Figures

(Figure 1.1) Distribution of Cambrian and Ordovician rocks in the principal terranes in Great Britain, after Bluck et al. (1992). HBF. Highland Boundary Fault: SUF, Southern Upland Fault.

(Figure 1.2) Palaeogeographical sketch-maps of the world, showing the changing relative positions of England, Wales and Scotland through the Cambrian and Ordovician. Adapted from maps generated by Dr David Lees using Atlaswinpro (Cambridge Paleomap Services).

(Figure 1.3) Simplified palaeogeographical maps for the Cambrian and Ordovician of England and Wales, modified from Cope et al. (1992). In (c) the active volcanic areas are A, Arenig; B, Builth; C, Cadair Idris; F, Fishguard. In (d) the volcanic areas are AM, Aran and Moelwyn; BVG, Borrowdale Volcanic Group; C, Cadair Idris; 13, Duncannon.

(Figure 2.1) Distribution of Cambrian outcrops in England and Wales.

(Figure 2.2) Cambrian chronostratigraphy and trilobite zones in England and Wales, with Lower Cambrian schemes for south-east Newfoundland and Morocco shown for comparison. The base of the Cambrian System is defined at the base of the Phycodes pedum Zone at Fortune Head, south-east Newfoundland. For sources of radiometric dates, see Davidek et al. (1998) and landing et al., 1998.

(Figure 3.1) Correlation of the principal Cambrian sequences in North Wales, modified from Rushton (1974, fig. 2). The arrows in this and succeeding figures indicate the stratigraphical ranges of individual GCR sites.

(Figure 3.2) Geological sketch-map of the Harlech Dome, after the British Geological Survey (1994b). Cambrian and Tremadoc GCR sites are as follows: 1, Ogof Ddû; 2, Tyn-llan and Wern; 3, Y Garth; 4, Afon Llafar; 5, Amnodd Bwll; 6, Nant-y-graean; 7, Bryn-llin-fawr; 8, Rhobell-y-big and Dol-cyn-afon.

(Figure 3.3) Geological map of St Tudwal's Peninsula, after Nicholas (1915) and Young et al. (1994).

(Figure 3.4) Barmouth, viewed from the west. To the left, greywackes of the Rhinog Formation (Lower Cambrian) form wooded outcrops above the town. The smoother terrain beyond is occupied by the Hafotty Formation (St David's Series), the manganiferous basal beds lying approximately along the line of the track that extends from the church (centre right, arrowed) obliquely up the hill to the left. The rougher highest ground consists of Barmouth Formation greywackes, with the Gamlan, Clogau and Maentwrog formations beyond. The far side of the Mawddach Estuary is made up of upper Cambrian and lower Ordovician rocks. (Photo: Cambridge University Collection of Air Photographs, BST 038: copyright reserved.)

(Figure 3.5) Cliffs on the west side of St Tudwal's Peninsula, looking north-east, showing the type development of the Hell's Mouth Formation. The headland on the right is Trwyn Carreg-y-tir, and the beds with trilobites lie near the top of the cliff. The Trwyn y Fulfran Formation underlies the smoother ground beyond and the trial-pits for manganese ore show dark. The cultivated ground behind is underlain by the Tudwal Sandstone Formation of Arenig age. (Photo: Cambridge University Collection of Air Photographs, 70K EH5: copyright reserved.)

(Figure 3.6) Middle–Upper Cambrian boundary at Porth Ceiriad, St Tudwal's Peninsula. The figures are standing in front of dark mudstones of the upper part of the Nant-y-big Formation. The composite calcareous unit at the boundary of the Nant-y-big and Maentwrog formations reaches beach level at the right of the photograph, and is overlain by sandstone beds of the Maentwrog Formation. (Photo: A.W.A. Rushton.)

(Figure 3.7) Geological map of Afon Llafar, after Allen et al. (1981, fig. 3).

(Figure 3.8) Cambrian trilobites from North Wales. (a, b) Ptychagnostus punctuosus, cephalon and pygidium, x4, from the Clogau Formation (St David's Series) in Afon Llafar. (c) Olenus micrurus Salter, x4, from Maentwrog Formation (Merioneth) of Nant Ganol. (d) Peltura scarabaeoides (Wahlenberg), x 3, from Dolgellau Formation of Rhobell-y-big.

(Figure 3.9) Stratigraphical succession east of Ogof Criccieth, measured by D.R.A. Ponsford (unpublished), with zonal stratigraphy from Howells and Smith (1997, fig. 5).

(Figure 3.10) Sketch map of the section east of Ogof Ddû, after Ponsford (unpublished).

(Figure 3.11) The Dolgellau Formation exposed at Ogof Ddû. (a) The thickest of the beds of volcaniclastic sandstone in the scarabaeoides Zone. Zircon crystals from this bed yielded an age of 491±1 Ma. (b) A large calcareous concretion in the scarabaeoides Zone, lying about 1 m below the sandstone in (a). Bedding lamination is visible dipping to the left just above the hammer handle; cleavage dips more steeply to the left. (Photos: J.K. Prigmore.)

(Figure 3.12) Geology of the section along the north side of Llyn Padarn through the Fachwen Syncline and Gallt-y-foel Anticline, after Wood (1969), with stratigraphical terminology modified by Reedman et al. (1984).

(Figure 3.13) Conglomerate and sandstone of the Fachwen Formation (Lower Cambrian), dipping to the left and cleaved almost vertically. Railway cutting, north side of Llyn Padarn, Snowdonia. (Photo: J.K. Prigmore.)

(Figure 3.14) Alexandra Quarry, Moel Tryfan, looking south-west, showing vertically cleaved Llanberis Slate dipping at about 45° to the left. The Dorothea Grit to the right does not show the cleavage conspicuously. (Photo: British Geological Survey photographic collection, L2333.)

(Figure 3.15) Cwm Graianog, Nant Ffrancon, looking north-west. Large ripples on the upper surface of a bed of quartzose sandstone in the Upper Cambrian Carnedd y Filiast Grit. (Photo: J.K. Prigmore.)

(Figure 3.16) The Cambrian–Ordovician unconformity at Cwm Graianog (arrowed). Overlying the Carnedd y Filiast Grit (close foreground, pale) are thinner bedded flags of the Marchlyn Formation (Merioneth Series) that are overstepped uphill by the base of the Graianog Sandstone of Arenig age. (Photo: J.K. Prigmore.)

(Figure 4.1) Correlation of the principal Cambrian sequences in South Wales and England, modified from Rushton (1974, figs 2, 3). The stratigraphical ranges of the GCR sites are indicated. For the location of Treffgarne Bridge, see (Figure 8.1).

