumulated in the Welsh Basin, sited close to the edge of Avalonia, were subjected to a south-easterly directed, bulk simple shear (Wilkinson, 1987, 1988). The effect of this shear was profoundly affected by a mosaic of basement blocks defined by north- and north-east-orientated lineaments which represented zones of repeated fault activity. Possibly, the Harlech and Snowdon blocks are the most clearly distinguished because of their respective accumulations of Cambrian and Ordovician strata, which indicate a northward migration of the main depositional centre.

It has been proposed (Wilkinson and Smith, 1988; Kokelaar, 1988; Howells et al., 1991) that the major north- to north-east-striking lineaments were propagated from these basement fractures and that they influenced patterns of sedimentation, volcanism, structure and metamorphism over a long period of time. The deformation was largely channelled into the cover rocks along these lineaments, as narrow zones of intense heterogeneous deformation, with the intervening areas showing less-intense, but consistent, deformation.

During late Tremadoc to early Arenig times, the thick basinal sequence of sedimentary rocks, which had accumulated in the area of the Harlech block during Cambrian times, began to be uplifted. Along the Vale of Ffestiniog, the reworked Tremadoc microfloras, the disrupted lower Ordovician strata and the mid-Ordovician unconformity indicate that by Caradoc times the area of the Harlech block had developed into a positive feature. Simultaneously, the Snowdon block, to the north, subsided to form the site of a major depositional centre during Ordovician times; the two major eruptive cycles reflect periods of crustal extension, with the development of the Snowdon graben, and corresponding magma movement.

The Caledonian deformation caused contrasting structural patterns in the Lower Palaeozoic sequence on the two blocks. The sequence on the older, uplifted Harlech block deformed along north- and north-north-east-striking fractures which were inherited from the earlier, end-Tremadoc, deformation. The predominance of north-striking cleavage suggests that the maximum compression was east-west. In contrast, the structural grain of the sequence on the Snowdon block is primarily north-east-south-west; the pre-existing volcano-tectonic structures controlled the formation of the folds which were com pressed about the more rigid Harlech block. In addition, the form of the folds were affected by the competency contrasts presented by the locally thick sequences of volcanic rocks and large intrusions within the sedimentary sequence. The arcuation of the cleavage and the fold axial plane traces, together with the variations in the cleavage transection across the district, are consistent with this model.

The structure of the Snowdon district (Figure 23) is dominated by large, upright to steeply inclined, southeast-verging folds with a mainly axial-planar cleavage. Both folds and cleavage were produced by end-Caledonian (late Silurian to early Devonian) deformation (Shackleton, 1959; George, 1963; Coward and Siddans, 1979). The folds and the cleavage (Figure 24) define a broad arcuate pattern which is considered to have been caused by the south-easterly directed compression of the Ordovician sequence, north of the Vale of Ffestiniog, against and over the Precambrian and Cambrian strata of the Harlech Dome. Some of the faults within the district were active repeatedly (Kokelaar, 1988).

In the south of the district, a gently dipping cleavage has been variously interpreted as a thrust-related fabric (Coward and Siddans, 1979), a tectonic fabric imposed on unlithified sediments (Lynas, 1970) or the result of large-scale, flexural slip during folding (Hawkins and Jones, 1981). More recently it has been recognised that this fabric is a variant of the main penetrative cleavage (Allen and Jackson, 1985; Campbell et al., 1987; Smith, 1988).

A tentative chronology of deformational events is shown in (Table 4). Helm et al. (1963) and Roberts (1967) invoked three phases of end-Caledonian deformation: D1, upright north-east-striking folds with axial planar cleavage; D2, rare, flat-lying folds and an S2 crenulation cleavage; and D3, steep north-west-striking folds with an S3 axial-planar crenulation cleavage. However, only two episodes of deformation are recognised in the Snowdon district: pre-Arenig faulting and end-Caledonian folding, faulting, cleavage development and low-grade metamorphism. Smith (1988) and Wilkinson (1988) accomplished detailed strain studies and structural analysis across the district.

Folds

The fold strike varies progressively across the district. In the south of the district, the broad, open, north-striking folds in Cambrian strata are the northern closures of the folds of the Harlech Dome. In the west and north-west, a series of tight to isoclinal and locally overturned, north-to north-east-striking folds occur in Cambrian strata and are similar to those in the Slate Belt, to the north-west (Wood, 1974). Between these two areas, in the central and eastern parts of the district, the outcrop of the Nant Ffrancon and the Snowdon Volcanic groups, defines a broad, generally north-east striking-syncline, termed the 'Snowdon Synclinorium' (Shackleton, 1959).

Within this structure, the major folds, with wavelengths of 2–3 km, are non-cylindrical or periclinal, and are essentially of two fold styles. The more competent sandstone and volcanic lithologies form concentric or flattened concentric folds (Class 1B folds of Ramsay and Huber, 1987), whereas similar-style folds (Class 1C), with pronounced thickening of in the fold hinges, occur in the less competent strata and in areas of high strain. There is a coincidence between the loci of major synclines and long-standing fractures which limited thick accumulations of sedimentary and volcanic rocks (Kokelaar, 1988). Examples include the Llwyd Mawr Syncline, the Snowdon Syncline, the Yr Arddu Syncline and the Moel Hebog Syncline (Howells et al., 1987, 1991). In general, mesoscale and outcrop-scale folds are rarely observed except within the upper formations of the Mawddach Group in Cwm Pennant and south of the Vale of Ffestiniog.

Folds of Upper Cambrian–Lower Ordovician strata

The north- to north-north-east-striking Cwm Pennant Anticline (Shackleton, 1959) folds sandstones and siltstones of the Marchlyn Formation and the Carnedd y Filiast Grit Member. In detail, the fold comprises several south-east-verging, en échelon periclines with short (0.51 km), sinuous traces. Steep, north-westerly dipping reverse faults cut out the eastern limb of some folds and others terminate against short faults with apparent sinistral, strike-slip displacements. Mesoscale folds (Class 1C to 3 of Ramsay, 1967) are common, for example at Cwm Dwyfor [SH 5406 5071] and Cwm Cipwrth [SH 5288 4797].

In Cwm y Ffynnon, Trum y Ddysgl and in the vicinity of the Mynydd Mawr microgranite a series of north-east-striking anticlines are defined by the outcrop of the Carnedd y Filiast Grit Member. These folds can be traced for nearly 5 km; they display intense limb attenuation and locally the southern limbs are overturned by as much as 40°.

To the west of Porthmadog, the Ynyscynhaiarn Anticline (Figure 23), in upper Cambrian to lower Ordovician strata, was identified by Fearnsides (1910), and Shackleton (1959) considered that it folded the 'Trerriadoc Thrust Zone'. The fold is closely comparable with the north-striking folds of the Harlech Dome; it is asymmetric with a steep, faulted western limb and a gently dipping eastern limb with attendant parasitic folds. Complex minor folds and faults within the core of the fold are particularly well displayed around Moel y Gadair [SH 5217 3907] and Graig Ddu [SH 525 376].

The northward extension of the north-plunging, Dolwen Pericline (Allen and Jackson, 1985) is poorly defined by a weak open fold, with limb dips of less than 20°, in Cambrian and lower Ordovician strata, to the east of the Tan y Grisiau granite (Figure 23). This fold, termed the Ffestiniog Anticline, plunges northwards and has little expression in the overlying Caradoc mudstones.

In both the Ynyscynhaiarn and Ffestiniog anticlines the regional S1 cleavage is non-axial planar. When this is considered together with the non-Caledonoid trend of the fold axial planes, and the tendency for the folds to have little affect in the overlying Ordovician sequence, it suggests that the folds are nucleated upon early fractures associated with east-west extension affecting the Harlech Dome.

Folds of Upper Ordovician strata

The outcrop of the Caradocian volcanic sequence defines several sinuous, south-east-verging, periclinal folds which form a broad arcuation from north-striking, as the Moel Hebog Syncline in the west of the district, through to approximately east-striking, as the Dolwyddelan Syncline in the east (Figure 25). These folds have axial traces more than 15 km long and wavelengths of 3–4 km. Along their length they vary from open periclinal structures, with rounded hinges, to tight and isoclinal, locally overturned, non-cylindrical folds.

The Moel Hebog Syncline can be distinguished for over 10 km, from west of Llyn Cwmystradllyn to the Snowdon summit, and is most clearly defined by the outcrop of the Pitts Head Tuff Formation. Along the length of the syncline several plunge culminations and depressions can be distinguished. Dips on the limbs of the fold vary from 45° in the south, where the fold termination plunges gently (12–40°) northwards, to 60° in the central outcrop; in the same distance, the fold axial plane changes from shallow to steeply inclined. In the central part of the outcrop, the north-west limb is locally overturned and the fold is near isoclinal with marked limb attenuation and hinge thickening.

In the Glaslyn valley, north of Tremadog, a weak, open syncline-anticline pair is distinguished by the outcrop of the Prenteg Sandstone Formation. Farther to the northeast, a series of tight, south-east-verging, north-east-plunging folds are similarly defined by the outcrop of the Moel Hebog Sandstone Formation. In the Nantmor valley they are closely associated with a complex network of steep normal faults and reverse faults. The main fold, the Glaslyn Valley Anticline, can be traced for 3 km and is complemented by an extension of the Yr Arddu Syncline immediately to the south-east. Stereogram data define weak girdle distributions and indicate that fold terminations plunge gently to the north-east and south-west.

The Yr Arddu Syncline is most clearly defined by the outcrop of the Yr Arddu Tuffs which are interpreted to have ponded above a north-north-east-trending fracture (Howells et al., 1987). The axial plane trace of the fold can be followed for some 2 km, from the folds of the Glaslyn valley, to the south of Yr Arddu, northwards into the Moel Meirch Syncline and further into the Dolwyddelan Syncline (Wilkinson, 1988). The latter is a major, locally overturned pericline (Williams and Bulman, 1931; Howells et al., 1978; Wilkinson, 1988) (Figure 25) which is defined by the outcrop of the Lower Rhyolitic Tuff Formation and contains, within its core, the youngest strata of the district, the Black Slates of Dolwyddelan. The axial plane trace, although sinuous, strikes east-north-east for about 11 km and, like the Moel Hebog Syncline, it displays marked variations along its length. In the east and west closures it is an open structure with dips of about 45° on the limbs and plunging 25–30° in the east, and 20° in the west. The fold tightens progressively and west of Dolwyddelan Castle, in the centre of the structure, it is an isocline with the northern limb overturned to 60°. To the east, about the Lledr valley, the syncline splays into a number of smaller folds.

On the steep slopes south of Y Lliwedd and Gallt y Wenallt on the Snowdon ridge, and above Cwm Gwynant, the complex outcrop patterns of the Lower Rhyolitic Tuff Formation and the Bedded Pyroclastic Formation define a number of minor folds, some of which are exposed in the scarps (Plate 11.2). Locally interlimb angles may be as low as 40° and strike-parallel faults commonly cut out the steep, common limbs of synclines and anticlines (Wilkinson, 1988).

Along the northern edge of the district are the northeast-striking Capel Curig Anticline, the Cribau Syncline and the Lledr Anticline with axial trace lengths of up to 5 km. The western termination of the Capel Curig Anticline plunges at up to 35° to the south-west and is typical of the open upright structures of the Bangor district. To the south and west, as the folds adopt a more easterly strike, they tighten and verge to the south with moderately dipping axial planes.

Folds within the Nant Ffrancon Group are difficult to distinguish. This suggests that the dominantly silty mudstone sequence deformed in a ductile fashion and that the regional strain was distributed over a wide area and not focussed into individual structures. However, rare bands of more competent strata locally define folds such as the periclinal fold pair distinguished by the dolerite sill at Clogwyn Brith, north-east of Tan y Grisiau [SH 665 470] (Plate 12.2). These folds, with a wavelength of about 2 km, and an amplitude of 1.5 km, form a mono-dine with a steep north-west-dipping axial plane which is locally displaced by north-east-striking normal faults. Around the fold closures an arcuate hinge cleavage dips 10–15° steeper than bedding.

Faults

Within the district, the fault patterns are complex (Figure 23). Faulting was mainly syn-depositional (Cambrian–Ordovician) and syn-tectonic (mid-Devonian). Later, Variscan (late Carboniferous) and Mesozoic–Tertiary movement almost certainly occurred on many of the faults but this is difficult to distinguish.

