Old Red Sandstone Rocks of Great Britain

Chapter 1

Introduction to the Old Red Sandstone of Great Britain

W.J. Barclay

From: Barclay, W.J., Browne, M.A.E., McMillan, A.A., Pickett, E.A., Stone, P. and Wilby, P.R. (2005) The Old Red Sandstone of Great Britain, Geological Conservation Review Series, No. 31, Joint Nature Conservation Committee, Peterborough, 393 pages, illustrations, A4 hardback, ISBN 1 86107 543 X For more information see: http://www.jncc.gov.uk/page-2936

Introduction

INTRODUCTION marine Devonian rocks of south Devon and Cornwall. The Old Red Sandstone is one of the two major The GCR sites described in this volume are ‘red-bed’ sequences of sedimentary rock in representative of the continental Old Red Sandstone Great Britain, the other being the younger facies in Great Britain. The rocks are mainly of what Permo–Triassic rocks that were formerly termed is now formally defined as Devonian age (about the ‘New Red Sandstone’ (see the companion 418 to 362 million years (Ma) old), but according GCR volume by Benton et al., 2002) to to modern definitions extend back into the Silurian distinguish them from the Old Red Sandstone, Period, perhaps locally into the Wenlock Series rocks that are about 150 million years (Ma) (424 Ma). They also extend upwards into what is older. In the early days of geological research in now defined as the early Carboniferous at less than the 1830s, the Old Red Sandstone was included 362 Ma (see Figure 1.4, ‘Stratigraphical framework in the Carboniferous System, but soon after was for the Old Red Sandstone’, this chapter). given separate status and accorded a Devonian The Old Red Sandstone crops out principally age, in recognition of its equivalence to the in five areas in Great Britain (Figure 1.1), which

Figure 1.1 Simplified sketch map showing the principal Devonian outcrops of Great Britain. Marine Devonian strata are confined to south-west England, the remainder being sedimentary rocks of Old Red Sandstone facies and volcanic rocks. Caledonian (Ordovician to Late Devonian) intrusive rocks are not shown.

3 Introduction to the Old Red Sandstone of Great Britain

broadly reflect the original sedimentary basins in lenticular, phosphatized ‘lag deposit’ marking which they were deposited. These are: the top of the Silurian Ludlow Series. However, the international agreement at the Montreal • the Orkney and Shetland islands and north- Devonian Symposium in 1972 to define the base east Scotland (the Orcadian Basin); of the Devonian System in the fully marine, • the Midland Valley of Scotland (in an graptolite-bearing succession exposed at Klonck amalgamation of several basins of which the in the Czech Republic, at the base of the largest was the Strathmore Basin); Monograptus ultimus Biozone (e.g. House, • the Scottish Borders and Northumberland 1977) now places the basal parts of the Old Red (the Scottish Border Basin); Sandstone in the modern Silurian System. The • the southern Lake District (the Mell Fell strata from the Ludlow Bone Bed up to the base Trough); and of the modern Devonian System, which is as yet • south Wales, the Welsh Borderland and Bristol poorly defined in the Old Red Sandstone, (the Anglo-Welsh Basin). belong to the PÍídolí Series, the fourth, upper- most series of the Silurian System (White and Figure 1.2 shows the stratigraphical distribution Lawson, 1989). The age intervals (or stages) of of the main Old Red Sandstone sequences. the Devonian Period, also defined in the marine Traditionally, the base of the Old Red rocks of Europe, are applied to the terrestrial Old Sandstone in the Anglo-Welsh Basin was placed Red Sandstone succession with some difficulty at the base of the Ludlow Bone Bed, a thin, because of its absence of marine fossils.

Figure 1.2 Stratigraphical distribution of the main Old Red Sandstone sequences of Great Britain. Tectonic events and their timing are from Soper and Woodcock (2003). Ages are from Williams et al. (2000). Small solid bars indicate the principal volcanic rocks. Individual chapter introductions provide more detailed stratigraphical distribution charts. (HV – Hoy Volcanic Member; MF – Mell Fell Conglomerate Formation; RC – Ridgeway Conglomerate Formation.)

4 Scope

GCR SITE SELECTION SCOPE

The selection of Geological Conservation Review The GCR sites were selected according to (GCR) sites described in this volume was carried thematic GCR ‘Blocks’, the present volume out in the 1980s and 1990s, following the describing the ‘Non-marine Devonian’ GCR criteria set out in Ellis et al. (1996). The main Block, which consists of 64 ratified GCR sites, reasons for qualification of a site for a particular together with a small number of potential GCR site selection category are: GCR sites. The site descriptions are arranged geographically, from north to south, in areas that • international importance – for example, the correspond to the original depositional basins. site may be important because it is a type The sites are listed in Table 1.1, together with the locality for a geological time period, rock unit principal criteria for their selection. Many of the or fossil species, or is of historical importance sites have features that satisfy several selection in the development of geological science; criteria. Furthermore, there are numerous Old • possession of unique or exceptional geological Red Sandstone sites that have been independently features; selected for other GCR palaeontological ‘Blocks’. • national importance because a site is represen- These sites are described in the companion GCR tative of a feature, event, process or rock body volumes on fossil fishes (Table 1.2; Dineley and that is fundamental to the understanding of Metcalf, 1999) and Palaeozoic palaeobotany the geological history of Great Britain. (Table 1.3; Cleal and Thomas, 1995).