(Figure 4.2) Sketch of the Cambrian geology between St David's and Solva, south-west Wales, after the British Geological Survey (1973), with locations of the GCR sites.

(Figure 4.3) St Non's Bay, a classic section in the St David's area, looking east. To the left of the sea-cave (centre) are Pebidian rocks (Precambrian). To its right is the basal conglomerate of the Caerfai Group, overlain by the St Non's Sandstone, both slightly overturned to the south. The outcrop of the red Caerfai Bay Shale is below where the stone wall descends to the cliff-top, and the dark cliffs to the extreme right are made up of Caerbwdy Sandstone. (Photo: British Geological Survey photographic collection, A6083.)

(Figure 4.4) Caerfai Bay, looking east. Thickly bedded Caerbwdy Sandstone dips steeply to the South, overlain by more thinly bedded units of the Solva Group, a considerable part of which is faulted out. At the extreme right is a dolerite intrusion associated with part of the Menevian Group. (Photo: British Geological Survey photographic collection, A6088.)

(Figure 4.5) Geological map of Trwyncynddeiriog, after mapping by Lewis (1987, unpublished).

(Figure 4.6) Geological map of Porth-y-rhaw, after Jefferies et al. (1987).

(Figure 4.7) Eastern side of Porth-y-rhaw, showing dark mudstones of the lower and middle Menevian Group intruded by a dolerite sill (near the caves). The upper Menevian is at the extreme seaward end of the outcrop. (Photo: R.M. Owens.)

(Figure 4.8) Middle Cambrian Trilobites from South Wales. (a) Onymagnostus davidis (Hicks), x4, from Solva Harbour. (b) Eodiscus punctatus (Salter), x8, from Porth-y-rhaw. (c) Bailiella lyelli (Hicks), x3, from Trwyncynddeiriog. (d) Plutonides hicksii (Hicks), x2.5, from Dwrhyd. (Photos: M. Lewis.)

(Figure 4.9) Geological sketch-map of Solva Harbour, after Lewis (1987, unpublished).

(Figure 5.1) Ercall Quarry (Quarry 3 of Toghill and Beale, 1994), looking north-east. The Ercall Granophyre (Precambrian), is the pale, non-bedded rock on the far left of the photograph. The Wrekin Quartzite, centre, is pale and thickly bedded, and is overlain on the right by a darker cliff of Lower Comley Sandstone. (Photo: C. D. Prosser.)

(Figure 5.2) Cambrian fossils from English sites. (a, b) Mobergella cf. radiolata Bengtson, x 12, from the basal Comley Sandstone (Comley Series, Tommotian) of Ercall Quarry. (c) Tomagnostus fissus (Linnarsson), x10, from the Abbey Shales (St David's Series), Illing's trenches, at Hartshill Hayes. (d) Olenus gibbosus (Wahlenberg), x6, from the Outwoods Shales (Merioneth Series), Purley Quarry.

(Figure 5.3) (a) Comley Quarry, looking south, showing the rock succession as drawn by Cobbold between 1906 and 1914. (b) Lower to Middle Cambrian succession seen in 1933 in Cobbold's 'Excavation No. 2', south of Comley Quarry, after Cobbold (1933). This section shows the full succession of Comley Limestones Ac3 to Ad. Note the difference in scale between (a) and (b). See also (Figure 5.4).

(Figure 5.4) Cobbold's Excavation No. 2, 200 yards south of Comley Quarry, photographed in August 1929 (see (Figure 5.3)b. The hammer rests on the Lapworthella Limestone (Ad), with the upper Comley Sandstone to the left. The Protolenus Limestone (Ac5) lies to the right of the hammer, underlain by the Strenuella Limestone (Ac4), which appears as two massive beds. The Eodiscus bellimarginatus Limestone (Ac3) is a little paler and forms a thicker bed. The rubbly beds to the right (with folding ruler lying across them) are the Red Callavia Sandstone (Ac2), with the lower Comley Sandstone (Ab4) beyond. (Photo: British Geological Survey photographic collection, A4857.)

(Figure 5.5) Boon's Quarry, north-west of Nuneaton. Spheroidally weathered Precambrian tuffs of the Caldecote Volcanic Formation overlain unconformably (at arrows) by immature conglomeratic sandstones of the Boon's Member of the Hartshill Formation (Lower Cambrian). (Photo: British Geological Survey photographic collection, A14973.)

(Figure 5.6) Woodlands Quarry Hartshill, Nuneaton area. Photograph taken in 1913, looking north. The calcareous Home Farm Member of the Hartshill Formation forms a bedding plane on the right, above the right-hand skip. The massive sandstones overlying are the Woodlands Member, succeeded by the basal beds of the Purley Shale Formation, which extend to the quarry floor behind the white-shirted quarryman. The massive rocks forming the left-hand quarry face are a lamprophyre intrusion. (Photo: British Geological Survey photographic collection, A1635.)

(Figure 6.1) Distribution of Ordovician outcrops in Wales, England and Scotland.

(Figure 6.2) Chronostratigraphy of the Ordovician of England and Wales, correlated with the graptolite zonation. Selected ages (in millions of years) from the study of radioactive isotopes are shown to the right. Sources: 1, Tucker et al. (1990); 2, Compston and Williams (1992); 3, Landing et al. (1997); 4, Davidek et al. (1998); 5, Tucker and McKerrow (1995).

(Figure 7.2) Outcrops of Tremadoc rocks in Wales and central England, after Bulman and Rushton (1973).

(Figure 7.1) Generalized sequences of Tremadoc rocks in North Wales and Shropshire, showing stratigraphical ranges of individual GCR sites. For locations of sites see (Figure 3.2), (Figure 7.9), (Figure 9.1) and (Figure 10.1).

(Figure 7.3) Bryn-llin-fawr, as excavated in 1978, looking south. The figure on the lett is working at the Rhabdinopora beds at the base of the Dol-cyn-afon Formation which dip to the left and strike towards the viewer. The next figure stands near the volcaniclastic sandstone beds that lie close to the boundary between the Merioneth and Tremadoc series. The other figures stand near the top of the Dolgellau Formation, on the beds with the Acerocare Zone fauna. (Photo: A.W.A. Rushton.)

(Figure 7.4) Section at Bryn-llin-fawr, as measured in 1978 by S.F. Tunnicliff, with fossil distribution from Rushton (1982, fig. 3) and Fortey et al. (1991, fig. 1).