North-north-west-striking faults

In the Harlech district, Allen and Jackson (1985) considered that north-north-west-striking faults predated late Tremadoc intrusions and that displacements in the Cambrian strata were greater than in the Ordovician, suggesting initiation prior to the emplacement of the Rhobell Volcanic Group (Tremadoc). Evidence from within the Snowdon district indicates that the main faults, at least, were reactivated during Ordovician times.

In the south and south-east of the district, north-northwest-striking, mainly normal faults affect Lower Cambrian to Lower Ordovican strata. They downthrow to the east and are part of a large fault system on the eastern side of the Harlech Dome (Allen and Jackson, 1985). The largest of these faults, the Trawsfynydd Fault (Matley and Wilson, 1946), has a downthrow of some 1200 m in the vicinity of Trawsfynydd. To the north it passes into a complex zone of faulting about the Cwm Bowydd Fault (Bromley, 1963) which at Blaenau Ffestiniog has an easterly downthrow of about 150 m but, a short distance farther, it cannot be distinguished in the silty mudstones and siltstones of the Nant Ffrancon Group. The fault also marks the eastern edge of the outcrop of the Tan y Grisiau granite.

North-north-west-trending faults displace the upper formations of the Mawddach Group at the south-west end of the Penrhyndeudraeth peninsula and south of the Vale of Ffestiniog where some are intruded by dolerite dykes, up to the level of the Ffestiniog Flags Formation. These normal faults, with displacements of few tens of metres, cannot be traced into the overlying Ordovician strata.

North-east-striking faults

The north-east-striking faults were most influential in the geological evolution of the district; they determined the morphology of the Snowdon graben, which influenced the development of the Caradocian volcanic activity, and later, during the Caledonian deformation, they accommodated and partitioned the compressive stress across the district.

In the north-west of the district, a zone of subparallel to en échelon faults represent the southerly extensions of faults at the eastern margin of the Cambrian Slate Belt. The faults have relatively short (less than 2 km) traces and are concentrated in the tight anticlinal folds of Drws y Coed and Cwm Pennant. Poor exposure makes it difficult to determine fault movement but in the Bangor district both vertical and strike-slip displacements have been distinguished (Howells et al., 1991).

A major zone of anastomosing faults, the Cwm Pennant Fracture Zone (Wilkinson and Smith, 1988), can be traced for 20 km northwards from Criccieth, along the eastern side of Cwm Pennant to the north-west flank of Snowdon. It defines the western margin of the Moel Hebog Syncline and, to the south, it cuts out the Dolgellau Formation on the western limb of the Ynyscynhaiarn Anticline. Near Bryncir [SH 530 440], offsets of a dolerite sill and local deflections in the cleavage pattern suggest a local component of dextral strike-slip movement.

A similar zone of anastomosing faults, up to 1 km wide, can be traced north-eastwards from Porthmadog, along the Glaslyn valley and into the Nantmor valley; it has been termed both the Nantmor Valley Fracture Zone (Howells et al., 1991) and the Glaslyn Valley Fracture Zone (Wilkinson and Smith, 1988). In the northern part of the Glaslyn valley and the Nantmor valley the complex array of faults is well defined within the Nant Ffrancon Group. Two subparallel, north-east-striking faults are linked by numerous short north-north-east-striking faults with sinuous traces but northwards, across the outcrop of the Caradocian volcanic rocks, they merge into a single fault.

The Glaslyn Valley Fracture has been linked to the Mochras Fault at the west side of the Harlech Dome (Campbell et al., 1985) which, in the vicinity of Mochras, has a vertical displacement of at least 2000 m (Woodland, 1971). However, in the Glaslyn valley there is no evidence for such a large displacement and it is possible that these Tertiary movements are distributed across a number of offshore faults (Dobson et al., 1973; Dobson and Whittington, 1988). Geophysical evidence (Blundell et al., 1969) indicates a buried rock basin beneath the Glaslyn estuarine alluvial flats and the strata on either side of the estuary, at Porthmadog [SH 5643 3728] and Boston Lodge [SH 5836 3742], are shattered and intensely veined. Here, in the Tremadoc and Arenig strata, a normal displacement of up to 1.5 km can be inferred but, farther north, in the Prenteg Sandstone Member, the displacement is minimal and possibly dissipated in minor strike-slip movements.

Post-Caradoc, dextral strike-slip displacements, of between 1 and 4 km, have been inferred on both the Cwm Pennant and the Nantmor Valley Fracture zones (Wilkinson, 1988; Howells et al., 1991). In the Gwynant valley, between Beddgelert and Llyn Dinas, there is much, locally intense, faulting between these two zones where they lie within the Snowdon Volcanic Group. Movement occurred from pre- to post-Snowdon Volcanic Group times and the faults distinguished the apical graben of the Lower Rhyolitic Tuff Formation caldera (Howells et al., 1986).

South of the Vale of Ffestiniog, a series of north-east-striking, normal faults displace the Ffestiniog Flags, Maentwrog and Clogau formations. These 'Meridional Faults' of Matley and Wilson (1946) include the extension of the Glyn Valley Fault which obliquely transects the western limb of the Ffestiniog Anticline; they cannot be distinguished north-east of Blaenau Ffestiniog.

East of the Cwm Bowydd Fault and in the Migneint area, a group of faults including the Cwm Teigl, Bryn Ddu, Fynnon Eidda and Oernant faults, displace Upper Cambrian and Lower Ordovician strata. The sinuous traces of individual faults can be followed for up to 10 m and locally they contain silicified, mineralised fault breccias. Both strike-slip and normal movements have been proposed (Lynas, 1973; Bromley, 1963) but are difficult to prove. Numerous subsidiary and minor splay faults are present and trend in a variety of directions.

North-west- and west-north-west-striking faults

These faults are common within the Ordovician strata throughout the district and, typically, they are less than 1 km in length. They are most clearly developed in the outcrop of the Dol-cyn-Afon and Allt Lŵyd formations with normal displacements of only a few tens of metres. In Drws y Coed and in the Gwynant valley, these faults are commonly quartz veined and mineralised. They offset major north-east-trending faults and are considered to have developed late in the tectonic evolution of the district. To the north of the district, the prominent topographic features of both the Pass of Llanberis and Nant Ffrancon Pass lie subparallel to the trend of these faults.

Cleavage

With the exception of the area south and west of the Tan y Grisiau granite, where an early pre-tectonic, bedding-parallel fabric (S0) is preserved, a single penetrative, end-Caledonian cleavage (S1) is present in all strata throughout the district (Figure 24). However, its orientation and attitude is highly variable. Locally, it is overprinted by a crenulation cleavage (S2). The combined effects of arcuation, transection and fanning confuse the relationship of cleavage development to folding. However, the occurrence of well-developed cleavage fans, such as those about the closures of the Dolwyddelan Syncline, strongly suggest that the main SI cleavage formed early during folding and that it was subsequently rotated.

S0

A pre-tectonic, bedding-parallel fissilty (S0) is present in the Upper Cambrian rocks south of the Vale of Ffestiniog and, locally, within the disturbed mudstones of the Lower Caradoc. It comprises subparallel, anastomosing planes defined by the alignment of fine shreds of white mica (illite) and chlorite. The fissilty is concordant with bedding around slump and later tectonic folds, it does not cut mineral veins or affect igneous rocks. Its preservation within Cambrian strata is probably the result of the shadowing effects of the Tan y Grisiau granite (see below).

Near Llyn Morwynion, the S0 fabric in Caradoc strata is overprinted by, and difficult to distinguish from, a gently inclined variant of the S1 regional cleavage.

S1

In the sedimentary rocks the S1 cleavage varies from a closely spaced disjunctive cleavage to a widely spaced (up to 10 mm) crenulation cleavage. In thin section it is defined by foliae of aligned phyllosilicate, opaque and secondary minerals which separate microlithons (Powell, 1979) of non-foliated quartz, chlorite or muscovite. In coarse-grained sandstones and tuffs, the (S1) cleavage forms spaced planes, 5–7 mm apart, of aligned chlorite flakes, which anastomose about quartz and feldspar crystals and lithic clasts.

The (S1) cleavage is typically a penetrative, subvertical planar fabric. It strikes north-north-east in Cwm Pennant, east in the vicinity of Dolweyddelan, and north-west in the Migneint; it transects many of the folds in the district, mainly in a clockwise manner. Where the S1 cleavage is steeply inclined, the cleavage-bedding intersection lineation plunges gently, both to the north and south. Locally, as in Cwm Pennant and the Glaslyn Valley, these lineations are parallel to a north-plunging, mineral lineation. In general, where the strike deviates from the north-east there is a reduction in dip, for example down to 10° in the Blaenau Ffestiniog area. South-west of Blaenau Ffestiniog, the S1 cleavage is weakly developed and lies close to bedding (less than 15°). Deformed thermal spots in the aureole of the Tan y Grisiau granite define a clear stretching lineation, plunging 15–20° to the north or north-north-west. In contrast, the local cleavage-bedding, intersection lineation plunges consistently to the north-north-east. In the vicinity of major faults the S1 cleavage is locally intense and imparts a phyllitic character to the silty mudstones.

On the north-east limb of the Ynyscynhaiarn Anticline, and north of Tremadog, the S1 cleavage strikes northwest, markedly oblique to the regional trend. To the north-west, into Cwm Pennant, it is deflected to the north-north-east and lies parallel to the S1 cleavage with no evidence of overprinting. Between Moel Hebog and the Glaslyn valley, along the outcrop of the Prenteg Sandstone Member, the S1 cleavage is deflected through 90° to lie parallel with the regional strike. This deflection cannot be solely attributed to refraction and must, in part, relate to complexities at major fold closures.

The large competence contrasts between the volcanic and the sedimentary rocks results in cleavage fanning across all the major folds. The angle of divergence is generally less than 10° but in the Dolwyddelan Syncline it is about 30°. Cleavage-bedding intersections show a systematic reversal across the Cwm Pennant Anticline, the Yr Arddu Syncline and the Moel Hebog Syncline and indicate clockwise cleavage transection of between 10° and 30°.

S2

The S2 cleavage is mainly restricted to two areas where it crenulates S1 cleavage planes. In Cwm Pennant, the sub-vertical S2 cleavage strikes east and deforms the steeply inclined S1 cleavage. In contrast, west of Blaenau Ffestiniog the S2 cleavage strikes north-east and deforms a gently inclined S1 fabric. In thin section, the crenulations are seen to be asymmetric microfolds with a spacing of 1–2 mm. Similar crenulation fabrics may be developed in the vicinity of major faults, for example the Glaslyn Nantmor Valley Fracture and the Craiglaseithin Fault, where the Si cleavage is intensely developed and the silty mudstones characteristically phyllitic.

Post-S2 cleavage deformation is only recognised in the local development of kink bands, as about Beddgelert, north of Llyn Gwynant and on the northern limb of the Dolwyddelan Syncline (Wilkinson, 1988)

Metamorphism

The Lower Palaeozoic rocks of the Welsh Basin have been referred to as part of the 'non-metamorphic' or 'paratectonic' Caledonides (Dewey, 1969; Phillips et al., 1976). Nevertheless, the complete sequence suffered low-grade regional metamorphism during the late Silurian-early Devonian orogeny; in addition, the pre-Arenig strata were probably affected by late Tremadoc movements. The metamorphism has been the subject of much research during recent years by considering both the the mineralogies of the basic rocks (Roberts, 1981; Bevins and Rowbotham, 1983) and the crystallinty of the white micas, illite, in the mudstones (Merriman and Roberts, 1985; Roberts and Merriman, 1985). An integral part of these studies has been the systematic sampling over large areas, which has enabled a regional pattern to emerge.

Metamorphosed basic volcanic rocks

Within the district, the metamorphic assemblages of the basic volcanic rocks belong to the prehnite-pumpellyite and greenschist facies. Roberts (1981) distinguished four zones about the isograds pumpellyite-in, pumpellyiteout/clinozoisite-in and biotite-in. The three highest grade zones were distinguished in the strata between the Snowdon and Dolwyddelan synclines. The biotite-in isograd is symmetrically disposed about, but discordant with, the axial-plane trace of the Snowdon Syncline. The isogradic surface was interpreted (Roberts, 1981) to be related to a thermal dome and to dip outwards at moderate to steep angles. Metamorphic crystallisation was a syntectonic and immediately post-tectonic phenomenon as is indicated by the growths of stilpnomelane and actinolite crystals across the regional (S1) cleavage.