5 Introduction to the Old Red Sandstone of Great Britain a and and mportant mportant stones, the portant for for portant ter Flagstone ter Flagstone on Dalradian Dalradian on . Also a fossil a fossil . Also GCR site mestone Member mestone he Dundee Flagstone lian facies. Also sections lian facies. Also rine Melby Formation. ones. ones. n igneous rocks formable junction with the Strath junction with the Strath formable and shrinkage cracks in the Lower Caithness and shrinkage cracks Also a Caledonia Also nformably overlain by Late Devonian sand d Red Sandstone facies in Scotland containing fish exposed part of the Orcadian Basin. Orcadian of the part exposed facies inin t the Midland Valley of Scotland n Group and of the apparently con of the apparently n Group and hropods and trace fossils. and trace hropods h the Caithness Flagstone Group. Flagstone Group. h the Caithness e Hoy Sandstone Formation, including the Hoy Volcanic Member. including the Hoy Volcanic Sandstone Formation, e Hoy Eday Marl in Formation Orkney. Eday ns of the Yesnaby Sandstone Group, including unique aeo Type locality of John o’Groats Sandstone Group. Sandstone Group. o’Groats of John Type locality sea-cliff exposures of cyclic lacustrine, fluvial and aeolian facies of the Brindis exposing sandstones Early Devonian unco fish GCR site. Classic fossil fish locality yielding whole, well-preserved specimens,fish GCR and i coastal sections of conglomerates and volcanics. and volcanics. conglomerates of coastal sections gnificent exposures in dramatic sea cliffs of conglomerates and sandst ntinuousof sea-cliff exposures Formation. Walls ssil plant GCR site. World renowned floral and arthropod lagerstätte.and arthropod floral renowned site. World ssil plant GCR preservation in Exceptional (Fish Bed), of importance regionally as a marker horizon between the Lower Caithness Flagstone horizon between regionally as a marker (Fish Bed), of importance Group. Caithness Flagstone and Upper Group Flagstone Group in an otherwise poorly otherwise in an Flagstone Group Formation. Also a fossil fish GCR site. fish GCR a fossil Also Formation. in the Orcadian Basin. stromatolites the best containing Flagstone Formation Stromness in the Lower Formation. fish GCR site. important arthropod important arthropod fauna. metasedimentary rocks and late Caledonian granites in the south-west of the Orcadian Basin. Caledonian granites Orcadian in the south-west of and late metasedimentary rocks Rory Group. Rory Group. Kinnesswood Formation. Kinnesswood Formation. Dundee Flagstone and Scone Sandstone formations. Formation containing fish, art containing Formation evidence on the nature of southern margin of the Orcadian Basin at the time of maximum extent. (Achanarras) lake unconformity),New Aberdour and Quarry Haven. units being separated by a spectacular unconformity. unconformity. units being separated by a spectacular hydrothermal spring deposit. spring hydrothermal Well-exposed, accessible section of the Eday Group. Group. Eday accessible section of the Well-exposed, Complete section of the Balnagow Sea cliffs exposing one of the best sections of mature calcrete development in Scotland the development calcrete Sea cliffs exposing one of the best sections mature Fossil fish GCR site, with superb coast sections at Gamrie Bay, Pennan (Lower ORS–Middle ORS ORS Bay, Pennan (Lower ORS–Middle at Gamrie Fossil fish GCR site, with superb coast sections onian the through Best section Devonian Carboniferous Mid-DevonianEarly (also Devonian) Mid-Devonian Best section in Orkney throug Mid-Devonian Group. Thickest, best-exposedof Eday section the GCR Old Red Sandstone sites and proposed sites, with main criteria for their selection. Continued on page 7. Section Early Devonian Early Section sectio sea-cliff Superb Table 1.1 Table South Fersness Bay, Eday Bay, Eday South Fersness Yesnaby and Gaulton Coast Mid-Devonian Site Age Selection criteria criteria Selection Site Age Basin Orcadian Rova Head Easter Footabrough Wick of Watsness to The Cletts, Exnaboe Mid-Devonian Mid-Devonian Co Mid-Devonian of conglomerates. exposures Spectacular sea-cliff Continuous (P) Toutties The Mid-Silurian Fossil fish GCR site. Oldest (Mid-Silurian) Ol Sarclet (P) Tarbat Ness Devonian Early Mid-Devonian and Late Group. Sea-cliff sectionsDevonian Sarclet in the Lower Achanarras Quarry (P) Quarry Achanarras Mid-Devonian Achanarras Li Fossil fish GCR site, the richest locality in Great Britain. Type of Melby: Matta Taing to Lang Rigg (P) Melby: Matta Taing to Lang Mid-Devonian Section Coast South Stromness Newark Bay Taracliff Bay to Greenan Nev Coast, Eday of lacust exposures and foreshore site. Spectacular sea-cliff Fossil fish GCR (P) Coast Hoy Old Man of Berstane Mid-Dev of Bay Red Point (Thurso–Scrabster) Pennyland Late Devonian (P) o’Groats John Mid-Devonian Mid-Devonian Quarries Wick cliffs of th Spectacular sea Unique onshore evidence of marine-influenced deposition in the Middle Devonian Eday Marl Flagstone Group) Well-exposed lacustrine cycles lake (Upper Caithness of the Orcadian Mid-Devonian Mid-Devonian Mid-Devonian deposits and basement-cover topography. Spectacular lake-margin site. Fossil fish GCR of fish-bearing lake deposits Spectacular exposures Dunnottar Coast Section Dunnottar (P) Coast Crawton Bay Mid-Silurian–Early Devonian Ma Late Silurian–Early Devonian Fine Tynet Burn (P) Mid-Devonian Fossil Loch Duntelchaig (Dun Chia Hill) Hill) Chia (Dun Duntelchaig Loch Mid-Devonian exposureOld Red Sandstone of Dramatic resting unconformably conglomerates Middle North Esk River Esk North Milton Ness (P) (P) Quarry Aberlemno late Early Devonian Devonian Early Late Devonian–Early Group the Strathmore sections of Best fossils.arthropod important for site. Also GCR palaeobotany Palaeozoic and Fossil fish Im Tillywhandland Quarry (P) Quarry Tillywhandland Devonian Early site. Unique lacustrine Fossil fish GCR Whiting Ness Devonian Early and Late Sea cliffs Den of Findon, Gamrie Bay and New Gamrie Bay Findon, of Den Aberdour (P) Rhynie (P) Devonian Early Fo Midland Valley of Scotland and adjacent areas and adjacent Scotland Valley of Midland