(Figure 7.5) Rhabdinopora flabelliformis cf. parabola (Bulman), X 2. This taxon, from the basal beds of the Dol-cyn-afon Formation at Bryn-llin-fawr, is characteristic of the lowest Tremadoc. (Drawing by Dr P Legrand, published with his permission.)

(Figure 7.6) Pen Benar, Abersoch. Laminated sandstone units in mudstones of the lower part of the Dol-cyn-afon Formation, such as are not commonly developed in the flabelliformis and tenellus zones. (Photo: J.K. Prigmore.)

(Figure 7.7) Tremadoc fossils from the site at Wern. (a) Niobella homfrayi smithi Stubblefield, x0.65. (b) Rhabdinopora flabelliformis socialis (Salter), x1.5, paralectotype from Cefn Cyfanedd.

(Figure 7.8) Y Garth Quarry, west of Penrhyndeudraeth, seen from the west. The quarry exploits a dolerite intrusion into the Upper Mudstone Member (or 'Garth Hill Beds') of the Dol-cyn-afon Formation. The Upper Mudstone dips north (to the left), making weak features on the wooded hillside between the quarry and river (Afon Glaslyn). (Photo: Cambridge University Collection of Air Photographs, 70Kn EN59: copyright reserved.)

(Figure 7.9) Fossils from Tremadoc sites. (a) Adelograptus tenellus (Linnarsson), x 3, Cwm Crymlyn. (b) Shumardia (Conophrys) salopiensis Callaway, x12, Sheinton Brook. (c) Beyrichona triceps Matthew, x 25, Coundmoor Brook, Evenwood.

(Figure 7.10) Geological map of the Shineton Shales of the Wrekin district, after Stubblefield and Bulman (1927, pl. 5). Coundmoor Brook includes the Tremadoc GCR site south-east of Evenwood and the type Harnage Shale locality south-west of Harnage (Chapter 10).

(Figure 7.11) Geological sketch-map of the Tremadoc rocks from the area of Granham's Moor to Linley Big Wood, with vertical sections exposed in each area, after Fortey and Owens (1992, fig. 1).

(Figure 7.12) Granham's Moor Quarry, 1 km east of Habberley [SJ 3886 0350]. The vertical beds of the Stiperstones Quartzite Formation (Arenig) strike SSW. The underlying flaggy beds of the uppermost Habberley Formation (Upper Tremadoc) are just beyond the left edge of the photograph. (Photo: British Geological Survey photographic collection, A4808.)

(Figure 8.1) Distribution of Ordovician (Arenig to Ashgill) rocks in south-west Wales, after British Geological Survey (1994c). Locations of GCR localities as follows: 1, Ogof Hên and Road Uchaf; 2, Pwlluog; 3, Abereiddi Bay; 4, Abergwaun; 5, Sholeshook; 6, Robeston Wathen; 7, Bryn-banc; 8, Llanfallteg; 9, Pontyfenni; 10, Mylet Road; 11, Meidrim; 12, Dan-lan-y-castell; 13, Glan Pibwr; 14, Allt Pen-y-coed; 15, Cwm yr Abbey; 16, Birdshill; 17, Crag; 18, Dynevor Park; 19, Ffairfâch; 20, Talar Wen. Also 21, Treffgarne Bridge (Upper Cambrian, see Chapter 4).

(Figure 8.2) Correlation of the principal Arenig to Ashgill successions in south-west Wales, showing the stratigraphical ranges of individual GCR sites. Note that the upper Caradoc stages are not separately distinguished in this figure.

(Figure 8.3) Geological map of Ogof Hên and Road Uchaf, including the type section of the Ogof Hên Formation, after Kokelaar et al. (1985).

(Figure 8.4) Fossils from Arenig and Llanvirn sites in South Wales. (a) Falcatodonta costata Cope, x4.5, from the Bolahaul Member (Moridunian) at Dan-lan-y-castell. (b) Merlinia selwynii (Salter), x2.5, Pibwr Member (Moridunian), Glan Pibwr. (c) Portedieldia punctata (Crosfield and Skeat), x8, Cwm yr Abbey Member (Moridunian), Cwm yr Abbey. (d) Hyolithid conch and operculum, x 5, and (e) Bergarnia rushtoni Fortey and Owens, x 3.5, both from the Pontyfenni Formation (Fennian), Pontyfenni. (I) Undulograptus austrodentatus (Harris and Keble), x6, Llanfallteg Formation (basal Abereiddian), Llanfallteg.

(Figure 8.5) Location of exposures in the lower part of the Pibwr Member, after Fortey and Owens (1978, fig. 4). The base of the Pibwr Member and of the Carmarthen Formation are taken here.

(Figure 8.6) Allt Pen-y-coed stream section, exposing the basal stratotype of the Cwmffrŵrd Member, the middle member of the Carmarthen Formation, after Fortey and Owens (1978, fig. 5).

(Figure 8.7) Cwm yr Abbey stream section, exposing the type section of the Cwm yr Abbey Member of the Carmarthen Formation and the stratotype base of the Whitlandian Stage, after Fortey and Owens (1978, fig. 5).

(Figure 8.8) Pwlluog, St David's, south-east corner. To the extreme left and on the foreshore are dark, almost vertical, shales of the Penmaen Dewi Formation. In the background, on the northern side of Trwynhwrddyn, are grey, silty sediments probably referable to the Abercastle Formation, faulted against the Penmaen Dewi Formation in the corner of the inlet. (Photo: R.M. Owens.)

(Figure 8.9) Fossil distribution across the Arenig–Llanvirn boundary in the Llanfallteg section, after Fortey and Owens (1987, fig. 9). 'B' shows the approximate level of the thin bentonite beds. The range of U. cumbrensis is from Mitchell (1992, fig. 7).

(Figure 8.10) Sketch-map of the geology around Abereiddi Bay and Llanvirn, after Hughes et al. (1982, figs 5, 6). This is the historical type area for the Llanvirn Series.

(Figure 8.11) Abereiddi Bay, looking north-west to Trwyncastell. To the right, the low black cliff near sea level is the Castell Limestone (basal Caradoc), sub-vertical but slightly overturned, with the Caerhys Shale (mainly Llandeilian) behind it and extending farther to the left, beyond the ruined black buildings. The paler cliffs behind and Trwyncastell (the headland with the tower) are made up of Llanrian Volcanic Formation (upper Abereiddian, murchisoni Zone). (Photo: R.M. Owens.)

(Figure 8.12) Didymograptus murchisoni (Beck), x2, from the Caerhys Shale Formation at the southern end of Abereiddi Bay.