Metamorphosed mudstones

X-ray diffraction analysis of the mineralogy and white mica crystallinity of the mudstones, has distinguished three stages of recrystallisation, from high diagenetic zone through anchizone to epizone conditions (Merriman and Roberts, 1985; Roberts and Merriman, 1985). The stages relate to the degree of deformation; the lowest grade, Stage 1, metamorphosed mudstones either lack or have only a very weak cleavage while the highest grade, Stage 3, mudstones are strongly cleaved.

From Porthmadog, in the south-west of the district, crystallinity decreases and metamorphic grade increases towards a central area. Within this area, isocrysts are widely spaced and crystallinity values fall from about 0.23 to 0.16. The area trends south-west and broadly coincides with the Snowdon Syncline (Howells et al., 1991; Roberts and Merriman, 1985). To the west, in a traverse from the limbs into the hinge zone of the Cwm Pennant Anticline, crystallinity values decrease from 0.36 to 0.19, and grade increases. On the western limb of the Ynyscynhaiarn Anticline, and close to its closure, a sharp change in crystallinity is almost coincident with the Tremadoc–Arenig boundary. The strong Caledonian deformation of the Ordovician mudstones in the hinge zone of the Cwm Pennant Anticline produced relatively high-grade rocks but the pre-Arenig burial metamorphism had previously indurated the Cambrian strata sufficiently to resist further metamorphism (Roberts and Merriman, 1985) From the metamorphic mineral associations it is suggested (Roberts and Merriman, 1985) that the highest temperatures attained during metamorphism lay in the range 330–380°C.

Between Tremadog and Rhyd, the metamorphic grade is anomalously low, with crystallinity down to 0.50 (diagenetic zone). Also, the determination of the minerals rectorite, corrensite and pyrophyllite in the mudstones are indicative of low-grade, thermal metamorphism. However, the mudstones are intensely cleaved with high strain values (Smith, 1988) and a higher metamorphic grade would be expected. It has been suggested that this inconsistency is due to thermal alteration, by a subsurface intrusion, preserving the early metamorphic, diagenetic signature (Roberts and Merriman, 1985).

Chloritoid

The occurrence of chloritoid has recently been reported by Brearley (1988) and Smith (1988), from Cwm Pennant and the Vale of Ffestiniog, in Llanvirn and Caradoc mudstones, of diagenetic to epizone grade. It occurs as randomly orientated, euhedral prismatic porphyroblasts, less than 1.0 by 0.2 mm, in isolated crystals and branching clusters. Its relationships to the tectonic fabrics suggest growth during cleavage development; its growth depends on the host rock chemistry, on a high Fe2+/Mg ratio (Halferdahl, 1961) and A12O3 in excess of that contained in co-existing micas and epidote (Miyashiro, 1973). It is believed to occur below 400°C.

Metamorphic resetting of Rb/Sr isotope systems

Evans (1990) has made a comprehensive study of the Rb/Sr geochronology of the extrusive, acid volcanic rocks and acid to intermediate, subvolcanic intrusions in northern Snowdonia and the Lleyn Peninsula; included were the Tan y Grisiau granite and Mynydd Mawr micro-granite intrusions from within the district.

Regression results for Ordovician slates gave ages of 407 ± 17 and 409 ± 15 Ma. Combined, these indicate an age for the regional metamorphism and cleavage formation of 408 ± 10 Ma, postdating the age of mudstone deposition by at least 50 Ma. Rb/Sr whole rock isochrons from extrusive rhyolites and tuffs gave comparable ages and indicate that the systems have been reset. Similarly, most of the subvolcanic intrusions gave late Silurian-early Devonian ages (Howells et al., 1991) and some, such as the Tan y Grisiau granite, a late Devonian age, 384 ± 10 Ma. However, a few analyses, including those of the Mynydd Mawr microgranite, 438 ± 4 Ma, gave ages which appear not to have been reset during the late Caledonian metamorphic event.

There is no correlation between the reset ages and the grade of metamorphism; the temperature reached at any particular locality does not appear to have been a factor in the re-homogenisation of the isotopic systems. However, Evans (1990) has shown that all the rocks with 'reset' Devonian ages display extensive replacement by secondary minerals resulting from hydration reactions during low-grade, regional metamorphism. This suggests that it is the extent to which the rocks have reacted with water that affects the Rb/Sr isotopic systems. The water was probably released from the host sediments as they were deformed and partially dehydrated; its influx into the igneous rocks, and the closed system behaviour of their isotopes during metamorphism, suggests that they acted as sumps.

Geophysical investigations and deep structure

The first significant geophysical work in North Wales was a gravity survey (Powell, 1956) which outlined the major anomalies, including the marked regional increase in Bouguer gravity anomaly values towards the Irish Sea, the gravity anomaly lows associated with the Tan y Grisiau granite and the sedimentary basin in Tremadog Bay. Within the district, the few stations delineated a gravity anomaly high over the Snowdon massif and a gravity anomaly low, from a single reading, over the Tan y Grisiau granite. Subsequently, systematic regional gravity and aeromagnetic surveys have indicated that the geophysical characteristics of the district are not very distinctive, probably because of the limited range of physical property contrasts within the Lower Palaeozoic sequence. However the magnetic anomaly values near the Tan y Grisiau granite and close to the Snowdon massif are some of the highest in England and Wales.

Gravity data

The Bouguer gravity anomaly data were collected at a uniform station distribution of approximately 1 station per 1.6 km2 throughout the district and are shown as a contour map in (Figure 26).

A summary of rock density determinations of the Lower Palaeozoic rocks is given in (Table 5). The grain density sets an upper limit to in-situ values which will be attained where the bulk porosity reduces to zero, as would be expected with increasing depth. Additional details are provided by Allen and Jackson (1985) and Entwistle (1982). The nature of the underlying Precambrian rocks remains uncertain but the Mona Complex on Anglesey has densities ranging from < 2.7 Mg/m3 for micaceous schists to >3.0 Mg/m3 in the hornblende schists.

Bouguer anomaly values increase north-westwards across the district, as part of the general regional gradient which culminates locally above the Precambrian basic complex in south-east Anglesey. This gradient partly reflects variations in the lower crust, such as crustal thinning or an increase in its density (Blundell et al., 1971). There is also some contribution from higher levels: reduced Bouguer anomaly values across the Harlech Dome are indicative of lower-density rocks within the Precambrian basement, while the Lower Palaeozoic mudstones and basic intrusions are relatively dense giving higher Bouguer anomaly values elsewhere.

The regional gravity anomaly trend is broken by an elongate anomaly low, A; (Figure 26.1) from near Blaenau Ffestiniog to Penmachno, which reflects the Tan y Grisiau granite. The effect of the Cwm Teigl Fault can be distinguished, separating two components of the low, but the influence of the Cwm Bowydd Fault is minor. The regional trend is also broken in the north-west of the district, B; (Figure 26.1) where values decrease towards a Bouguer anomaly low above the fault-controlled Arfon Basin (Reedman et al.,1984), whose western margin lies close to the Menai Straits.

A narrow Bouguer anomaly low, C; (Figure 26.1) coincident with the Glaslyn valley has been attributed to a relatively deep Tertiary channel (Blundell et al., 1969). To match the depths to bedrock interpreted from seismic results on Morfa Harlech, a density of 2 Mg/m3 was calculated for the channel fill. The anomaly is mainly of near-surface origin but, to the south, it extends into the gradient associated with the Mochras Fault (Allen and Jackson, 1985), which juxtaposes Cambrian and Jurassic strata at depth. A similar gradient would not be expected above the Glaslyn valley, even if it is developed along a branch of the same fault, as bulk density contrasts within the Lower Palaeozoic rocks are much smaller.

On a filtered Bouguer gravity anomaly map (Figure 26.2) the anomalies are more clearly correlated with the exposed geology although, on the existing data, they cannot be interpreted in detail. The outcrop of the Snowdon Volcanic Group is associated with lower gravity anomaly values around Snowdon and a similar relationship is seen near Llwyd Mawr, a; (Figure 26.2), near Capel Curig, b; (Figure 26.2) and the Dolwyddelan Syncline, south of g; (Figure 26.2). A prominent south-west-trending anomaly, c; (Figure 26.2) near Snowdon, between Hoel Hebog and Cwm Idwal, lies parallel to the northern part of the Cwm Pennant Fault Zone, and a north-south branch, d; (Figure 26.2) coincides with the southern part of the Cwm Pennant Fault Zone. Some north-east trends within the Snowdon Volcanic Group outcrop probably reflect the dominant fractures which controlled its eruption and emplacement. The Nantmor Fracture can be distinguished adjacent to the local gravity anomaly low, coincident with the Yr Arddu Tuffs, e; (Figure 26.2); the basaltic centre near Snowdon coincides with an anomaly high, f; (Figure 26.2) on the north-west side of the Beddgelert Fracture Zone.

The response of the Mynydd Mawr microgranite is difficult to distinguish due to its location on the steep anomaly gradient between Snowdon and the Arfon basin. In contrast to the Bouguer anomaly low over the Tan y Grisiau granite, however, it appears to be weak, indicating its smaller size and slightly different composition. Individual basic intrusions are too small to generate discrete positive anomalies but they would increase the bulk density of the host formations as, for example, in the Nant Ffrancon Group, near Moel Siabod, g; (Figure 26.2).

Aeromagnetic data

The total field aeromagnetic data are shown as a contoured map (Figure 27). Over the rugged terrain of Snowdonia, some distortion would be expected above near-surface magnetic rocks.

Evans and Greenwood (1988) reported magnetic susceptibility measurements of a range of rocks from Snowdonia. The relatively few rocks with mean values >10 10−3 SI units included both sedimentary rocks and a variety of volcanic rocks; the Tan y Grisiau granite was slightly lower at 3–7 10−3 SI units. The acid volcanic rocks are essentially non-magnetic (Table 5), although some differences are observed between the major units. With few exceptions, the dolerites are also non-magnetic. The highest values (600 10−3 SI units) were obtained from magnetite-rich sandstones in the Cwm Eigiau Formation. A significant contribution to the magnetic anomaly pattern over the Cambrian strata of the Harlech Dome was also attributed to sedimentary strata with disseminated pyrrhotite (Allen and Jackson, 1985).

The high aeromagnetic anomaly levels throughout the district are part of an arcuate belt from the Bala Fault, through the Harlech Dome and Snowdonia to the southeast of Anglesey, and south-west along the Lleyn Peninsula. This broad feature is unlikely to be the reflection of a set of disparate, near-surface sources and it is suggested that there is a significant contribution from the underlying basement. Upon this positive background, a number of local anomaly highs can be distinguished; for example, above the outcrop of the Tan y Grisiau granite, A; (Figure 27), to the north of Mynydd Mawr, B; (Figure 27) and at the southern edge of the Snowdon massif, C; (Figure 27). The reduced-to-pole image of the aeromagnetic data shows a broad elliptical high anomaly across both the Snowdon centre and the Tan y Grisiau granite, which is similar in area to the Bouguer anomaly low. In contrast, the Llwyd Mawr tuffs are weakly magnetic and while short wavelength anomalies occur above inliers of the Snowdon Volcanic Group to the north-east, the background magnetic field level is lower, D; (Figure 27).

A narrow anomaly low, E; (Figure 28) south of the Vale of Ffestiniog is marked by steep gradients along an eastnorth-east-trending discontinuity. In detail, the aeromagnetic signature is more complex, comprising several components which transect the north-south axis of the Harlech Dome. These anomalies are of short wavelength, indicating a shallow origin, but without additional ground traverses their precise location, and cause, cannot be distinguished. One anomaly, between Ynys Gifftan [SH 601 371] and Llynnau Gamallt [SH 741 449] correlates in part with local intrusions and faults. The eastern end of the principal positive anomaly in this area, F; (Figure 27) overlies the narrow granite outcrop near Creugu [SH 655 420].

The northward extension of the Trawsfynydd Fault crosses the anomalies along the Vale of Ffestiniog, G; (Figure 27) and confines the higher values from the Tan y Grisiau granite. A number of north-westerly trending anomalies across the Vale of Ffestiniog partly reflect intrusions and minor faults. The large anomalies within the Harlech Dome were variously attributed to the sedimentary strata, intrusions and, in the Dolwen Pericline, to a core of magnetic rocks (Allen and Jackson, 1985).