6 Scope stone tionally hologies. Psammosteus in. nes Formation. n’ boundary exposure. n’ boundary d ?aeolian sandstones at sandstones at d ?aeolian ided river sandbody morp d floodplain mudstone facies, interna ‘Downtonian’–‘Dittonia ídolí and lowermost Devonian strata. ídolí and lowermost ¯ mity). Fossil fish GCR site at Hawk’s Heugh. Hawk’s site at Fossil fish GCR mity). rmation) of aeolian origin. ídolí and basal Devonian strata. Devonian strata. ídolí and basal ¯ g superb sections of fluvial an ne unconformities recognized by James Hutton. by James ne unconformities recognized of the Psammosteus Limestone horizon. Limestone Psammosteus of the ate palaeosols of the Chapel Point Calcretes Memberate palaeosols ( GCR site, with important sandstones illustrating fluvial, bra sandstones illustrating vian channel sandstones an vian . ídolí Sandy Haven Formation and of the Townsendand of Sandy Haven Formationídolí Tuff Bed. ¯ Sandstone facies conglomerates England. in north-west continued cumented, accessible section ofcumented, accessible the Brownstones Formation. sh GCR site. Also important for exposure of the Plateau Beds Formation. Plateau Beds Formation. of the for exposure important Also sh GCR site. – R site with superb exposure of the lowermost strata of the Brownsto of the strata lowermost of the site with superb exposure R mplete inland section through topmost P sea-cliff and foreshore exposures of Wenlock marine strata and the overlying Old Red the and strata marine Wenlock exposures of sea-cliff and foreshore sandstones (the Knox Pulpit Sandstone Fo sandstones (the Knox extension of GCR site to include higher P GCR site extension of fossil plant GCR site. Also important for exposure of the Senni Formation.Also importantofplant GCR site. exposurefossil for the Table 1.1 Table Proposed extension to include the intervenin Formation. Formation. Sandstone. strata. Pease Bay. known for its arthropod trackways. trackways. arthropod its known for calcrete development and providing an important stratigraphical marker horizon. horizon. marker stratigraphical an important calcrete development and providing Limestone). Fossil plant GCR site yielding a well-preserved assemblage of land plants in the Teith Sand Type locality of the Campsie LimestoneType locality Member, including the Stanley Limestone, of representing mature Classic section at Siccar Point (Hutton’s Unconfor Point Classic section at Siccar Excellent strike section of the entire Old Red Sandstone succession. entire section of the Excellent strike Continuousunderlying Old Red Sandstoneof section succession and overlying of the entire and the Old Red Sandstone succession. exposing the entire dip section Superb, accessible Representative section of the Quartz Conglomerate Group. Group. Conglomerate Quartz Representative section of the Type locality of the Grey Grits Formation. the Grey Grits Formation. of Type locality an Sandsto classic Old Red three One of the nian Excellent exposure of Lochko vonian analysis. sedimentological site with well-documented Fossil fish GCR Carboniferous Late Silurian–Early Carboniferous Late Silurian–Early Devonian Magnificent Carboniferous Carboniferous Carboniferous Carboniferous Wolf’s Hole Quarry (P) Hole Quarry Wolf’s Devonian Early Bas Strathmore of the side on the southern site exposing the highest lava Fossil fish GCR Auchensail Quarry (P) Heugh (E) Hawk’s Siccar Point to Late Devonian–Early Devonian Early Dingle, Tugford (P) Oak The Scar Dingle (P) Cusop Sawdde Gorge (E) De Early Late Silurian–Early Devonian Best, most co Late Silurian–Early Devonian Proposed Late Silurian Formation. Raglan Mudstone of the inland exposure Good Site Tay Bank Glen Vale (P) Devonian Early Late Devonian Age Cutting Palmers Hill Rail Pooley Bridge Anglo-Welsh Basin Porth-y-Mor (P) Devil’s Hole Late Devonian Important criteria Selection Devonian Early Basin. Border in the Scottish of calcrete Exposures Late Silurian–Early DevonianDevonian Early Historically important fossil fish Old Red Best section of the Anglo-Welsh Basin. sectionsaccessible Old Red Sandstone in best, most One of the palaeontology) for their GCR sites confirmed are sites of these (most site P Potential site to extension E Proposed Llansteffan Llansteffan DevonianSilurian–Early Late exposures of stacked carbon Superb Freshwater East–Skrinkle Haven Freshwater East–Skrinkle Haven (‘Tenby Cliffs’) Lydney Gateholm Island and Albion Sands (P) Little Castle Head (P) (North) West Angle Bay (P) Freshwater West Late Silurian–Early Devonian Late Silurian–Early P Reference section of the Late Silurian–Early Devonian site with good section Fossil fish GCR Late Silurian–Early Largs Coast, Ayrshire Ayrshire Largs Coast, Late Devonian exposures in Important coastal Ross-on-Wye, Royal Hotel Wilderness (Land Grove) Quarry Devonian Early Early Devonian Fossil fish GC Excellent, well-do Pantymaes Quarry (P) Devo Early Waen (P) Afon y Duffryn Crawnon (P) Craig-y-cwm (P) Late Devonian Late Devonian Late Devonian–Early possible aeolian Formation, including facies. the Plateau Beds of Type locality Potential fossil fi Heol Senni Quarry Caeras Quarry Craig-y-Fro Quarry (P) Abercriban Quarries Devonian Early Devonian Early Late Devonian–Early Devonian Early Formation. Senni of the representative Also site. fish GCR fossil Classic Formation. in the Brownstones pebbly facies of local Best exposure Portishead Glenthorne Devonian Early and Late Mid-Devonian Estuary. of Severn succession east Sandstone Old Red the Best section of Channel. Bristol of the facies south Old Red Sandstone Best section of North Newton Shore, Arran Shore, Newton North Late Devoni Southern Scotland and the Lake District District Lake the and Scotland Southern