(Figure 8.13) Llanvirn Quarry, south of Abereiddi Bay, looking south-east, photographed in 1932. The Aber Mawr Formation (lower Abereiddian, artus Zone) dips steeply north, the cleavage here being nearly coincident with bedding. (Photo: British Geological Survey photographic collection, A6108.)

(Figure 8.14) Abergwaun, immediately east of Fishguard, looking north. The Afon Gwaun provides a dip-section across the southern limb of the Goodwick Syncline. The foreground is underlain by Abereiddian slates. The acidic and basic lavas of the Fishguard Volcanic Group form bluffs at the mouth of the inlet. In the distance the cliffs are of acid tuffs on the northern limb of the Goodwick Syncline. (Photo: British Geological Survey photographic collection, A6129.)

(Figure 8.15) The geology around Ffairfâch and Afon Cennen, after Williams (1953) and Wilcox and Lockley (1981). The Cennen section is historically the type section for the Llandeilo Series (now Llandeilian Stage).

(Figure 8.16) Lithological and faunal succession in the type section for the Ffairfâch Group, after Williams et al. (1981).

(Figure 8.17) Geological map of Dynevor Park, after Williams (1953).

(Figure 8.18) Marrolithoides simplex (Williams), x4, from the Middle Llandeilo Flags, Talar Wen, Bethlehem.

(Figure 8.19) Geological map south of Meidrim, after Addison (in Bassett et al. 1974, fig. 6).

(Figure 8.20) Road section at Pengawse Hill, west of Whitland. Black Mydrim Shales dipping uphill are overlain conformably by paler Sholeshook Limestone. The transition is marked by alternations of shale with layers of cal-creous nodules and thin beds of limestone. (Photo: J.A. Zalasiewicz.)

(Figure 8.21) Geological sketch-map and cross section around Sholeshook, after Price (1973a).

(Figure 8.22) Sholeshook railway cutting, north-east of Haverfordwest. Photograph taken in 1909, looking east, showing northward-dipping Sholeshook Limestone in its type development. (Photo: British Geologcial Survey photographic collection, A892.)

(Figure 8.23) (a) Normalograptus sp., x 3, a typical graptolite that proliferates in the upper beds of the Mydrim Shales at Pengawse Hill. (b) Transverse section of the bryozoan Kuckersella borealis (Bassler), x30, Slade and Redhill Beds, Pengawse Hill. (c) Eucystis pentax Paul, x4, Sholeshook Limestone, Sholeshook. (d) Archegocystis stellulifera (Salter), x2, Sholeshook Limestone, Sholeshook. (e) Atractopyge verrucosa (Coalman), holotype cranidium, x1.5, from the Crûg Limestone, Crûg.

(Figure 8.24) The geological succession north of Robeston Wathen, after Price (1973a).

(Figure 8.25) Distribution of the principal divisions and generalized vertical section in the Builth–Llandrindod inlier, modified after British Geological Survey (1994c) and Davies et al. (1997, fig. 4). Localities: 1, Howey Brook; 2, Newmead; 3, Bach-y-Graig; 4, Llanfawr Quarry; 5, Gwern yfed fâch Quarry.

(Figure 8.26) Geological map of the Gilwern Anticline and Howey Brook including the 'main feeder' tributary, after Davies et al. (1997, fig. 5).

(Figure 8.27) Geological map of the area around Newmead, after Jones and Pugh (1949) and Earp (1977). The massive basalt forms small inliers, interpreted by Jones and Pugh as stacks that protrude through the Newmead Formation.

(Figure 8.28) Sketch-map of the Bach-y-graig stream section, from Sheldon (1987b, unpublished).

(Figure 8.29) Faunal distribution about the base of the teretiusculus Zone at Bach-y-graig, after Hughes and Sheldon (in Fortey et al., 1991, fig. 5).

(Figure 9.1) Distribution of Ordovician (Arenig to Ashgill) rocks in North Wales, after British Geological Survey (1994c), showing the location of GCR sites. For the Tremadoc site at Pen Benar, see Chapter 7.

(Figure 9.2) Schematic section to show the general stratigraphical relations of the main divisions that make up the Ogwen Group in North Wales, after Rushton and Howells (1998). The positions of GCR sites are shown as follows: 1, Cadnant Cutting; 2, Rhiwlas; 3; Gelli-grîn; 4, Nant Aberderfel; 5, Wîg; 6, Nant y Gadwen; 7, Trwyn Llêch-y-doll; 8, Afon Seiont; 9, Bryn Glas and Hafotty Ffilltirgerig; 10, Llynnau Cregennen.

(Figure 9.3) Correlation of the principal Arenig to Ashgill sites in North Wales, showing the stratigraphical ranges of the GCR sites. Note that the Actonian and Onnian substages of the Streffordian Stage are not separately distinguished in this figure.

(Figure 9.4) Bryn Glas, east of Ffestiniog, looking east over the glaciated plateau of the Migneint (here underlain by Merioneth Series, repeated by faults) towards Arenig. The massive basal Arenig sandstones of the Garth Grit paraconformably overlie silty slate of the Upper Mudstone Member of the Dol-cyn-afon Formation (Tremadoc). The base of the Garth Grit juts out over a shrub that is growing on the top bed of the Dol-cyn-afon Formation. The height of the section is about 3 m. (Photo: A.W.A. Rushton.)

(Figure 9.5) Hafotty Ffilltirgerig, north-west flank of Arenig Fawr. Sandstones with mudstone laminae of the Allt bkyd Formation (Llyfnant Flags of Fearnsides), dipping eastwards, away from the observer. Such sandstones are typical of the lower Arenig and are developed widely around the Harlech Dome. (Photo: A.W.A Rushton.)

(Figure 9.6) Trwyn-llêch-y-doll, St Tudwal's Peninsula, looking north-east. Bioturbated sandstones of the Tudwal Sandstone Formation (Arenig, probably Fennian) form a vertical cliff and overlie the basal beds of the Ceiriad Formation (Cambrian, probably St David's Series) with unconformity. The total height of the cliff is about 45 m. (Photo: A.W.A. Rushton.)

(Figure 9.7) Stratigraphical section through the base of the Wîg Bach Formation at Wîg, with an enlargement (left) of the basal unconformity on the Sarn Complex (Precambrian). The base of the mudstones of the Wîg Member appear above the 'junction bed'. After Beckly (1985, unpublished).

(Figure 9.8) Distribution of rocks of the three stages of the Arenig Series in Nant y Gadwen, after Beckly (1985, unpublished).