The Snowdon centre anomalies are separated by the Hebog–Idwal Fracture Zone, H; (Figure 27) from a block of relatively high anomaly values to the north-west, B; (Figure 27). The Mynydd Mawr microgranite lies at the edge of the block in a similar relationship as that of the Tan y Grisiau granite to its more extensive magnetic anomaly. This group of anomalies extends to the Menai Straits where another small granite crops out at Twt Hill and it is therefore possible that granitic rocks are more widespread at depth. The western margin of the caldera is clearly defined by weakly magnetic basement, J; (Figure 27) to the west of the Cwm Pennant Fault.

The large magnetic anomalies on the southern flank of the Snowdon massif were investigated by ground traverses. Anomalies exceeding 3000 nT in amplitude near Bwlch Cwm Llyb [SH 593 523] were related to sandstones in the Cwm Eigiau Formation, with more than 10 per cent magnetite; the source of the magnetite has not been identified. The high angle between the strike of the sandstone beds and the flight lines, combined with rugged relief and the wide spacing between the lines, inhibited detailed correlation of ground traverse data with anomalies in the airborne survey. However, the latter indicate a more extensive source at depth than the zones, less than 10 m wide, mapped near surface over the sandstones.

Two weak, but persistent, south-easterly trending aeromagnetic anomalies, K and L; (Figure 27) more clearly expressed as a variation in the horizontal gradient, cross the district and are interpreted to reflect Tertiary dolerite dykes which are commonly reversely magnetised. They form part of a set of anomalies which can be traced across Anglesey and the north Irish Sea to the Mourne granites. Few corresponding outcrops are seen in the district, but anomaly 'K.' is linked to an exposure of dolerite in the Berwyn Hills, farther to the south-east (information from Mr A D Evans, 1993).

Seismic data

Shallow-marine reflection data from Tremadoc Bay (Dobson et al., 1973) indicate the presence of a deep Tertiary basin south-west of Morfa Harlech although, because the lithological variations are too small to produce good reflections, it is difficult to estimate the thickness of Tertiary strata. The basin extends for about 18 km to the west-south-west of the Mochras Fault which bounds it to the east; to the north it lies in Lower Palaeozoic strata near Porthmadog.

The existence of a deep buried valley interpreted from the gravity data, extending northwards from Morfa Harlech into the Glaslyn valley, has been confirmed by seismic refraction data (Blundell et al., 1969). The Tertiary infill, up to 400 m with a velocity of 2.2 km/s, overlies a basement of velocity 5.2 km/s which is interpreted as Cambrian strata.

Laboratory sonic velocity measurements on rock samples (Entwistle, 1982) did not reliably reproduce those measured in the field. The velocities for both the Lower Palaeozoic sedimentary rocks and the igneous rocks are generally high, in the range 5.5–6.5 km/s, and they cannot be separated; the Mesozoic–Tertiary strata have values of 2–4 km/s from seismic refraction surveys, contrasting with 5–5.5 km/s shown by the Lower Palaeozoic strata.

Following the Lleyn earthquake (Turbitt et al., 1985) in 1984 a local seismic network was established to monitor the aftershocks. The network, used in conjunction with local quarry blasts, provided information on the upper crust. Arrivals at Ynys-wen [SH 558 433] from blasts at Minffordd Quarry, near Porthmadog, had minimum velocities of 5.3 km/sec, which are consistent with other values obtained for the Ordovician sedimentary rocks.

Other ground surveys

Within the district, geophysical work has been undertaken during commercial mineral reconnaissance surveys and by postgraduate students in areas of known mineralisation. Induced polarisation data allowed rhyolites, with a combination of high chargeability and low resistivity, to be distinguished from acid tuffs. Some specific anomalies were considered as possible ore targets but no major discoveries were reported. Electromagnetic surveys distinguished anomalies associated with known mineral veins and they also proved useful in positioning lithological boundaries and faults beneath drift.

Magnetic traverses at Cwm Dwyfor [SH 541 507] located anomalies of 500 nT associated with pyrrhotite veins with susceptibilities up to 130 10−3 SI units; values up to 275 10−3 SI units were measured on magnetite-rich sandstones at Cwm Tal y Migneidd [SH 545 529]. At Hafod y Llan [SH 622 521] similar anomalies, close to the contact between the Lower Rhyolitic Tuff and Bedded Pyroclastic formations, probably contribute significantly to the observed aeromagnetic anomaly high.

Geophysical interpretation

A two-dimensional model (Figure 28) illustrates some of the main features of the gravity and magnetic fields across the district. It should be noted that this model is not unique and that the thickness interpretations result from poorly constrained density and magnetic susceptibility values.

A combination of crustal thinning and mid-crustal variation at depths of 15–20 km (not shown) has been used to account for the regional gravity gradient. A relatively shallow, magnetic 'basement' beneath the Lower Palaeozoic rocks of the Harlech Dome and Snowdonia is the source of the observed, high background anomaly levels. The main features of the upper crust are discussed briefly below.

Low-density, acidic ash-flow tuffs (Padarn Tuff Formation) are thought to infill the Arfon Basin and cause the gravity anomaly low at the north-western end of the profile (Reedman et al., 1984). The mean density of 2.76–2.79 Mg/m3 adopted here for the basement of North Wales is lower than that used by Reedman et al. (1984) on the assumption that the denser rocks (such as the hornblende schists of the Mona Complex) are localised. Thus, to maintain the thickness of the tuffs at about 2 km, this model also requires low-density rocks beneath the Arfon Basin. The granite intrusions of Twt Hill and Mynydd Mawr lie on the flanks of the basin and similar bodies underlying it may contribute to the Bouguer gravity anomaly low if a discrete basement block is not present. Alternatively, the thickness of tuff has to be increased to about 4 km which, in terms of basin development, is less probable. The high gravity anomaly values between Mynydd Mawr and the Idwal Fault Zone can be explained by thick, dense sedimentary strata such as the Llanberis Slates Formation. A coincident magnetic anomaly low may reflect local deepening or alteration of the magnetic basement.

A mean density of 2.73 Mg/m3 has been estimated for the thick sequence of Caradoc, acid and basic, volcanic rocks and intercalated sedimentary rocks; the density of the underlying Cambrian strata, with their associated intrusive rocks, is possibly slightly higher. The model suggests a relatively shallow basin, 1.5–2 km thick, with the highly magnetic sandstones and mineralised veins causing the magnetic anomaly high, at 25 km, on the south-west flank of Snowdon. The discrete responses from these narrow, near-surface sources are not resolved in the observed values because of the limitations of the regional airborne survey.

A distinctive Bouguer gravity anomaly low associated with the Tan y Grisiau granite and its thermal aureole, indicates a significant subsurface body. Cornwell et al. (1980) considered that the granite was sheet-like, dipping and wedging out to the north-north-west and extending for 8 km to the east-north-east of its outcrop. The associated aeromagnetic anomaly indicates a more extensive feature, also prominent in the Euler solution interpretation (MacDonald et al., 1992), and possibly represents a local rise in the basement which separates the relatively high-density basement of Snowdonia from lower density, but still magnetic basement beneath the Harlech Dome. The latter is similar to that beneath the Arfon Basin and implies a more granitic composition than elsewhere. The thickness of Cambrian strata cannot be determined geophysically but borehole information (Allen and Jackson, 1978) suggests that magnetic basement is overlain by higher density Precambrian volcanic rocks (Bryn-teg Volcanic Formation).

Chapter 5 Economic geology

The Snowdon district has been exploited for a wide range of raw materials through historical times to the present. Slate has been the most extensive material extracted as can be witnessed by the great spoil tips in the vicinity of Blaenau Ffestiniog. However copper ore has been extracted from veins in the volcanic rocks around the Snowdon massif over many centuries and minor amounts of lead and zinc have been extracted over the last 150 years. There are small workings for sand, gravel and building stone throughout the district, although extraction has been limited. The only current working quarry for hard rock aggregate is that in dolerite, at Y Garth near Porthmadog.

Slate

There is no unequivocal evidence to support the much-quoted statement, that slate has been extracted in Snowdonia since Roman times, but it is almost certain to be true. The area is the site of the greatest slate extraction in the United Kingdom and this occurred mainly during the 19th century. The increase in annual output from 26 000 tons in 1793 to 489 000 tons in 1898 (Lewis, 1979) can be directly related to the massive expansion of the great industrial conurbations in England and Wales. The decline in extraction, apart from a few minor pauses, in the 20th century has been spectacular — an estimated 20 000 tons was extracted in 1973.

Within the Snowdon district there are numerous trial pits and small quarries for slate, probably some in every sedimentary formation. The drift-covered Llanberis Slates Formation, of Cambrian age, which crops out in the north-west corner of the district lies just on the edge of the Nantlle–Llanberis–Bethesda slate belt which forms such an impressive topographic feature to the north and west. However, the sites of main extraction are entirely of Ordovician age, within the Nant Ffrancon Group about Blaenau Ffestiniog and the Cwm Eigiau Formation about the Dolwyddelan Syncline. At Dolwyddelan, the Black Slates overlying the Snowdon Volcanic Group are the highest strata quarried in the district.

The largest quarries within the district are those in the lower Ordovician sequence about Blaenau Ffestiniog, in the complexes of Oakley, Llechwedd and Maen Offeren. Here the worked cleavage dips 10–35°, in an arc, to the north and east. This low-angle cleavage has been interpreted (Campbell et al., 1985) as a local variant of the regional cleavage that was caused by thrusting across the roof of the Tan y Grisiau granite. The slates in the zone of shallow-dipping cleavage were excavated initially in open quarries and then were pursued further in underground mines which continued to be termed quarries. The well-exploited tourist facility at Llechwedd is probably the best example of such development.

The zone of low-angle cleavage extends westwards and the large quarry complex at Rhosydd, at the head of the Croesor valley, exploited it in a series of large caverns (Lewis and Denton, 1974). To the east of Blaenau Ffestiniog, the cleavage inclination steepens slightly into the open Manod Quarries [SH 732 470] to [SH 733 455] and the large quarry at Cwm Penmachno [SH 751 469]. However, at Croes y ddwy afon Quarry, recent excavations have exposed large underground caverns in slates of Tremadoc age, in the hanging wall of the Garth Grit member

In the vicinity of Dolwyddelan, slates have been worked, close to the base of the Snowdon Volcanic Group in a series of quarries along the southern limb of the syncline. The development reflects the mudstone lithologies at this stratigraphical level which are in sharp contrast to the sandstone-dominated sequences, at a similar level, in central Snowdonia. The cleaved black mudstone overlying the Snowdon Volcanic Group, in the core of the syncline, was also exploited for slate, in a quarry on the south side of the castle.

Metalliferous deposits

Within the Snowdon district the earliest reference to the mining of metalliferous deposits is in an account of a visit to the Drws y Coed area by Edward I in 1284 (Bick, 1985). The main period of exploitation, of copper, with minor amounts of lead and zinc, occurred in the 19th and early 20th centuries, but the veins were small, low grade and commonly with fine-grained, intergrown sulphides which caused mineral dressing problems. The mines tended to be small, sporadic and rarely profitable. There was a brief upsurge in mineral exploration in the early 1970s that was facilitated by financial support from government agencies. A number of major companies examined prospects within the district. The results of some of this exploration can be found in Open File Reports of work accomplished under the Mineral Exploration and Investment Grants Act 1972 (MEIGA). Reports relating to the district are Nos. 75 Ffestiniog, 76 Hafod y Llan, 77 Prysor Gamallt, 87 Nantmor, 88 Nantlle and 93 Drws y Coed. They are available from the National Geological Records Centre at BGS Keyworth.

There are a large number of small mines and trial pits for metals throughout the district but there are two main concentrations. The first lies generally on the south side of the Snowdon massif and east of Beddgelert, and the other, in the Nantlle valley, close to Drws y Coed. About Snowdon, the mineralisation, within Ordovician strata, is volcanogenic and is related to the final stages of activity in the Snowdon caldera (Howells et al., 1991). The mineralisation in the Nantlle valley is mainly of vein type and is largely confined to Cambrian strata.

Synsedimentary metalliferous deposits

The only synsedimentary deposits worked within the district are the pisolitic ironstone deposits in the disrupted lower Ordovician sequence in the vicinity of Tremadoc. The ore occurs in lenticular masses, up to many tens of metres in length, within sheared, dark grey silty mudstones and the outcrop is marked by a series of small trial pits and quarries. The lenticular masses are interpreted to be rafts of a disrupted bed within the melange. The ore was mainly extracted prior to 1860 as eventually, at deeper levels, the sulphur content reduced its quality. The ooliths, as recognised by Fearnsides (1910), comprise a core of sericite and chlorite with a thick carapace of magnetite. Pulfrey (1933) described chlorite ooliths replaced by magnetite from rafts within the Rhyd Melange, at Blaen y plwm, and Hallimond (1925) distinguished chamosite ooliths traversed by stilpnomelane, an iron-rich mica.