7 Introduction to the Old Red Sandstone of Great Britain

Table 1.2 GCR sites in the Old Red Sandstone described in the fossil fishes GCR volume. After Dineley and Metcalf (1999). Continued on page 9. Site Stratigraphy Criterion Treatment in this volume Full description FD Summary description SD Not described ND Orcadian Basin Westerdale Quarry Mid-Devonian; Eifelian One of oldest fish-bearing ND horizons in Orcadian Basin; complete specimens Achanarras Quarry Mid-Devonian; Eifelian– Richest Old Red Sandstone FD Givetian boundary fish site in Britain Cruaday Quarry Mid-Devonian; Eifelian– Best Old Red Sandstone fish ND Givetian boundary* site in Orkney Black Park, Edderton Mid-Devonian; Eifelian– Fish well preserved in three ND Givetian boundary* dimensions Den of Findon, Gamrie Mid-Devonian; Eifelian– Prolific fish fauna SD Givetian boundary* Tynet Burn, Elgin Mid-Devonian* Rich fish fauna and FD historically important Melby Mid-Devonian; Eifelian– Northernmost occurrence of FD Givetian boundary* Achanarras horizon Papa Stour Mid-Devonian; Eifelian– Fish in sedimentary rocks in ND Givetian boundary* predominantly volcanic sequence Dipple Brae Mid-Devonian Fish fauna younger than that ND of the Achanarras horizon Spittal Quarry Mid-Devonian Rare fish fauna, including only ND Mid-Devonian cephalaspid Banniskirk Quarry Mid-Devonian First ORS site to yield fishes ND Holborn Head Quarry Mid-Devonian; mid- 10–11 fish species, including ND Givetian Osteolepis panderi Weydale Quarry Mid-Devonian Well-preserved Osteolepis ND panderi and Dipterus valenciennesi Pennyland Mid-Devonian; Givetian Many fish specimens from FD several fish-bearing horizons John o’Groats Mid-Devonian; late Youngest fish fauna in SD Givetian Caithness The Cletts, Exnaboe Mid-Devonian; late Northernmost late Givetian FD Givetian fish site Sumburgh Head Late Mid-Devonian; late Possibly youngest fish fauna ND Givetian of the Orcadian Basin Midland Valley of Scotland The Toutties Late Wenlock Oldest Old Red Sandstone FD facies rocks in Scotland; unique fish fauna Tillywhandland Quarry Early Devonian One of best Early Devonian FD fish sites in Scotland Aberlemno Quarry Early Devonian Best surviving of the famous FD Turin Hill fish sites; also a fossil plant GCR site (Table 1.3) Wolf’s Hole Quarry Early Devonian Unique pteraspid fish fauna FD Whitehouse Den Early Devonian Fossil acanthodian fish ND Grampian Highlands Ardmore–Gallanach Late Silurian–Early Unique early fish fauna in ND Devonian sediments associated with Lorne lavas Bogmore, Muckle Burn Earliest Late Devonian Diverse fish fauna with over ND (Frasnian) 15 species Scaat Craig Late Devonian Diverse late Devonian fish ND fauna and a distinctive tetrapod * Achannaras Fish Bed horizon

8 Scope

Table 1.2 – continued.