(Figure 9.9) Fossils from Nant y Gadwen. (a) Cranidium of the Moridunian trilobite Merlinia selwynii (Salter), x 2.5. (b) The Fennian graptolite Isograptus caduceus gibberulus (Nicholson), x 4.

(Figure 9.10) Stratigraphical succession showing the Arenig to Llanvirn succession along Afon Seiont, after Fortey et al. (1990, fig. 3).

(Figure 9.11) Llynnau Cregennen, viewed from Barmouth, looking ESE towards Cadair Idris (the highest ridge). The dark scarp below and to the right of Cadair Idris is Tyrrau Mawr, made up of Aran Volcanic rocks intruded by microgranite. The lakes Llynnau Cregennen appear as dark expanses directly below the summit of Cadair Idris; and, below them, steep wooded slopes around the settlement of Arthog are mostly of Dol-cyn-afon Formation (Tremadoc). The ground left of Llynnau Cregennen and Arthog is well featured, showing successively higher ridges of the Dol-cyn-afon (quarried for slate), Allt Lŵyd Formation (Arenig), Offrwm Volcanic Formation (Arenig–Llanvirn), and the Bryn Brith and Cefn Hir members of the Cregennen Formation (Llanvirn). (Photo: Cambridge University Collection of Air Photographs, BMZ 22: copyright reserved.)

(Figure 9.12) Llynnau Cregennen, looking north-east towards Pared y Cefn Hir. The lower ground to the left is occupied by fossiliferous Llanvirn slates of the Cregennen Formation, overlain by basic and acid tuffs of the Cefn Hir Member dipping to the right, with debris from a microgranite intrusion littering the dip-slope. (Photo: R.M. Owens.)

(Figure 9.13) Distribution of Ordovician rocks on Anglesey, from British Geological Survey (1994b), with details of the Gynfor inliers from Bates (1972).

(Figure 9.14) Cliffs of Ogof Gynfor viewed from the sea. On the extreme right, surmounted by pale-weathering quartzite, is the Gwna Melange of the Mona Supergroup. It is overlain unconformably by thick sandstones and conglomeratic horizons of the Torllwyn Formation (Arenig), which are disposed in a faulted syncline; within this is a faulted wedge of dark cherry shales of early Caradoc age. The left-hand fault is vertical, whereas the right-hand one is an overthrust. (Photo: D.E.B. Bates.)

(Figure 9.15) Salterolithus caractaci paucus Cave, x3, showing healed injury on the right-hand side of the fringe. Trilobite Dingle.

(Figure 9.16) Sketch map of the Trilobite Dingle Formation in Bron-y-buckley Wood, Welshpool, based on Cave (1957, fig. 1) and including an east-west fault suggested by Dr J. K. Ingham (pers. comm., 1996) that separates the Lower Harnagian mudstones at the northern end of the section from Upper Harnagian strata farther south.

(Figure 9.17) Geological map of the type development of the Gelli-grîn Formation west of Gelli-grîn Farm, from Bassett et al. (1966, fig. 4).

(Figure 9.18) Dorsal valve of Dolerorthis duftonensis prolixa Williams, x2, Gelli-grîn.

(Figure 9.19) Geological map showing the Cadnant Shale Formation and contiguous units in Cadnant railway cutting, and the eastward extension of the lower part of the succession in the temporary road section documented by Wood and Harper (1962, fig. 1).

(Figure 9.20) (a) Geological map showing the occurrence of the Nod Glas Formation and the overlying Trawscoed Formation (Ashgill) to the south of the Gwern-y-brain site, from Cave and Price (1978, fig. 3); (b) more detailed map of the Nod Glas Formation and contiguous units in the Gwern-y-brain section, after Cave (1965, fig. 3).

(Figure 9.21) 'Graveyard' of Gravicalymene arcuata Price, x 5, Cynwyd.

(Figure 9.22) Ashgill fossils from sites in North Wales. (a, b) Ventral and dorsal valves of Vellamo sp., x1.5, Cynwyd. (c, d) Tetraeucystis munita (Forbes), x4, Rhiwlas. (e-g) Brachiopods of the Hirnantia Fauna, Hirnant Limestone of Aber Hirnant: (e) Dorsal valve of Eostropheodonta hirnantensis (M'Coy), x1.5; (1) Dorsal valve of Hirnantia sagittifera (M'Coy), x1.5; (g) Ventral valve of Plectothyrella crassicostis (Dalman), x2.

(Figure 9.23) Geological map of the type locality of the Rhiwlas Limestone along Afon Tryweryn and the largely concealed outcrop at Rhiwlas to the north, after Bassett et al. (1966, fig. 9); see also Paul (1973, fig. 7).

(Figure 9.24) Simplified sedimentary log of the uppermost part of the Deganwy Mudstone Formation and the lower half of the Conway Castle Grits at the southwest end of Deganwy Quarries, after Cullen (1986, unpublished).

(Figure 10.1) Map showing the distribution of Ordovician rocks in south Shropshire and eastern central Wales, from British Geological Survey (1994c). GCR sites as follows: 1, Gwern-y-brain; 2, Trilobite Dingle; 3, Spy Wood and Aldress dingles; 4, Meadowtown; 5, Betton Dingle; 6, Hope Valley; 7, Shelve Church; 8, Bergam Quarry; 9, Mytton Dingle; 10, Granham's Moor (Tremadoc, see Chapter 7); 11, Coundmoor Brook (Harnage); 12, Hope Bowdler; 13, Soudley Quarry; 14, Marshwood; 15, Onny River; 16, Coston Farm; 17, Linley Big Wood (Tremadoc, see Chapter 7).

(Figure 10.2) Correlation of the chronostratigraphical standard and the graptolite zonal succession with the lithostratigraphical succession in the Shelve area, following British Geological Survey (1991); the Tremadoc is from Fortey and Owens (1992).

(Figure 10.3) View eastwards along Mytton Dingle. The Mytton Flags Formation, cropping out on the north side of the valley, dips steeply towards the viewer and strikes obliquely south-west. The ridge on the left is Blakemoorflat; Green Hill, on the right, slopes down into Perkins Beach. (Photo: Cambridge University Collection of Air Photographs, BM 13: copyright reserved.)

(Figure 10.4) Scarp of the Mytton Flags Formation on the east side of Hope Valley, looking north-east. The lower ground is occupied by the Hope Shale Formation, the road following the Mytton–Hope boundary quite closely. The white tip-heap is spoil from Snailbeach Mine. The next nearer cleft is Crowsnest Dingle. (Photo: Cambridge University Collection of Air Photographs, BM 8: copyright reserved.)