Volcanogenic mineralisation

The minor base metal, quartz-sulphide vein deposits are mainly concentrated in the outcrop of the Snowdon Volcanic Group (Reedman et al., 1985) and especially at the contact between the Lower Rhyolitic Tuff Formation and the overlying Bedded Pyroclastic Formation. The narrow, irregular veins trend north-east or north-west and are especially common within the Beddgelert Fracture Zone, a possible apical graben of the Snowdon Caldera (Howells et al., 1991). The caldera mineralisation is associated with intensive hydrothermal alteration which caused extensive silicification and chloritisation with the removal of calcium, sodium and strontium, and the addition of silica, iron, manganese, fluorine and base metals. Such alteration is typical of feldspar dissolution and the growth of clay minerals. The hydrothermal activity caused circulation of mineralising fluids along faults during the waning stages of the volcanic activity (Reedman et al., 1985)

The veins comprise quartz, pyrite, chalcopyrite, sphalerite and galena. Magnetite and haematite occur in a small group of veins in Cwm Tregalan [SH 622 520] (Colman and Appleby, 1991) and pyrite, magnetite and fluorite occur in a vein near Mynydd Mawr [SH 5486 5569], 5 km west of Snowdon (Colman, 1990). Pyrrhotite is locally common in some veins at the Lliwedd, Cwm Llan and Hafod y Porth mines in the Gwynant valley. Calcite, dolomite and marcasite occur in the veins of the Brittannia Mine, above Llyn Glaslyn. Traces of gold (up to 1 ppm) in the sulphides and a lead-bismuth-silver mineral were determined in the Hafod y Porth Mine, near Beddgelert. Cobalt/nickel ratios of >1 determined in the pyrite indicates its volcanogenic source (Bralia et al., 1979). Baryte or arsenopyrite, both of common occurrence in some volcanogenic deposits, are rare. The veins predate the cleavage development and locally occupy synvolcanic faults.

Brittannia Mine

Brittannia Mine [SH 612 548], about the Pyg and Miners tracks above Llyn Glaslyn, was the largest in the area. A total of about 3000 tons of copper ore had been extracted when, in 1916, the mine finally closed (Bick, 1985). The mine worked a single vein, up to 3 m wide, on a number of levels over a strike length of 300 m. The vein comprises an early quartz-pyrite-chalcopyrite-sphalerite-galena assemblage cut by a later vein with a calcite-marcasite-dolomite-sphalerite assemblage which followed the same structure. Both assemblages show evidence of hydraulic brecciation, caused by ore-fluid pressure on the vein walls, with rounded to angular fragments within a sulphide-rich matrix. Some sphalerite in the calcite-bearing veins is finely banded, light and dark brown, reflecting variations in the iron content (Colman and Laffoley, 1986). The workings consist of a number of small ore shoots, mainly above the Lower Rhyolitic Tuff Formation/Bedded Pyroclastic Formation boundary which may have acted as a physical or chemical control to cause mineral deposition. The ore shoots are commonly curved in plan.

Lliwedd Mine

Lliwedd Mine lies at the head of Cwm Meirch [SH 632 532]. The extraction was entirely from the Bedded Pyroclastic Formation but close to the contact with the underlying Lower Rhyolitic Tuff Formation. A series of open excavations, trending at 125° and 75°, can be traced for over 300 m. The vein dips at more than 75° to the north. In the lower slope, the vein, trending at 125°, crosscuts the strike of the bedding, 50°, and the cleavage, 75°, but on the upper slope the vein, trending at 75°, lies subparallel to them. Quartz veins parallel to the cleavage are generally undeformed while those crosscutting veins are folded and offset. The mineralisation is mainly quartz-pyrite-pyrrhotite-chalcopyrite-sphalerite with minor galena, within a dilated fault zone with no evidence of early brecciation. However, the sulphides are cleaved and strained indicating later, regional deformation. About 100 m to the south of the main vein, a small vein, trending at 130°, containing cleaved massive pyrrhotite and chalcopyrite also indicates pre-cleavage deposition.

Hafod y Porth Mine

This mine, at the head of Cwm y Bleiddiad [SH 611 508], consists of a series of small adits, shafts and trial pits over an area of about 700 by 200 m. A total of about 300 m of adits have been driven, but the effort was to little avail as only 58 tons of copper ore were extracted (Bick, 1985). The mine workings are within beds, near the base of the Bedded Pyroclastic Formation, which are mainly tuffaceous silty mudstones with many tuff and rhyolite clasts. A recent survey (Agnew, 1989) detected strong VLF–EM and magnetic anomalies over an unworked part of the area which could possibly be related to near-surface, pyrrhotite-bearing sulphide mineralisation.

Cwm Bychan Mine

This small mine in Cwm Bychan, 1 km south-east of Beddgelert [SH 602 472] was connected with the Welsh Highland Railway in Aberglaslyn Pass by an aerial cableway, the remains of which can still be seen (Laffoley and Rex, 1986). The mine worked a quartz-sulphide vein, trending at 030° and dipping at 70° to the east, from three adits. The hanging wall of the vein is sharp and planar, whereas the footwall contact is irregular. The mineral assemblage is quartz-pyrite-chalcopyrite-sphalerite and the vein contains slabs of the adjacent siltstones. The sulphides have crystallised preferentially about the clasts of wallrock and, in places, the replacement is complete, with the development of a quartz-sulphide supported breccia. In places, the vein quartz has crystallised in fine fibrous crystals, up to 70 mm long, lying normal to the vein. A prominent fault is marked by a 0.30 m zone infilled with white clay gouge and, in the southernmost adit, several curved fractures infilled with kaolinite (information from Ms C R Hallsworth) are exposed. The kaolinite cannot be of Ordovician age because it would not have survived the Caledonian metamorphism and it is probably a Tertiary infill of a pre-existing fracture.

Sygun Mine

The Sygun copper mine, 3 km north of Beddgelert, was the site of some of the earliest experiments by Elmore to separate different sulphide minerals by froth flotation (Bick, 1985). However, the low grade of the ore and the low price of copper made profitable operation, on such a small scale, impossible. In recent years the mine has been reopened as a tourist attraction.

Mines in the Nantlle valley

The Nantlle valley is crowded with mines and trial pits. Drws y Coed mine was the most extensive operation with a total recorded production of 13 000 tons of copper ore (Bick, 1985). The sulphide minerals are hosted in quartz veins within Cambrian sandstones or, as at Cwmffynon [SH 534 522], within siltstones at their contact with a diorite intrusion (Colman, 1990). The mineralisation consists of chalcopyrite, pyrite, sphalerite and galena. Old records show that the sandstone-associated mineralisation was enriched at fault intersections. The veins, with up to 10 per cent chalcopyrite (Anon, 1973), form a series of steep, easterly dipping ore shoots, up to 7 m wide and 100 m long, which have been worked up to 200 m below the surface.

Aggregate

Within the district there are a large number of large and small disused quarries which were worked for aggregate. Currently the only working quarry is in the dolerite at Y Garth [SH 595 392] although the Moel y Gest Quarries [SH 556 388] have been worked for dolerite in the recent past and the rhyolite at Foel Gron [SH 743 428] is worked, albeit on a small scale, intermittently. The hard-rock resource is enormous, although in most of the district any proposal for extraction would be subjected to stringent planning requirements because of its situation in the National Park.

Building stone

Stone for building is not presently being worked. In the past the main quarries were in Blaenau Ffestiniog working the quartz latite in the well-jointed, steep margin of the Manod intrusion.

Chapter 6 Tertiary and Quaternary

Tertiary deposits are not exposed within the district but geophysical investigations suggest that they do occur at depth. Shallow-marine reflection data in Tremadoc Bay (Dobson et al., 1973) indicate the presence of a Tertiary basin and the deposits may extend along a buried valley, into the area of the Glaslyn estuary (Blundell et al., 1969).

The area of north-west Wales was one of the first in Britain to be investigated in an attempt to prove or disprove the Glacial Theory. Since that activity, in the middle of the 19th century, the area has continued to attract a succession of geomorphologists to its glacial problems (see Campbell and Bowen, 1989, and references therein). The most comprehensive analysis of the possible evolution of the glaciation in Snowdonia has been produced by Addison (1988) and this account draws heavily on this work.

During the Quaternary there were many glacial episodes (stadia) that were interspersed with warmer periods (interstadia). Within the district, the glacial features are entirely of the final glaciation (Late Devensian), from around 27 000 yrs BP to 10 000 yrs BP, which probably removed all evidence of the earlier episodes. The earliest radiocarbon age of vegetation at an ice-free site is 13 670 ± 280 yrs BP near Llyn Dywarchen. Organic sediments within the moraines about Llyn Llydaw, dated at 9930 ± 120 yrs BP probably mark the start of the Post-glacial (Flandrian) climatic improvement. The district lay just to the north of the thickest ice accumulation in Wales, between Arenig Fawr and the Rhinog Mountains (Campbell and Bowen, 1989). This ice was entirely of local accumulation, about the higher ground of northwest Wales, and throughout Quaternary times it obstructed the southerly movement of the larger Irish Sea Ice Sheet about the north and west sides of Wales (Figure 29). The erosional effects of the Welsh ice sheet are nowhere more dramatically displayed than in the high ridges and cwms of the Snowdon district (Plate 11.2).

The till deposited by the Welsh ice sheet is typically grey-brown with much silt and gravel-sized debris and boulders of Lower Palaeozoic rocks. Nowhere in the district has the till of the Irish Sea ice been recognised, although it has been determined in boreholes in Cardigan Bay and has been dated as pre-Ipswichian (Garrard, 1977). It is possible that it occurs beneath the estuarine alluvial flats in the Glaslyn estuary, north-east of Tremadog. However, within the district there is evidence, in the shelly sands on Moel Tryfan, of the incursion of the Irish Sea ice sheet.

Landforms

A systematic analysis, or description, of the glacial land-        forms which occur throughout the district is beyond the scope of this account. The most dramatic features are probably those that are concentrated in the Snowdon massif, which Addison (1988) referred to as 'a masterpiece of cirque glaciation'. Here, within the core of the Snowdon Horseshoe, a series of three rock basins descend north-eastwards, from Cwm Glaslyn, beneath the steep north-east-facing cliff below the summit of Snowdon, to Cwm Llydaw and Cwm Dyli (Plate 11.2). Both Glaslyn and Llydaw are occupied by rock-dammed lakes and each basin is separated from the next by rock lips at 710 m above OD, 640 m above OD and 470 m above OD respectively. The basins are bound by steep cliffs which are commonly deeply striated and gouged by debris that had been incorporated at the base of the ice sheet. It has been suggested (Gray and Lowe, 1982) that two sets of intersecting striae in Cwm Llydaw reflect flow during the last ice advance and an earlier event. Asymmetric rock mounds, roche moutoneés, are common, in the valley floors, as for example in Cwm Dyli and below the Lliwedd cliff, with their smoothly eroded surfaces facing upstream and steeper, plucked surfaces facing downstream. Similarly sculpted surfaces are well exposed along the line of the Pyg Track, below the scree on the south side of Crib Goch ridge. Perched blocks, many precariously placed, are particularly prominent.

The lip of Cwm Dyli hangs high above the Gwynant valley which descends sharply, in a south-west direction, from Pen y Pass at the head of the Gwyryd valley. The form and features of both these valleys reflect intense glacial scouring. The line of the Gwynant valley is marked by a series of rock steps, lakes and infilled basins and was clearly fed, between Llyn Gwynant and Llyn Dinas, by a glacier in Cwm Llan. However, for the valley to have been fed, at Cwm Dyli, from the main Glaslyn glacier would have needed the latter to have turned through 120–150°, to flow south-west. Addison (1988) argued that the main excavation of the Gwynant valley occurred at the height of the Devensian glaciation, by downcutting at the base of the main ice sheet, as a result of the obstruction caused by the Snowdon massif in its north-west movement. Such contrary movements between the basal and upper parts of the ice cover explain similar breaches between valleys throughout Snowdonia as, for example, the upper levels of the Glaslyn glacier fed southwards, through Bwlch y Saethau, into Cwm Llan, while the lower levels were excavating in a north-easterly direction.