Site Stratigraphy Criterion Treatment in this volume Full description FD Summary description SD Not described ND Southern Uplands Oxendean Burn Late Devonian Abundant fragments of ND Bothriolepis Hawk’s Heugh Late Devonian Only British occurrence of FD Remigolepis Anglo-Welsh Basin Ludford Lane and Ludford Silurian; P¯ídolí Internationally renowned for ND Corner rich fish fauna; see also Table 1.4 Ledbury cutting Silurian; P¯ídolí Historical site yielding ND complete specimens of Auchenaspis and Hemicyclaspis Temeside, Ludlow Silurian; P¯ídolí Historical site in Temeside ND Mudstone Formation yielding a rich fish fauna including Hemicyclaspis murchisoni Tite’s Point (Purton Silurian; Ludlow–P¯ídolí Thelodus parvidens fish Passage) fauna, allowing correlation with Ludlow Bone Bed, and source of Cyathaspis Lydney Late P¯ídolí–Early Sequence of vertebrate faunas, FD Devonian including specimens of Sabrinacanthus Downton Castle area Early P¯ídolí Several quarries in Downton ND (network of 4 sites) Castle Sandstone yielding vertebrate remains Bradnor Hill Quarry Late P¯ídolí Late P¯ídolí thelodont ND fauna Devil’s Hole P¯ídolí–Lochkovian Fish fauna straddling SD Downtonian–Dittonian boundary Oak Dingle, Tugford Lochkovian (Dittonian) Near-strike section of fish- SD bearing beds; earliest record of Weigeltaspis Cwm Mill Lochkovian (Dittonian) Unique preservation of ND complete cephalaspids, including three new species; also specimens of Rhinopteraspis crouchi Wayne Herbert Quarry Lochkovian (Dittonian) Well-preserved, diverse fish ND fauna Besom Farm Quarry Lochkovian (Dittonian) Rich, diverse fish fauna, ND including 7 type specimens and sole occurrence of 5 of them Heol Senni Quarry Lochkovian–Pragian Only occurrence of Althaspis FD senniensis Portishead Late Devonian Unique fish fauna, including FD only British occurrence of Groenlandaspis Prescott Corner Late Devonian (Frasnian) Extensive Late Devonian fish ND fauna Afon y Waen Late Devonian Bothriolepis and FD Holoptychius in Upper Old Red Sandstone

9 Introduction to the Old Red Sandstone of Great Britain

Table 1.3 GCR sites in the Old Red Sandstone described in the Palaeozoic palaeobotany GCR volume. After Cleal and Thomas (1995). Site Stratigraphy Criterion Treatment in this volume Full description FD Summary description SD Not described ND Orcadian Basin Sloagar Mid-Devonian; Late Only occurrence of ND Givetian Svalbardia in Britain Bay of Skaill Mid-Devonian Important floral assemblage ND in Sandwick Fish Bed; type locality of Protopteridium thomsonii Rhynie Early Devonian Renowned Devonian FD palaeobotanical site; 22 species unique to this site Midland Valley of Scotland Turin Hill Early Devonian Best example of FD (as ‘Aberlemno Quarry’) Zosterophyllum Zone flora in world and type locality of Cooksonia caledonica Ballanucater Farm Early Devonian; Emsian Best Emsian floral assemblage ND in Britain Auchensail Quarry Early Devonian; Emsian Well-preserved Emsian floral FD assemblage Anglo-Welsh Basin Targrove Quarry Early Devonian; Most diverse rhyniophytoid ND Gedinnian plant assemblage in world Capel Horeb Quarry Late Silurian; Oldest vascular plants in ND Ludfordian–P¯ídolí world in Ludlow Series; Long Quarry Formation yielded some rhyniophytoids including Cooksonia Perton Lane Late Silurian; P¯ídolí Classic locality and type ND locality of Cooksonia Freshwater East Late Silurian; P¯ídolí Most diverse Silurian flora in FD the world Llanover Quarry Early Devonian; Classic locality yielding one of ND Siegenian most diverse Psilophyton Zone flora in Britain Craig-y-Fro Quarry Early Devonian Some of best preserved SD Devonian plants in Britain; locality second only to the Rhynie site in Britain

The GCR sites provide representative Red Sandstone strata are present in addition to localities for the entire stratigraphical range of the contemporaneous igneous rocks for which the Old Red Sandstone. The initial selection of they are cited. All of the Old Red Sandstone sites GCR sites for the ‘Non-marine Devonian’ GCR described in the other GCR volumes are listed in Block included sites in the Anglo-Welsh Basin in Tables 1.2, 1.3, 1.4 and 1.5, along with the level strata that extended down from the base of the of detail in which they are described in the Devonian System to the Ludlow Bone Bed. present volume. Only some of these ‘over- These strata, comprising the Downton Group lapping’ sites are given full descriptions in the (the former Downtonian Stage), are of Silurian present volume, which emphasizes the sedi- (PÍídolí Series) age and the sites (Table 1.4) are mentological and lithostratigraphical features. described in the GCR volume on Silurian strati- The Freshwater West potential Old Red graphy (Aldridge et al., 2000). The GCR volume Sandstone GCR site, and part of the Freshwater on Caledonian igneous rocks (Stephenson et al., East–Skrinkle Haven GCR site are also Variscan 1999) includes sites (Table 1.5) in which Old to Alpine structures GCR sites.