(Figure 10.5) Geological map of the type area of the Hope Shale Formation in the Hope Valley, from the British Geological Survey (1991).

(Figure 10.6) Geological map of the area of Betton Dingle and Meadowtown, from the British Geological Survey (1991).

(Figure 10.7) Geological map of Spywood and Aldress dingles and the adjoining dingles, from a map prepared by Dr R. Cave.

(Figure 10.8) Vertical section exposed in the Spy Wood and Aldress areas, prepared by Dr R. Cave.

(Figure 10.9) Geological map of the Onny Valley showing the Caradoc succession in the southern Caradoc area, based on Savage and Bassett (1985, fig. 3), with stratigraphical amendments proposed by Owen and Ingham (1988) and Fortey et al. (1995).

(Figure 10.10) Stratigraphical succession for part of the northern Caradoc area, showing the stratigraphical range of the Coundmoor Brook site near Harnage.

(Figure 10.11) Coston Farm. The Coston Formation (the local equivalent of the Hoar Edge Grit Formation) in the old quarry north of Coston Manor. (Photo: J. K. Ingham.)

(Figure 10.12) Brachiopods from the type Caradoc area. (a, b) Harknessella vespertilio (J. de C. Sowerby), x2, Coston. (c, d) Dinorthis flabellulum (J. de C. Sowerby), x2, Coston. (e, t) Heterorthis alternata (J. de C. Sowerby), x 1.5, Soudley.

(Figure 10.13) Geological map of the area around Coundmoor Brook, south-west of Harnage, after Pocock et al. (1938, fig. 28). For location, see (Figure 7.10).

(Figure 10.14) Roadside section at Hope Bowdler, showing the pebbly lower part of the Harnage Shale Formation resting unconformably on an irregular surface of the Uriconian complex (Precambrian). The hammer head lies at the unconformity. (Photo: J. K. Ingham.)

(Figure 10.15) Sedimentary log of the Alternata Limestone Formation and the lower part of the Cheney Longville Formation at Soudley Quarry, based on Hurst (1979b, fig. 4).

(Figure 10.16) Soudley Quarry. Massive colour-banded Horderley Sandstone overlain by the Alternata Limestone, here consisting of shelly limestones alternating with siltstones. (Photo: British Geological Survey Photographic collection A8888.)

(Figure 10.17) Sedimentary log through the Crosspipes Member of the Cheney Longville Formation in Marshwood, based on Hurst (1979b, fig. 14), showing the base of the Actonian Substage and the three faunal associations recognized by Hurst (1979a; see also Lockley, 1983).

(Figure 10.18) Geological map of the eastern part of the Onny River section showing the bases of the Actonian and Onnian substages and their relationship to the upper Caradoc formational boundaries, after Hurst (1979b, fig. 8), and Toghill (1992, fig. 4), with the amendment to the Onnian lithostratigraphy proposed by Owen and Ingham (1988).

(Figure 10.19) The cliff section at the eastern cnd of the Onny River section, showing how difficult it is to see the slight angular unconformity of the Lower Silurian (upper Llandovery) Hughley Shale Formation above the shales of the uppermost Caradoc Onny Formation (Caradoc, Streffordian, Onnian Substage). (Photo: J. K. Ingham.)

(Figure 10.20) Onnia gracilis (Bancroft), x3, from the Wistanstow Member of the Acton Scott Formation (Streffordian, Onnian Substage).

(Figure 11.1) Geological sketch-map of the Skiddaw Group in the main outcrop of the English Lake District, after Cooper et al. (1995, fig. 2). GCR localities: 1, Trusmadoor; 2, Blaze Bridge and Scawgill Quarry; 3, Barf; 4, Randel Crag; 5, Outerside; 6, River Calder (Tremadoc, Chapter 7).

(Figure 11.2) Generalized vertical section for the Skiddaw Group in the Northern Fells Belt, showing the ranges of the GCR sites and correlation with the British and Scandinavian graptolite zones and the Australasian stages (after Cooper et al. 1995, figs 2, 3).

(Figure 11.3) Araneograptus murrayi (Hall), x 1, from the uppermost Tremadoc strata at Trusmadoor.

(Figure 11.4) Scawgill Quarry, High Lorton. Turbidite sandstones of the lower part of the Loweswater Formation, dipping west at 16°. Some thicker units show complete Bouma a–e cycles and some thinner bedded units are fossiliferous. (Photo: British Geological Survey photographic collection, D3832.)

(Figure 11.5) View of south-east face of Barf, north-west of Keswick, looking north, with Bassenthwaite Lake in the background. At the top of the hill the upper beds of the Loweswater Formation dip steeply to ESE and lower down are inverted. They form big screes from which many fossils have been collected. 'The Bishop' is the white-painted rock near the centre and Slape Crag is the vertical face near the top of the hill. (Photo: British Geological Survey photographic collection, D3838.)

(Figure 11.6) Correlation chart of the upper Ordovician (Dent Group) in the main crop of the Lake District, the Cautley and Dent inliers and the Cross Fell Inlier, and the stratigraphical range shown by the GCR site described herein, based on Kneller et al. (1994, fig. 5). In all areas the Dent Group lies unconformably on the Borrowdale Volcanic Group. The correlation of the graptolite zones with the stages of the Ashgill Series is subject to revision (R. B. Rickards pers. comm. 1999).

(Figure 11.7) Ashgill Beck, showing the type development of the Troutbeck Member of the Ashgill Formation, with the Old Man of Coniston (Borrowdale Volcanic Group) in the background. (Photo: A.W. Owen.)

(Figure 11.8) The southern face of Ashgill Quarry showing the Skelgill Formation in the topmost part of the quarry conformably overlying shales of the uppermost Ordovician Ashgill Formation. (Photo: A.W Owen.)

(Figure 11.9) Geological map showing the Dufton Shale Formation in Harthwaite Sike, based on Burgess and Holiday (1979, fig. 6) and Dean (1959a, fig. 3).

(Figure 11.10) Fossils from the Dufton Shale Formation in Pus Gill. (a, b) A brachiopod of the corona facies (Longvillian), Trematis corona Davidson, x3, with enlargement of sculpture, x8. (c, d) Two views of the Pusgillian trilobite Petaspis moeldenensis Cave (sensu lato) , x 2.

(Figure 11.11) Geological map showing the Dufton Shale Formation in Pus Gill (based on Burgess and Holliday (1979, fig. 8) and Dean (1959a, fig. 1).