Throughout the district, the complex sequence of contrasting lithologies, and igneous intrusions, profoundly affected the topographical expression of the glacial erosion. The competent volcanic rocks and the less competent sedimentary rocks formed the dramatically serrated profile between Moelwyn Mawr and Yr Arddu. Clearly, the well-developed cleavage planes in the fine-grained sedimentary rocks further facilitated their erosion. The boss-like intrusions at Mynydd Mawr and Manod Mawr are probably the best examples to demonstrate the resistant effects of igneous intrusions. Although the main effects of erosion throughout the district are those of grinding the substrate, by the debris-charged ice at the base of the ice sheets, there is much evidence which indicates river erosion. This may have occurred either as subglacial drainage or during post-glacial times. The profiles of both Cwm Bychan and Cwm Nantmor, south-east of Beddgelert, suggest that they were probably subglacial drainage channels. Similarly the profile of the Lledr valley suggests that it was overdeepened by water drainage and the rock barriers, with broad alluvial flats on their upstream sides, suggest that the meltwater was, at some time, dammed in small lakes.

Deposits

Because of the dominant mountainous terrain within the district, the deposits of the glaciation are, in general, less impressive than the effects of ice erosion. Also, in many instances the deposits are difficult to differentiate as, for example, moraine grades into boulder clay (till), and scree grades into head and steep alluvial cones.

Moraine

The moraines have been a focus of interest for the succession of geomorphologists into Snowdonia since Darwin in the early 19th century. In recent years, Gray (1982) claimed to have systematically mapped their distribution and from this he constructed the size and volume of the last glaciers in the district. More recently, Addison (1988) distinguished the salient features of the moraines and their relationship to the landforms, and outlined a reasonable pattern of their evolution.

Within the district, moraines are particularly well featured at the sites of the late-stage, valley glaciation in the Snowdon massif. They form both in local concentrations of small mounds and ridges, as about Llyn Llydaw, and as single large mounds, for example abutting against the steep slope on the north side of Craig Cwm Silyn. Many deposits, such as the delicate transverse ridges in Cwm Glas [SH 618 562], indicate that, in spite of the extensive solifluction processes, their original form has been retained. However, elsewhere, as in parts of Cwm Dyli, the features are less well defined and the deposits grade into boulder clay and solifluction debris. South of the Snowdon massif, moraines are generally restricted to small remnants in the upper cwms of the Moelwyn and Manod hills.

Boulder clay (Till)

Boulder clay is found mainly on the gentle lower slopes, as valley fill and preserved in small hollows. The broadest areas within the district lie north-east of Llyn Trawsfynydd and in the Pennant valley, north-north-west of Porthmadog. There are few natural sections through the boulder clay but most streams expose up to 6m locally. Other sections have been exposed during road excavations and particularly good examples occur close to the Pen y Gwyryd Hotel, on the roads to Beddgelert and Pen y Pass, although these are slowly being hidden by the construction of retaining walls.

The boulder clay is typically grey-brown in colour and ochreous, when weathered. The clast content is entirely locally derived and as a result is very variable. Where it overlies cleaved silty mudstone formations the fragments are generally small and angular and impart a gravelly character to the deposit. Whereas boulder clay associated with extensive outcrops of blocky jointed volcanic rock contain larger, subrounded and striated clasts in a more distinctly clayey matrix.

Blocky boulder clay, with little or no clayey matrix, developed from material that was transported on or within the ice sheet and was dumped when the ice melted. These deposits generally smear the surface and were particularly prone to solifluction, accumulating into lobes and terraces as displayed on the south-west side of Snowdon and in Cwm Pennant.

Drumlins are uncommon, even on the wider tracts of boulder clay, although the odd isolated mound occurs locally. Less-well-developed moundy features, as seen on the east side of Llyn Trawsfynydd, are probably the result of erosion.

Fluvioglacial gravel

Small lenses of gravel are common in the boulder clay and were probably laid down by temporary meltwater channels. However there are no extensive, mappable deposits of fluvioglacial gravels within the district. Small remnant high terraces have been distinguished (Howells et al., 1978) near Penmachno and in the Lledr valley, near Gethin's Bridge.

The small patch of fluvioglacial sand and gravel, exposed at Alexandra Slate Quarry [SH 519 561], on Moel Tryfan, in the north-west corner of the district, has been referred to as an 'internationally famous site' (Campbell and Bowen, 1989). Such a status has been conferred because the sands, at higher than 300 m above OD, contain marine shells and their discovery by Trimmer (1831) initiated a controversy which Reade (1893) described as 'a battleground of contending theories'. Proponents of the theory that all glacial drift was deposited from floating ice, following a major rise in sea level and a widespread inundation of the land, found strong support in the occurrence of these shells. However, it was subsequently realised that there was a more plausible explanation; that the sand and shells were incorporated into the base of the Irish Sea ice sheet and were transported inland as far as Moel Tryfan. Eventually, when the ice melted, the sand and shells were deposited by meltwater streams.

The Moel Tryfan shelly sands are now restricted to a small outcrop close to the quarry edge in the centre of the complex (Howells et al., 1981) and should not be disturbed as there is a wealth of published information (see Campbell and Bowen, 1989). Prior to the expansion of the quarry, in the early years of the century, about 7 m of drift was exposed above the slate but descriptions were variable with some suggestion of sand and gravel being interdigitated with the boulder clay. Currently the small outcrop of laminated shelly sands contain slabs of cemented sands with well-preserved flute-mark casts on their bases (Plate 13).

The first detailed faunal analysis by Darbyshire (1863) distinguished 56 different species of molluscs. The age of the deposit continues to be a matter of debate but both radiocarbon and amino acid measurements of the faunas indicate a Late Devensian age.

Scree and head

Scree deposits mantle most of the steep slopes in the district and, although they were initiated in periglacial times, many are still active. Typical are the screes accumulating about either side of Crib Goch, which are derived from the rhyolite in the ridge and facilitated by the closely spaced joints, the frequently harsh winter climate at this elevation and the continuous dislodgement of blocks by the walkers along the very restricted track. However, all the steep walls of the high cwms have extensive accumulations of active screes.

Thin head is invariably present on the higher slopes, not covered by boulder clay, and is also common, but difficult to define, on the boulder clay. The processes of solifluction, in-situ decrepitation and possible superficial movement, are affected by many factors and, as a result, the deposits are highly variable. For example, on the broad col, between Mynydd Mawr and Craig Cwm Bychan, up to 3 m of head is exposed, derived entirely from decomposed microgranite, and a small tongue can be traced down the eastern slopes into Cwm Planwydd. Elsewhere, as for example where cleaved siltstones lie close to the surface on steep slopes, the effects of hill creep and the initiation of head accumulation and solifluction can be more clearly distinguished. Road sections through the Forestry Commision's plantations, for example those on the east-facing slopes of Moel Hebog, have afforded many exposures with overturned bedding, and fragmentation in the 1–2 m of siltstones closest to the surface.

By their very diffuse nature, many of the features produced by these periglacial processes are difficult to categorise and interpretations are often subjective and controversial. Typical are two landforms near Moelwyn Mawr. The first is a series of ridges, littered with boulders and flanked by moraines, which extends into Cwm Croesor and has been interpreted to be a 'fossil rock glacier' (Gray et al., 1981). The second feature comprises a complex of well-developed, vegetated ridge sand furrows with wavelengths of 0.8 m and amplitudes of 0.2 m which are considered to indicate contemporary freezing processes (see Campbell and Bowen, 1989).

Storm gravel beach, marine and estuarine alluvium

A low, 1 m, storm gravel beach can be distinguished between Craig Dhû and Ogof Ogof Dhû, to the east of Criccieth. The dominant alluvium feature in the district is the broad swathe of estuarine alluvium in the lower reaches of the rivers Glaslyn, Nantmor and Croesor, to the north of Porthmadog. The boundary betwen river and estuarine alluvium has not been distinguished but, as the tidal reach on Mon Glaslyn is within 1 km of Pont Aberglaslyn, then most of the flat must be an estuarine feature. The occurrence of rock 'islands' as Hir Ynys, Ynysfor etc. indicates that the drowned relief was very irregular. A similar, but smaller area of estuarine alluvium occurs at the mouth of Mon Dwyryd, south of Penrhyndeudraeth, and here the tidal reach is up to Maentwrog.

Alluvial cone

Alluvial cones mainly occur where streams pass from steep to more-gentle gradients, for example where a hill stream joins the main valley or where the stream course passes from steep solid ground to more-gentle drift-covered slopes. Throughout the district there are numerous examples of both processes. Probably the largest cone systems are those that developed by streams issuing from the Snowdon massif into the flat-bottomed, breached valleys such as the Gwynant and Colwyn valleys. In the Gwynant valley, a complex of cones is particularly well developed on the north side of Llyn Gwynant; in the Colwyn valley, a very large cone is developed south of Ffridd Uchaf [SH 580 515], where the stream gradient decreases.

On the north side of Cwm Penmachno, steeply inclined cones developed on the south side of Craig Blaen y Cwm [SH 745 480], where the streams emerge from rock gorges on to the steep drift-smeared, lower slopes. Such groups of cones superficially resemble scree slopes and they occur at the mouths of steep gulleys in the rock cliffs at the back and sides of the high cwms, such as Cwm Dyli and Cwm Glas.

The composition of the cones reflect the source area of the streams; the grade and shape of the debris is determined by the composition of the clasts, the size of the stream and the distance of transport. The large cones on the north side of Cwm Gwynant lie approximately at 60 m above OD, yet its main source, out of Cwm Dyli, has dropped from about 850 m above OD, in less than 6 km. This youthful active source was capable of shaping and rounding the coarser clasts, even in such a short distance. In the steep cones derived out of the rock gulleys, the transport distances are short, the water ephemeral, and the clasts are generally angular.

River terrace deposits and alluvium

Narrow strips of alluvium are present along most rivers and streams and are particularly prominent in the Machno, Lledr and Gwynant valleys. Thicknesses are not known as the only sections are probably less than 2–3 m of coarse sand, gravel and clays in the current river banks. However it is possible that most valleys contain much greater thicknesses interdigitated with cone deposits. River terraces have been distinguished along many alluvial tracts although there is no indication of any regional significance. Typical are those in the Gwynant valley, to the west of Llyn Dinas, but here they have developed at the confluence with a stream from the vicinity of Hafod y Porth, to the north.

Lacustrine deposits

There are numerous lakes throughout the district as would be expected in an intensely glaciated terrain. The largest is the northern part of Llyn Trawsfynydd with the ailing nuclear power station on its shore. To the north the most prominent are Llyn Dinas and Llyn Gwynant in the Gwynant valley, and Llyn Cwellyn in the Gwyrfai valley, east of Mynydd Mawr. Elsewhere, there are many small lakes in hollows in the glacially scoured slopes and valleys.

The scouring of the valley floor is particularly well displayed in the profile of the Gwynant valley (Addison, 1988). Three rock basins can be distinguished all of which contained, at some time, large lakes. The two lakes remaining, Llyn Gwynant and Llyn Dinas, are gradually being silted up by successive deltas of the Mon Glaslyn. The lowest lake-basin, impounded just to the east of Beddgelert, has been completely infilled.

A similar, though more steeply inclined profile, with rock basins, is that which extends down from Cwm Glaslyn into Cwm Dyli. The lakes, Llyn Glaslyn and Llyn Llydaw, are a distinctive feature although the lacustrine deposits are largely covered by peat in the hollows, between the moraines that skirt the valley sides. The peat has provided a detailed record of vegetational and environmental changes during the Holocene (Ince, 1981, 1983). Elsewhere, as for example in the Gwynant valley, the lacustrine deposits are largely covered by alluvial cones from tributary streams.

Peat

Peat deposits are common throughout the district, in rock basins and hollows commonly marking the sites of old lakes and stream courses. Elsewhere, peat covers extensive slopes of boulder clay although in these situations it is generally thin. However, at Migneint, in the south-east of the district, south of Llyn Conwy, there is probably one of the thickest, if not the biggest, peat deposits in the whole of upland Wales. Its name means swamp, and the high, gently swelling breasts of moorland, around 500 m above OD, is virtually just that, as it is difficult to traverse, for any distance, without almost total immersion. The hill, Carnedd Iago, 538 m OD, seems to be the heart of the ombrogonous bog, as it is from here that the streams radiate, mostly to Afon Conwy, although its connected source is at Llyn Conwy.