10 History of research

Table 1.4 GCR sites in the Old Red Sandstone described in the Silurian stratigraphy GCR volume. After Aldridge et al. (2000). Site Stratigraphy Criterion Treatment in this volume Full description FD Summary description SD Not described ND Anglo-Welsh Basin Marloes Wenlock–P¯ídolí Classic site showing early ND (included with report transition from marine to Old for Albion Sands and Red Sandstone facies Gateholm Island) Albion Sands and Ludlow–P¯ídolí– Complete, conformable SD Gateholm Island Lochkovian succession from Ludlow into early Devonian Freshwater East (South) Wenlock–P¯ídolí Wenlock marine strata ND overlain by Old Red Sandstone; faulted/ unconformable relationship Ludford Lane and Ludford Ludlow–P¯ídolí Classic, internationally ND Corner renowned site traditionally regarded as reference section for Silurian–Devonian boundary; earliest known land animals, early plants (see Table 1.3), unusual arthropods and fish remains in Ludlow Bone Bed Brewin’s Bridge/Canal Ludlow–P¯ídolí– One of few sites in central ND Carboniferous England exposing marine Silurian–Old Red Sandstone junction, including Ludlow Bone Bed Capel Horeb Quarry Ludlow–P¯ídolí Good section of unconformity ND between Ludlow and P¯ídolí; internationally important plant site (Table 1.3) Little Castle Head P¯ídolí Old Red Sandstone facies SD rocks; Townsend Tuff Bed Lower Wallop Quarry Ludlow–P¯ídolí Marine to Old Red Sandstone ND transition later here, well into P¯ídolí

Site selection is inevitably subjective, but the HISTORY OF RESEARCH aim of the GCR is to identify the minimum number and area of sites needed to demonstrate The name ‘Old Red Sandstone’ appears to have the current understanding of the diversity and been first applied to the red rocks below the range of features within each GCR ‘Block’. The Mountain (Carboniferous) Limestone in the preferred sites are generally those that are least mistaken belief that they were the equivalents vulnerable to the potential threat of destruction, of the Permian Rotliegendes of Germany are more accessible and are not duplicated else- (Jameson, 1821; Simpson, 1959). It was initially where (Ellis et al., 1996). The original selection mapped and named in southern Wales and the of sites was made over 20 years ago, and all of Welsh Borderland (Phillips, 1818; Conybeare these sites are included in this volume. In and Phillips, 1822) and included as the addition, a small number of sites were identified lowermost part of the Carboniferous System. during the course of the compilation of the Murchison was the first to champion the Old volume as representing stratigraphical units or Red Sandstone as a separate geological entity. unique features not included in the original GCR According to Miller’s account (1841), a visiting site selection, and are described as ‘potential’ foreign geologist advised Murchison that ‘you GCR sites. must inevitably give up the Old Red Sandstone:

11 Introduction to the Old Red Sandstone of Great Britain

Table 1.5 GCR sites with Old Red Sandstone sedimentary rocks described in the Caledonian igneous rocks volume. After Stephenson et al. (1999). Site Stratigraphy/ GCR selection criteria radiometric age

Eshaness Coast Mid-Devonian Representative of Eifelian Eshaness volcanic succession, NW Shetland. Ness of Clousta to the BrigsMid-Devonian Representative of Givetian Clousta volcanic rocks, Walls, Shetland. Point of Ayre Mid-Devonian Representative of Givetian Deerness Volcanic Member, mainland Orkney. Too of the Head Mid-Devonian Representative of Givetian Hoy Volcanic Formation, Isle of Hoy, Orkney. South Kerrera Late Silurian to Early Representative of Lorn Plateau Volcanic Formation. Devonian Exceptional examples of subaerial lava features and interaction of magma with wet sediment. Ben Nevis and Allt Mid-Silurian Representative of Ben Nevis Volcanic Formation. a’Mhuilinn 425 Ma Exceptional intrusive tuffs. Internationally important as example of exhumed root of caldera, and historically for development of cauldron subsidence theory. Stob Dearg and Cam Mid-Lochkovian Representative of succession in eastern part of Glencoe Ghleann 421 ± 4Ma caldera, including basal sedimentary rocks. Exceptional rhyolites, ignimbrites and intra-caldera sediments. Possible international importance for radiometric dating in conjunction with palaeontology close to Silurian– Devonian boundary. Crawton Bay* Late Silurian–Early Representative of Crawton Volcanic Formation. Devonian Scurdie Ness to Usan Early Devonian Representative of ‘Ferrydean’ lavas and ‘Usan’ lavas, Harbour comprising lower part of Montrose Volcanic Formation. Black Rock to East Comb Early Devonian Representative of ‘Ethie’ lavas, comprising upper part of Montrose Volcanic Formation. Balmerino to Wormit Early Devonian Representative of eastern succession of Ochil Volcanic (Lochkovian) Formation. Possible international importance for 410.6 ± 5.6 Ma radiometric dating in conjunction with palaeontology close to Silurian–Devonian boundary. Sheriffmuir Road to Early Devonian Representative of western succession of Ochil Volcanic Menstrie Burn 416 ± 6.1 Ma Formation. Exceptional topographic expression of Ochil fault-scarp. Tillycoultrie Early Devonian Representative of diorite stocks, intruded into Ochil 415–410 Ma Volcanic Formation, surrounded by thermal aureole and cut by radial dyke swarm. Exceptional examples of diffuse contacts due to metasomatism and contamination, with ‘ghost’ features inherited from country rock. Port Schuchan to Dunure Early Devonian Representative of Carrick Hills volcanic succession. Castle Exceptional features resulting from interaction of magma with wet sediment are of international importance. Culzean Harbour Early Devonian Representative of inlier of Carrick Hills volcanic succession. Exceptional features resulting from interaction of magma with wet sediment are of international importance. Turnberry Lighthouse to Early Devonian Representative of most southerly inlier of Carrick Hills Port Murray volcanic succession. Exceptional features resulting from interaction of magma with wet sediment are of international importance. Pettico Wick to St Abb’s Early Devonian Representative of volcanic rocks in the SE of the Southern Harbour c. 400+ Ma Uplands. Exceptional vent agglomerates, block lavas, flow tops and interflow high-energy volcaniclastic sediments. * described in this volume