(Figure 11.12) Geological map showing the Dufton Shale Formation and Swindale Limestone in and adjacent to the Melmerby Road section, based on Burgess and Wadge (1974, fig. 7) and Arthurton and Wadge (1981, fig. 9).

(Figure 11.13) Geological map showing the Dufton Shale Formation, Swindale Limestone and Ashgill Formation in Swindale Beck, Knock, based on Burgess and Wadge (1974, fig. 6).

(Figure 11.14) Geological map of the Keisley area, after Burgess and Holliday (1979, fig.13). The Ordovician–Silurian boundary section in the trench across the track to the New Quarry was described by Wright (1985).

(Figure 11.15) Keisley New Quarry, the north part, showing several metres of southward-dipping limestone. The lower beds to the left are nodular with siltstone partings. The more massive overlying beds are disturbed by faulting. (Photo: British Geological Survey photographic collection, L1057.)

(Figure 11.16) Geological map of the area around Sally Beck, the River Rawthey and Wandale Beck, in the Murthwaite Inlier of the Cautley district, based on Ingham (1966, pl. 25) and Ingham (1970–1977, fig. 3).

(Figure 11.17) (a) Sphaerocoryphe kingi Ingham, x 5, Zone 6, Wandale Beck, Murthwaite Inlier (Figure 11.16). (b, c) Mucronaspis mucronata (Brongniart), x2, Ashgill Shale Formation, Fairy Gill, Taythes Inlier (Figure 11.19).

(Figure 11.18) Geological map of the Backside Beck area of the Westerdale Infier, Cautley district, based on Ingham (1966, pl. 26) and Ingham (1970–1977, fig. 4).

(Figure 11.19) Geological map of Ecker Secker Beck, Taythes Gill and Fairy Gill in the northern part of the Taythes Inlier, Cautley district, based on Ingham (1966, pl. 27) and Ingham (1970–1977, fig. 6).

(Figure 12.1) Distribution of Cambrian and Ordovician rocks in Scotland, showing the general location of key sites.

Figure 12.2 Stratigraphical succession in the north-west Highlands of Scotland, correlated with Laurentian (North American) chronostratigraphy; the scheme for the Ibex and Whiterock follows Ross et at(1997), the older stages of the Canadian Series being retained for reference to the succession of cephalopod faunas. The Avalonian standard is also shown for comparison. The Leny and Dounans limestones occur along the Highland  Boundary fault complex, and their stratigraphical settings are discussed in the text.

(Figure 12.3) Geological sketch-map of the Durness area, after Geological Survey of Scotland (1893), and schematic section from near Daill to Sangobeg (about 10 km), after Peach et al. (1907, fig. 20).

(Figure 12.4) Balnakeil Bay section, Durness. Algal growth in pale dolomitic limestone of the Eilean Dubh Formation. (Photo: J.K. Prigmore.)

(Figure 12.5) Balnakeil Bay section, looking north. Resistant carbonates of the Sangomore Formation forming a cliff above an eroded platform of the Sailmhor Formation. (Photo: J.K. Prigmore.)

(Figure 12.6) Smoo Cave, east of Durness. Cliffs of flat-lying Sailmhor Formation overlain in conformable succession by the lower beds of the Sangomore Formation. The height of the cliffs is about 30 m. (Photo: British Geological Survey photographic collection.)

(Figure 12.7) Simplified section across Loch Eriboll at An t-Sròn, from Peach et al. (1907, figs 20, 21). The length of the section is about 7 km.

(Figure 12.8) An t-Sròn. Pipe Rock Member of the Eriboll Formation overlain by the Fucoid Beds Member of the An t-Sròn Formation. The contact lies at the base of the thinly-bedded weathered unit a little below the hammer-head. (Photo: J.K. Prigmore.)

(Figure 12.9) An t-Sròn Formation at An t-Sròn. Thinly-bedded unit at the top of the Fucoid Beds Member overlain by massive arenites of the Salterella Grit Member. (Photo: J.K. Prigmore.)

(Figure 12.10) Summit of Meall a' Ghiubhais viewed from the south-west, showing the Kinlochewe thrust plane (K–K). Above the thrust is a nappe of deformed Lewisian, Torridonian and Cambrian rocks. Below it, Torridonian rocks are overlain unconformably by the Eriboll and An t-Sròn formations. (Photo: British Geological Survey photographic collection, C30.)

(Figure 12.11) Fucoid Beds Member in the stream section at Fuaran Mor, Meall a' Ghiubhais. (Photo: J.K. Prigmore.)

(Figure 12.12) Loch Awe Quarry, showing the Fucoid Beds Member. The lower beds, massive dolomitic sandstones, are overlain by thinly bedded units which include fossiliferous beds with Olenellus (Photo: British Geological Survey photographic collection, D1284.)

(Figure 12.13) Simplified sketch-map of the area of Ord, southern Skye, after Bell and Harris (1986, fig. 16a).

(Figure 13.1) Geological map of Leny Quarry, after Pringle (unpublished).

(Figure 13.2) Pagetiid trilobites from Leny Limestone, Leny Quarry. Three cephala, (a) x10, (b) x12, (c) x15, and a pygidium, (d) x15.

(Figure 13.3) Geological map of Lime Craig Quarry showing the faulted Highland Border complex and Lower Old Red Sandstone, from Bluck et al. (1984, fig. 7) and Bluck and Lawson (1992, fig. 10.2).

(Figure 14.1) Schematic cross-section in the Girvan area to show the stratigraphical and structural relationships across the major south-facing growth faults. The GCR sites are shown in bold type. After Ingham (1992b, fig. 30.5).

(Figure 14.2) Ordovician graptolite zones used in Scotland, based on the graptolitic succession in the Moffat Shale Group, and supplemented for older rocks by reference to the Australasian standard stages. British standard chronostratigraphy is shown for comparison.

(Figure 14.3) Geological map of the Ballantrae complex between Balcreuchan Port and Bennane Head, from Stone and Smellie (1988, fig. 16). Numbers 1 to 7 refer to units mentioned in the text.

(Figure 14.4) The graptolite Tetragraptus approximatus Nicholson, x3, from sandstone unit 1 (basal Bendigonian, lower Arenig) of (Figure 14.3).

(Figure 14.5) Gully 500 m south-west of Balcreuchan Port. Pillows of feldspar-phyric basalt lava (unit 5 of figure 14.3) dip steeply towards the observer and their upper surfaces protrude through a thin layer of red chert which contains Tetragraptus approximatus. (Photo: British Geological Survey photographic collection, D3587.)