The peat contains grasses, sedges and trees and it is commonly reworked into alluvium in the upland streams. In the higher peat flats, such as in Cwm Dyli, the surface of the peat is pocked by small circular peat swells (pingos) caused by freezing. At the top of the swells the peat has dried out and contraction fractures are well developed. Similar features have been recognised on the surface of the broad swathe of peat on the south-east side of Llyn y Gadair [SH 573 513].

Blown sand

Within the district, blown sand is well featured only in low dunes about Morfa Bychan.

Appendix 1

Borehole records

At the time of going to press, 66 borehole records of the district are held in BGS archives, eight of these are more than 30 m deep. Additional information is added as it becomes available.

An index to these records is available on workstation computers at each of the BGS offices and provides rapid access to the location of individual boreholes or to a set of boreholes for any defined area. A list of boreholes can be retrieved either by a place name, National Grid reference or map sheet number. The index contains information on the location and depth of the boreholes and the method of drilling. Plans to add to this data base are at an advanced stage.

The full records are held at BGS, Keyworth. Most are available for inspection by prior appointment, and copies of records may be made; a charge is made for these services, which reflects the storage and maintenance of the archive. Enquiries should be addressed to British Geological Survey, Keyworth, Nottingham, NG12 5GG

Appendix 2

BGS photographs

Over 150 photographs illustrating the geology of the Snowdon district are deposited for reference in the headquarters library of the British Geological Survey, Keyworth, Nottingham NG12 5GG; in the library at the BGS, Murchison House, West Mains Road, Edinburgh EH9 31A; and in the BGS Information Office at the Natural History Museum Earth Galleries, Exhibition Road, London SW7 2DE. The photographs depict details of the various rocks and sediments exposed and also include general views and scenery. A list of titles can be supplied on request. The photographs can be supplied as black and white or colour prints and 2 x 2 colour transparencies, at a fixed tariff.

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Fossil inventory

Chapter 2 — Cambrian

Chapter 3 - Ordovician

Figures, plates and tables

Figures

(Figure 1) Location of the Snowdon district and simplified geology of North Wales.

(Figure 2) Physical features of the Snowdon district.

(Figure 3) Lithostratigraphical subdivisions of the Cambrian sequence.

(Figure 4) Sketch map of the section at Ogof Ddû and graphic log.

(Figure 5) Graphical log of the Dolgellau Formation at Ogof Ddû with faunal distribution.

(Figure 6) Lithostratigraphical subdivisions of the Ordovician.

(Figure 7) Graphic log of the Dol-cyn-afon Formation (Tremadoc Series) at Ogof Ddû with faunal distribution.

(Figure 8) Distribution of the Dol-cyn-afon Formation in southern Snowdonia. Type sections for informal members described in text. Dark tint indicates main sandstone units; paler part is predominantly mudstone.

(Figure 9) Dol-cyn-afon Formation, correlation of members. Stipple indicates main sandstone units; unornamented part is predominantly mudstone.

(Figure 10) Stratigraphical logs illustrating the variable character of the Tremadoc–Arenig boundary in southern Snowdonia. See (Figure 8) for key.

(Figure 11) Selected graptolites from the Snowdon district. All X 5 except for figure d. BGS = British Geological Survey collections. SM = Sedgwick Museum, Cambridge. a. 'Glyptograptus' euglyphus Lapworth. SM A.18852. Nant Ffrancon Group, gracilis Biozone, Tyddyn deucwm, Tremadog. b. Climacograptus bicornis (Hall). SM A.18791. Nant Ffrancon Group, gracilis Biozone, Tyddyn deucwm, Tremadog. c. Dicranograptus celticus Elles & Wood. SM A.18596. Nant Ffrancon Group, gracilis Biozone, Tyddyn deucwm, Tremadog d. Nemagraptus gracilis (Hall), fragment, X 3. SM A.18408. Nant Francon Group, gracilis Biozone, Tyddyn deucwm, Tremadog. e. Dicellograptus cf. exilis Elles & Wood. SM A.118543. Nant Ffrancon Group, gracilis Biozone, Pensyflog, Tremadog. f. Didymograptus cf. murchisoni (Beck). BGS LZA 5729.Nant Ffrancon Group, murchisoni Biozone, Cwm Pennant[SH 5244 4803]. g, h. Didymograptus artus Elles & Wood. SM X.23504, 23505. Nant Ffrancon Group, artus Biozone, 'Llynbywydd' (Llyn Bowydd) near [SH 729 466]. i. Eoglyptograptus dentatus (Brongniart). SM X.23506. Allt Lŵyd Formation, Arenig, hirundo Biozone, Cwm Teigl about [SH 7302 4452] or [SH 7270 4434]? j, k. Aulograptus cucullus (Bulman). SM X.23507, 23508. Allt Lŵyd Formation, Arenig, hirundo Biozone, Cwm Teigl about [SH 7302 4452] or [SH 7270 4434]? l. Acrograptus cf. infrequens (Kraft). SM X.23509, partly restored from counterpart X.23510. Allt Lŵyd Formation, Arenig, hirundo Biozone, Cwm Teigl, about [SH 7302 4452] or [SH 7270 4434]? m. Acrograptus cf. acutidens (Lapworth). SM X.2311. Allt Lŵyd Formation, Arenig, hirundo Biozone, 'slate trial, E flank of Manod Mawr, Cwm Teigl', about [SH 7302 4452] or [SH 7270 4434]? n. Azygograptus lapworthi Nicholson. BGS Zi 1672. Allt Lŵyd Formation, Arenig, Fennian Stage, Pant-y-wrach, north of Penrhyndeudraeth [SH 617 402].

(Figure 12.1) Geological map of the area about the road section on the A 4085, north of Penrhyndeudraeth.

(Figure 12.2) Log of section exposed on A 4085 north of Penrhyndeudraeth showing the sub-Caradoc unconformity and main fossil horizons. See (Figure 8) for additional key. See Smith et al., 1995, fig.3.

(Figure 13) Geological sketch map of the vicinity of Rhyd showing distribution of blocks within the Rhyd Melange.

(Figure 14) Allochthonous raft of Garth Grit and its contact relationships at Rhyd [SH 6340 4191].

(Figure 15) Geological sketch map of the area between Rhyd and Pen y Clogwyn showing the distribution of the mélange and the disturbed beds.

(Figure 16) Cwm Eigiau Formation: graphic log and environmental interpretation of the Moel Hebog Sandstone Member, at Moel Hebog [SH 5705 4680] to [SH 5683 4690] (after Orton, 1988).

(Figure 17) Snowdon Volcanic Group: outcrop of the Lower Rhyolitic Tuff Formation, its various facies and its substrate.

(Figure 18) Sketch map of the outcrop of the Pitts Head Tuff Formation and its relationship with the Llwyd Mawr Tuff.

(Figure 19) Sketch map showing disposition of the fault zones and the shoreface prior to onset of the eruption of the Snowdon Volcanic Group.

(Figure 20) Sketch map of the outcrop and measured sections of the Lower Rhyolitic Tuff Formation.

(Figure 21) Graphic logs and environmental interpretation of the reworked facies of the Lower Rhyolitic Tuff Formation (after Fritz et al., 1990).

(Figure 22) Restored cross-section of the Bedded Pyroclastic Formation and related intrusions in the north-east face of Snowdon, (after Kokelaar, 1992).

(Figure 23) The principal folds and faults within the Snowdon district.

(Figure 24) The distribution and orientation of the cleavage within the Snowdon district.

(Figure 25) The Dolwyddelan Syncline: its form and sections. Stereographic projections (equal area) of poles to bedding. Cross indicates plunge of fold axis.

(Figure 26.1) Bouguer gravity anomaly contour map of the Snowdon district. Gravity values were referred to the National Gravity Reference Net of 1973 (Masson Smith et al., 1974). Bouguer gravity anomaly values were calculated against the Gravity Reference System of 1967 (Woollard, 1979) using a reduction density of 2.7 Mg/m3-

(Figure 26.2) Residual Bouguer gravity anomaly contour map after application of a high-pass (15 km) filter to remove the background field.

(Figure 27) Aeromagnetic anomaly contour map of the Snowdon district. The data were obtained at a nominal ground clearance of 305 m from north-south flight lines, 2 km apart, controlled by west-east tie lines every 10 km.

(Figure 28) Cross-section of the Snowdon district modelled from the geophysical anomalies. See figures 26.1 and 27.1 for profile location. A datum adjustment of -40 nT has been applied to aeromagnetic values.

(Figure 29) Sketch map showing the distribution of ice flow in North Wales.

Plates

(Front cover) Cover photograph Snowdon viewed from the south-west at Moel Hebog.

(Frontispiece) Snowdon face from Miner's Track just below Llyn Glaslyn

(Plate 1.1) Bedding in Maentwrog Formation, Craig Dhû [SH 5227 3745].

(Plate 1.2) Clastic dykes in Maentwrog Formation [SH 5227 3745].

(Plate 1.3) Bedding in Ffestiniog Flags Formation, Portmeirion [SH 5920 3718].

(Plate 1.4) Bedding in Upper Sandstone member, Tremadoc Series [SH 6712 4617].

Plate 2 Examples of fossils from the Mawddach Group. All the species are from the Dolgellau Formation, except for figures t and u which are from the Ffestiniog Flags Formation. All the figured specimens are in the collections of the British Geological Survey. a. Parabolinites? longispinus (Belt), cranidium Hr 1129, X 4, Peltura scarabaeoides Biozone, Ogof Ddû [SH 5135 3795]. b, c. Spaerophthalmus major Lake. Free cheek and cranidium from concretion, RU 9014B and RX 2850, X 8; scarabaeoides Biozone, Ogof Ddû [SH 5135 3795]. d. m. Lotagnostus trisectus (Salter). Latex cast of cephalon Hr 1067A and pygidium Hr 1084, both X 6; scarabaeoides Biozone, Ogof Ddû [SH 5135 3795]. e. i. Trilobagnostus rudis (Salter). Two of Salter's syntypes, GSM 8728, 8723, X 8; scarabaeoides Biozone, Penmorfa Church, about [SH 542 403]. f. Pseudagnostus obtusus (Belt), latex cast of pygidium from concretion, RU 9014A, X 8; scarabaeoides Biozone, Ogof Ddû [SH 5135 3795]. g, h, l. Richardsonella? invita (Salter). g, free cheek Hr 1140A, X 6; scarabaeoides Biozone, Ddû [SH 5135 3795]. h, 1, are two of Salter's syntypes, cranidium GSM 10821, X 4, and pygidium GSM 10823, X 6, both from scarabaeoides Biozone, Penmorfa Church, about [SH 542 403]. j, k. Eoorthis? sp. Internal moulds of brachial and pedicle valves showing strong ribs but (unlike Orusia) no concentric sculpture; Hr 1046, Hr 1044, both X 8; scarabaeoides Biozone, Ogof Ddû [SH 5135 3795]. n. Maladioidella abdita (Salter), cranidium Zv 9508, X 2; Parabolina spinulosa Biozone, Cwm-y-ffynnon cliff [SH 5403 5141]. o, p. Parabolina aff. mobergi W estergard. Cranidium associated with pygidium and two thoracic segments, Zv 9484, X 4; external mould of free cheek (curvature of genal spine emphasised by frontal compression), Zv 9473, X 3; spinulosa Biozone, Cwm-y-ffynnon cliff [SH 5403 5141]. q. Huenella sp. Internal mould of pedicle valve, RU 7123, X 2; scarabaeoides Biozone, Ogof Ddû [SH 5135 3795]. Photograph by courtesy of M G Bassett. r, s. Orusia lenticularis (Wahlenberg). Internal moulds of large brachial valve and small brachial and pedicle valves, showing concentric sculpture; both on Hr 884, both X 5; spinulosa Biozone, Ogof Ddû [SH 5135 3795]. t, u. Lingulella davisii (McCoy) from the Ffestiniog Flags Formation. t, internal mould of pedicle valve, GSM 16856, X 2, from 'Penmorfa'; u, external of brachial (?) valve, GSM 85087, X 2, from 'Tremadoc' (exact locality unknown).

(Plate 3) Complete but: (a) typically deformed example of Angelina sedgwickii Salter, GSM 102629, X 1, with strain-ellipse, and (b) retrodeformed image derived as described by Rushton and Smith (1993). Upper part of the Dol-cyn-afon Formation (Upper Tremadoc, A. sedgwickii Biozone), Y Garth [SH 598 394].

(Plate 4.1) Tremadoc–Arenig series contact, planar. Tan y Grisiau [SH 6800 4510].