12 Old Red Sandstone palaeogeography

it is a mere local deposit, a doubtful (but predominantly Devonian) age (e.g. Friend accumulation huddled up in a corner, and has and Williams, 2000). The term ‘Old Red no type or representative abroad.’ Sandstone’ is also applied in an informal litho- ‘I would willingly give it up if Nature would,’ stratigraphical sense. The three subdivisions replied Murchison, ‘but it assuredly exists, and I of the Old Red Sandstone recognized by cannot’. Compared to the richly fossiliferous Murchison – Lower, Middle and Upper – are rocks of the Silurian System below and the similarly retained as informal, but long- Carboniferous System above, the Old Red established lithostratigraphical terms onshore in Sandstone seemed relatively barren to the early the United Kingdom and as formal groups Victorian workers, but as the remains of early offshore in the North Sea. fishes were discovered, first in Scotland by the young Swiss naturalist Louis Agassiz, and later in south Wales and the Welsh Borderland, interest OLD RED SANDSTONE gradually increased. PALAEOGEOGRAPHY The Devonian System was established by Sedgwick and Murchison (1839) for the pre- The Old Red Sandstone represents a period Carboniferous marine rocks of Devon. These when ocean closure and continental collisions rocks were readily correlated with the Rhenish resulted in a world geography vastly different nearshore rocks and the Bohemian deep-water to that of much of early Palaeozoic times. The rocks of mainland Europe (House, 1977). With drift of the early Palaeozoic continents and their the recognition of large tracts of Old Red relative positions can be estimated from the Sandstone in North America, Norway, Siberia, correlation of geological successions and their Poland and Russia (the last containing many of faunas, with palaeomagnetism providing data the same fish species as Great Britain), the strata on palaeolatitudes. The Iapetus Ocean, which assumed a new importance (Geikie, 1879). At separated the northern (Laurentian) and southern the same time, Murchison (1839), impressed by (Gondwanan) continents, closed throughout the the great thickness of Old Red Sandstone strata Ordovician and Silurian periods as the smaller in the Welsh Borderland, and the difference continent of Avalonia fragmented from Gondwana between them and the overlying Carboniferous and drifted northwards (Figure 1.3). As the rocks, with which they had hitherto been Iapetus Ocean closed north of Avalonia, the merged, applied the status of ‘system’ to the Old Rheic Ocean opened behind it. To the east, the Red Sandstone. This situation held, in the UK at continent of Baltica also drifted northwards and least, for over 130 years, with the term used in a eastwards and the Tornquist Sea, an arm of the quasi-chronostratigraphical sense for rocks of Iapetus Ocean, slowly closed. The timing and continental facies and Devonian age. However, nature of the convergence of the three compo- with the advent of more precise stratigraphical nents that were to make up the Old Red procedures and classification, and, in 1972, the Sandstone continent remain matters of debate. new definition of the base of the Devonian Trench and Torsvik (1992) considered that System at a higher level, equivalent to a horizon Baltica and the eastern part of the Avalonia within the Old Red Sandstone (see below), the microcontinent collided first, in late Ordovician term is now no longer used in a quasi- times, moving northwards together to make first chronostratigraphical sense. Biostratigraphers contact with Laurentia by late Silurian time at tend not to use the term at all; the glossary in about 420 Ma (Torsvik et al., 1996; see Dewey the companion GCR volume on Silurian strati- and Strachan, 2003, fig. 1). However, Dewey and graphy, for example (Aldridge et al., 2000), Strachan (2003) interpret the Scandian Orogeny defines the Old Red Sandstone as ‘a classic term as the result of collision, by sinistral transpres- still applied to the terrestrial, largely clastic sion, of Baltica and Laurentia from about 435 Ma facies of the late Silurian to earliest to 425 Ma, with a soft collision between Avalonia Carboniferous in Britain’. Sedimentologists and Laurentia/Baltica (Laurussia) at about retain the name as a facies (or magnafacies) term 425 Ma (Soper and Woodcock, 2003). By late for all the terrestrial red beds and lacustrine Silurian (Ludlow) time, the continents had fully deposits of Silurian to early Carboniferous docked, with the Iapetus Ocean closed along the