(Figure 14.6) The south-east face of Aldons Quarry showing the Benan Conglomerate overlying the thin Superstes Mudstone (<2m thick), which in turn overlies the Stinchar Limestone. (Photo: A.W. Owen.)

(Figure 14.7) Geological map of the area near Dalfask, partly after Harper and Owen (1986, fig. 1). The tributary of Laggan Burn yielding the fossil faunas lies immediately north of the thrust which brings the Benan Conglomerate over mudstones of the Balclatchie Group.

(Figure 14.8) Simplified map of the Ordovician rocks along the Girvan Foreshore and the area immediately inland, after Bluck and Ingham (1992a, fig. 28.1a, b).

(Figure 14.9) 'Henderson's Unconformity' near Kennedy's Pass at the southern end of the Girvan Foreshore. Siltstones of the Ardwell Flags Formation overlie the Kilranny Conglomerate at the base of a large channel (scale bar is 1 metre long. (Photo: A.W. Owen.)

(Figure 14.10) Girvan foreshore. Box-fold in the Ardwell Formation, north-east of Kennedy's Pass. (Photo: British Geological Survey photographic collection, D1563.)

(Figure 14.11) Girvan foreshore west of Ardmillan, looking ENE. Steeply dipping fine-grained turbidites of the Three Mile Formation (Lower Whitehouse Group), younging to the left. Hammer (arrowed) above centre for scale. (Photo: British Geological Survey photographic collection, D3527.)

(Figure 14.12) Simplified geological map of Craighead Main Quarry, after Williams (1962, pl. 4) and Ingham (1992c, fig. 31.1). Inset map shows the position of the two smaller quarries to the south-west (see Williams, 1962 for details).

(Figure 14.13) Fossils from the Craighead Limestone, Craighead Quarry. (a, b) Lyopora favosa M'Coy, x 2. (c, d) Rostricellula lapworthi (Davidson), x 3.

(Figure 14.14) Fossils from the Starfish Beds, South Threave. (a) Rhombiferan cystid Pygecystis quadrata Bather, x 2. (b) Asteroid starfish Mesopalaeaster primus (Spencer), x 4. (c) Cornute calcichordate Scotiaecystis curvata (Bather), x 1.5. (d) Trilobite Toxochasmops bisseti (Reed), x 1.5.

(Figure 14.15) Burn, east of South Threave farmhouse showing divisions of the upper Drummuck Group, after Harper (1982b, fig. 11). Note the marked overstep of the Mulloch Hill Conglomerate, the local base of the Silurian.

(Figure 15.1) Correlation of fault-bounded stratigraphical successions in the Southern Uplands Terrane of Scotland that include GCR sites, based on Floyd (1996) and Rushton et al. (1996a, fig. 4). Columns represent the following: 1, Leadhills Imbricate Zone, Crawford district; 2, tract north of the Glen App Fault, Rhins of Galloway; 3, Morroch Bay Fault Zone, Rhins of Galloway; 4, north of lauriston Fault, south of Moffat; 5, Moffat Valley north-east of Moffat.

(Figure 15.2) Corsewall Point, east of the lighthouse, looking north. Beds of coarse conglomerate, dipping steeply and slightly overturned. Most of the large pale boulders are acid volcanic rocks and various granites. (Photo: British Geological Survey photographic collection, D4070.)

(Figure 15.3) Geological sketch-map of Morroch Bay, showing the distribution of graptolite faunas, after Rushton and Tunnicliff (1996, fig. 63).

(Figure 15.4) Port of Spittal Bay, south of Morroch Bay. The pale bed is a quartzose sandstone of the Glenwhargen Formation interbedded with darker greywackes of the Portpatrick Formation. See (Figure 15.5) (Photo: British Geological Survey photographic collection, D3756.)

(Figure 15.5) Comparison of the stratigraphical succession at and north of Morroch Bay with that at Port of Spittal Bay, showing that the onset of deposition of the Portpatrick Formation greywackes was markedly diachronous.

(Figure 15.6) Sketch-map of Black Linn, Glenkiln Burn, showing the principal graptolite localities, based on Lapworth (1879a), Williams (1994), and unpublished work by the British Geological Survey.

(Figure 15.7) Graptolites from Glenkiln Burn (a) and Dob's Linn (b-h). All figures x2. (a) Nemagraptus gracilis (Hall), gracilis Zone. (b) Dicellograptus morrisi Hopkinson, clingani–linearis zones. (c) Dicranograptus ziczac Lapworth, peltifer Zone. (d) Climacograptus wilsoni Lapworth, wilsoni Zone. (e) Dicellograptus anceps (Nicholson), anceps Zone. (f) Lasiograptus harknessi (Nicholson), wilsoni Zone. (g) Climacograptus supernus Elles and Wood, anceps Zone. (h) Orthograptus calcaratus (Lapworth) sensu lato clingani–linearis zones.

(Figure 15.8) Simplified geological map of Dob's Linn, based on Williams and Ingham (1989, fig. 20).

(Figure 15.9) Dob's Linn, Linn Branch, looking west. The stream makes a waterfall over Gala greywackes (Llandovery) and enters a gorge in Birkhill and Hartfell shale formations. The exposure high on the right is in Llandovery shales and the rill extending obliquely down to the Linn Branch approximately follows the strike. The trench across the Ordovician–Silurian boundary was made (after the photograph was taken) on the near side of the rill and to the right of the stream. The Barren Mudstones of the Upper Hartfell Shale (Ashgill) are nearest the viewer. (Photo: British Geological Survey photographic collection, D3559.)

(Figure 15.10) Dob's Linn, Linn Branch, looking east towards the junction with the Long Burn which flows to the right along the line of the main fault. The beds in the left foreground are inverted Lower Birkhill Shales, mainly lower Llandovery, with the uppermost exposures being close to the Ordovician–Silurian boundary. The Ordovician–Silurian boundary trench was excavated in the bluff of the North Cliff which descends to the Linn Branch. See (Figure 15.11). (Photo: British Geological Survey photographic collection, D3560.)

(Figure 15.11) View of the North Cliff on the north bank of the Linn Branch of Dob's Linn, after Williams and Ingham (1989, fig. 22). The internationally recognized boundary between the Ordovician and Silurian systems is in the Linn Branch Trench.

Tables

(Table 3.1) General stratigraphical successions proposed for the Arfon area

(Table 5.1) The trilobite faunas recognized by Ming, correlated with the Scandanavian zonal succession (Rushton, 1979).

(Table 7.1) Divisions of the Tremadoc in Wales, showing their approximate correlation.

(Table 15.1) Stratigraphy devised by Lapworth (1878), slightly modified.

References