(Plate 4.2) Tremadoc–Arenig series contact, angular unconformity, 10 m east of Tan y Grisiau.

(Plate 4.3) Garth Grit Member, Allt Lŵyd Formation, Moel Llys, Rhyd [SH 6404 4188].

(Plate 4.4) Sandstones, slump folded, Allt Lŵyd Formation, Tan y Grisiau [SH 6800 4510].

(Plate 5.1) View south-east from the edge of Manod Mawr, across the Teigl valley, well-featured ground, in the middle distance, acid tuffs, rhyolites, and interbedded silty mudstones, of the Rhiw Bach Volcanic Formation with quartz latite sheets. (A14648).

(Plate 5.2) Acidic ash-flow tuff, with internal planar lamination, Rhiw Bach Volcanic Formation, Manod Slate Quarry [SH 7268 4565] (A14652).

(Plate 5.3) Top of acidic ash-flow tuff overlain by cleaved silty mudstones with thin beds of crystal-rich, tuff turbidites. Manod Slate Quarry (A14653).

(Plate 6) Fossils from the Dol-cyn-afon and Allt Lŵyd formations, and the lower part of the Nant Ffrancon Group. The figures specimens are all in the collections of the British Geological Survey, except g and h which are in the Sedgwick Museum, Cambridge (SM). a, b, d, e. Microparia caliginosa (Salter). Lower part of the Nant Ffrancon Group (basal Caradoc?), Ty-obry, about [SH 6049 3964]; a, lectotype (laterally compressed), GSM 35265, X 2, with the inferred strain-ellipse; b, the same retrodeformed (Rushton and Smith, 1993); d, a paralectotype (obliquly compressed), GSM 32826, X 2, with strain-ellipse; e, the same retrodeformed. c. Dionide atra (Salter), lectotype pygidium, with cephalon of Platycalymene cf. duplicata (Murchison), GSM 25282, X 1.5; Lower part of the Nant Ffrancon Group (basal Cardoc?), Ty-obry, about [SH 6049 3964]. f. Niobella homfrayi (Salter), GSM 59424, X 1; a topotypic specimen from the lower part of the Dol-cyn-afon Formation (Lower Tremadoc, Rhabdinopora flabelliformis Biozone) of Penmorfa Church, about [SH 542 403]. g. Tropidodiscus? llanvirnensis [Hicks), latex cast of SM X.23513, x 3; top part of Allt Lŵyd Formation (upper Arenig, hirundo Biozone), 'Cwm Teigl', about [SH 7302 4452] or[SH 7270 4434]? h. Ogygiocaris cf. seavilli Whitford, SM X.23512, X 1, associated with Didymograptus artus in the lower part of Nant Ffrancon Group (Llanvirn, artus Biozone), 'Llynbywydd'(= Llyn Bowydd) near [SH 729 466]. i. Rhabdinopora flabelliformis socialis (Salter), synrhabdosome, GSM 37503, X 2; lower part of the Dol-cyn-afon Formation (flabelliformis Biozone), Cefn-cyfanedd S of Wern,near [SH 5434 3942].

(Plate 7) Acritarchs (a-i, k-m) and chitinozoan (j) from the Dol-cyn-afon, Y Foel and Nant Ffrancon formations. All specimens are housed in the MPK collection of the Biostratigraphy and Sedimentology Group, BGS, Keyworth. Magnification x 1000 unless indicated otherwise. a. Ancanthodiacrodium ovatum Rasul. MPK 9741. Sample number MPA 24957, Dol-cyn-afon Formation, Cwm Pennant [SH 5359 4896]. b. Cymatiogalea sp. MPK 9742. Sample number MPA 24973, Dol-cyn-afon Formation, Dol-Ifan Githin Quarry, Cwm Pennant [SH 5373 4953]. c. Vulcanisphaera britannica Rasul. MPK 9743. Sample number MPA 24957. d. Stelliferidium fimbrium (Rasul) Fensome et al., 1990. MPK 9744. Sample number MPA 24957. e. Stellechinatum celestum (Martin) Turner, 1984. MPK 9745. Sample number MPA 24562, Y Foel Formation, west of Y Foel, Cwm Pennant [SH 5197 4557]. f. Frankea sartbernardensis (Martin) Colbath, 1986. MPK 9746. Sample number MPA 24576, Y Foel Formation, Cwm Llefrith [SH 5458 4662]. g. Striatotheca quieta (Martin) Rauscher, 1973. MPK 9747. Sample number MPA 24559, Y Foel Formation, south-west of Y Foel, Cwm Pennant [SH 5214 4533]. h. Arkonia? sp. MPK 9748. Sample number MPA 24560, Y Foel Formation, south-west of Y Foel, Cwm Pennant [SH 5214 4533]. i. Baltisphaeridium sp. MPK 9749 . Sample number 23018, Nant Ffrancon Formation, Moel Dyrnogydd [SH 6970 4973]. j. Spinachitina bulmani (Jansonius). MPK 9750. X 400. Sample number MPA 23017, Nant Ffrancon Formation, Moel Dyrnogydd [SH 6974 4979]. k. Dicrodiacrodium anconforme var. minutum Burmann, 1968. MPK 9751. Sample number MPA 23018. 1. Veryhachium triangulatum Konzalova-Mazancova. MPK 9752. X 400. Sample number MPA 23024, Nant Ffrancon Formation, Moel Dyrnogydd [SH 6968 4959]. m. Orthosphaeridium ternatum? (Burmann) Eisenack, Cramer and Diez. MPK 9753. X 400. Sample number MPA 23024.

(Plate 8.1) Raft of Garth Grit member in Rhyd Mélange see (Figure 14) (A14633).

(Plate 8.2) Disturbed beds [SH 6226 4172].

(Plate 8.3) Asymmetric ripples. Moel Hebog Sandstone Member. Mynydd Gorllwyn (A14626).

(Plate 9) Caradoc shelly fossils from the Nant Ffrancon Group and Cwm Eigiau Formation. All specimens figured are in the collections of the British Geological Survey. a. Broeggerolithus broeggeri (Bancroft), fragmentary cranidium DJ 4928, X 4; Cwm Eigiau Formation (Soudleyan Stage), Moel Meirch [SH 6614 5028]. b. Dinorthis berzvynensis angusta Williams, brachial valve, DJ 5083, X 2; Cwm Eigiau Formation (Soudleyan Stage), Moel Meirch [SH 6590 5000]. c. Glyptarca' quadrata (Salter), internal mould Zv 638, X 1.5; Cwm Eigiau Formation (Longvillian?), S of Pant-y-pwll, about [SH 808 549]. d. Flexicalymene planimarginata (Reed), internal mould of cranidium, DJ 5246, X 3; Cwm Eigiau Formation (Longvillian Stage), Mon Dylfi [SH 6218 4512]. e, f. Sowerbyella sericea permixta Williams, internal moulds of pedicle and brachial valves DJ 4632, DJ 4633, both X 2; Cwm Eigiau Formation (low Soudleyan Stage?), W of Cnicht [SH 6384 4678]. g. Siluraster caractaci (Gregory) with Cyrtolites nodosus (Salter) to left; latex cast of GF 3548, X 2; Cwm Eigiau Formation (Soudleyan Stage), south-east of Roman Bridge Halt[SH 7163 5121]. h. Broeggerolithus broeggeri (Bancroft), latex cast of fringe, DJ 6326, X 2; Nant Ffrancon Group (Soudleyan Stage?), Moel Dyonogydd [SH 6922 4948]. i. Plaesiomys multifida (Salter), internal mould of pedicle valve DJ 4438, X 1.5; Cwm Eigiau Formation (upper Soudleyan Stage), Hafodydd Brithion [SH 6444 4900]. j, n. Dolerothis tenuicostata Williams, internal moulds of pedicle and brachial valves, DJ 5349 and DJ 5340, X 1.5. Nant Ffrancon Group (Costonian Stage?), Ceseiliau Duon [SH 6571 4450]. k, o. Nicolella humilis Williams, external of brachial valve and internal mould of pedicle valve, DJ 5363 and DJ 5291, X 2; Nant Ffrancon Group (Costonian Stage?), Ceseliau Duon [SH 6571 4450]. l, m. Salopia? globosa (McCoy), internal moulds of pedicle and brachial valve, RU 4117, RU 4125, both X 2; Cwm Eigiau Formation (Soudleyan Stage), Cwm Penamnen [SH 7353 5117]. p, q. Harknessella cf. subquadrata Bancroft, internal moulds of brachial and pedicle valve, DJ 5346 and DJ 5318, X 2; Nant Ffrancon Group (Costonian Stage?), Ceseiliau Duon [SH 6571 4450].

(Plate 10.1) Parataxitic foliation [SH 568 467] (A14667).

(Plate 10.2) Brecciated tuff [SH 568 469] (A14674).

(Plate 10.3) Yr Arddu Tuffs. Siliceous nodules at top of acidic ash-flow tuff, near Llyn Du'r Arddu [SH 628 467] (A14434).

(Plate 10.4) Flow-banded intrusive rhyolite. Yr Arddu [SH 627 463] (A14440).

(Plate 11.1) Y Lliwedd, peak and face in Lower Rhyolitic Tuff Formation, overlain, to left along ridge, by Bedded Pyroclastic Formation, with Upper Rhyolitic Tuff Formation in core of small syncline [SH 645 548] (A14391).

(Plate 11.2) View south-eastwards along the southern edge of the Snowdon Horseshoe. Llyn Glaslyn and Llyn Llydaw and the Lliwedd ridge (A14385).

(Plate 12.1) Dolerite sills intruding lower Ordovician silty mudstones in scarp above Tremadog village [SH 5918 3804] (A14618).

(Plate 12.2) Clogwyn Brith fold.

(Plate 13) Sole marks on bedding plane of cemented fluvioglacial sand. Alexandra Quarry [SH 518 560] (L 2337).

(Back cover)

(Succession) Geological succession in the Snowdon district.

Tables

(Table 1) Chronostratigraphical and lithostratigraphical subdivisions of the Cambrian in northern (N) and southern (S) part of the Snowdon district.

(Table 2) Lithostratigraphical subdivisions of the Tremadoc Series in the Snowdon and Harlech districts.

(Table 3) Ordovician chronostratigraphy with an approximate correlation of the shelly stages with the graptolite zones.

(Table 4) Correlation of proposed deformations across North Wales.

(Table 5) Summary of laboratory-determined, physical-property values of rock samples from Snowdonia.

Tables

(Table 4) Correlation of proposed deformation across North Wales.

Shackleton (1952, 1954) Helm et al. (1963), Roberts (1967) Lynas (1970) Coward and Siddans (1979) Harlech district Snowdon district
F3: open folds, strain-slip cleavage (S3); NW strike F4: small-scale folds, axial-planar strain-slip cleavage; NW strike F4: late cross-cleavage (S4) NE-trending folds (end-Silurian) F2: late crenulation cleavage; variable strike NW to W, to N–S
F2: low-angle, axial-planar strain-slip cleavage (S2), NE strike F3: not seen
Main Caledonian deformation; NE strike Main deformation and cleavage (S2), NE strike F2: Main deformation folds and cleavage F2: main deformation, F1: Main deformation folds and cleavage (S1), down-dip grain elongation, variable arcuate orientation
F1: early low-angle cleavage; NW strike F1: early flat-lying cleavage
End-Ordovician movement; NE strike NNW-trending folds, late Ordovician
Mid-Ordovician movments Main extensional phase; Ordovician faulting and subsidence; NE–SW strike
Pre-Arenig movments; NE strike Pre-Arenig N–S-trending faults, pre-Tremadoc NNE–N-trending folds Pre-Arenig N–S trending folds

(Table 5) Summary of laboratory-determined, physical-property values of rock samples from Snowdonia.

Rock type [no. of samples] Saturated density Mg/m3 Grain density Mg/m3 Magnetic susceptibility 10−3 SI units
Acid tuff [19] 2.64 ± 0.04 2.67 ± 0.03 0.3–1
Rhyolite [6] 2.65 ± 0.05 2.69 ± 0.03 0.2–1
Diorite [3] 2.75 2.77 0.5–4
Granodiorite [6] 2.70 ± 0.04 2.73 ± 0.03 1–22
Granite [6] 2.59 ± 0.03 2.64 ± 0.01 0.5–10
Quartz latite [6] 2.71 ± 0.04 2.76 ± 0.01 10–30
Dolerite [10] 2.92 ± 0.05 2.97 ± 0.03 1–40