13 Introduction to the Old Red Sandstone of Great Britain

14 Old Red Sandstone palaeogeography

=Figure 1.3 Sketch maps showing the movements extent of these movements remain the matter of and amalgamation of the early Palaeozoic continents debate, largely centred on whether there was a that produced the Old Red Sandstone (Laurussia) major, orogen-wide sinistral megashear or continent. (a) and (b) are global views to illustrate the fragmentation of Avalonia from Gondwana and its whether basins were controlled by strike-slip drift northwards as the Iapetus Ocean closed movements of different sense and at different (adapted from Torsvik et al., 1992, by Trench and times during the Caledonian orogenic cycle. Torsvik, 1992). (c), (d) and (e) show the later stages Another debate concerns the possible role of of the Caledonian Orogeny. Sinistral strike-slip move- gravitational collapse of the uplifted, granite- ments in relation to the Laurentian margin culminated in the Acadian Orogeny in late Early buoyed Caledonian Orogen in the formation of Devonian (Emsian) times (after Stephenson et al., some at least of the internal basins (e.g. 1999, adapted from Soper et al., 1992). Woodcock, 2000a). Dewey and Strachan (2003) conclude that the diachronous closure of Iapetus, and subsequent deformation and line of subduction (the Iapetus Suture) under basin formation were controlled by sinistrally the Southern Uplands. Thus, the Caledonian– dominated relative movement between the Appalachian Orogen (or North Atlantic Laurentian and Avalonia–Baltica plates. The Old Caledonides) and the newly amalgamated Old Red Sandstone basins probably formed as a Red Sandstone continent (Laurussia or result of sinistral transtension, with an estimated Euramerica) were formed. Continuing 1200 km of strike-slip movement between compression and shortening of the continental Laurentia and Baltica. Rheic convergence in crust resulted in the filling to sea level of the the Emsian Age (late Early Devonian) from Silurian marine basins, their inversion to upland 400 Ma to 390 Ma resulted in the Acadian areas and the establishment of terrestrial Orogeny, which affected the basins south of the conditions in newly developing basins. Highland Boundary Fault (Soper and Woodcock, Palaeogeographical reconstructions (e.g. 2003). Scotese, 2001) suggest that the continent lay in The Old Red Sandstone basins were formerly tropical to sub-tropical latitudes from the divided into two main groups on the basis of equator to about 30°S, with the Anglo- their positions relative to the Caledonian Orogen Welsh Basin lying approximately 5°S to 15°S. (e.g. Allen, 1977; Woodcock, 2000a). Those Palaeomagnetic data from the Lower Old within it (internal or intramontane basins) Red Sandstone in southern Wales suggest a include the Orcadian Basin, the basins of the latitude of 17 ± 5°S (Channell et al., 1992). Midland Valley of Scotland (but see below) and Sedimentological studies of the Old Red the Scottish Border Basin. The Anglo-Welsh Sandstone, and particularly of its fossil Basin was regarded as an external, or extra- carbonate soils (calcretes) confirm, by analogy montane basin, open to the sea to the south. with modern calcretes, a warm to hot, semi-arid However, the recent models, invoking major tropical to sub-tropical setting (e.g. Allen, 1986) orogen-parallel, sinistral movement and three with rainfall confined to wet seasons (e.g. separate, temporally discrete collision events Marriott and Wright, 1993). Uplift of the orogen (Grampian, Scandian and Acadian) collectively may have caused broad variations in the rainfall making up the Caledonides have revised the pattern, producing periods of wetter and drier former view of a continuously prograding climate. Caledonian mountain front. Woodcock (2000a), Friend et al. (2000), The recent models (Dewey and Strachan, Dewey and Strachan (2003) and Soper and 2003; Soper and Woodcock, 2003) envisage Woodcock (2003) presented recent overviews of that the highly oblique, sinistral closure of the tectonics and kinematics of Old Red the Iapetus Ocean resulted in, sequentially, Sandstone basin formation. Superimposed on transpression, strike-slip and transtension. The the broadly compressive stresses associated with area of maximum uplift in the Scandian Orogen convergence of the Avalonian and Laurentian was to the north of Britain, in Scandinavia, in continental margins, the oblique angle of an orogen of Himalayan proportions (Dewey closure produced strike-slip transpressive and and Strachan, 2003). The compression in the transtensional movements. The nature and Laurentian crust, of which the Scottish

15 Introduction to the Old Red Sandstone of Great Britain

Highlands were part, caused thrusting along points for continuing volcanic extrusion (e.g. major NE-trending faults, granitic intrusion, Bluck, 2000). The preserved sequences thus andesitic volcanicity and low-grade metamor- represent the deposits of separate pull-apart phism in northern Britain (Stephenson et al., basins, formed and brought together in a strike- 1999). The volcanic rocks were probably slip faulted collage. The Stonehaven Basin in extensive, their eroded remnants being seen at the north-east is the earliest, its sedimentary fill Ben Nevis, Glen Coe, Lorn and just north of the dating perhaps from the Wenlock Epoch Highland Boundary Fault. Volcanic rocks also (Marshall, 1991). It and its larger successor occur extensively within the Midland Valley of basins, the Crawton and Strathmore basins, Scotland, at Montrose, in the Sidlaw, Ochil and formed by sinistral strike-slip along the Highland Pentland hills, and in Ayrshire. They also occur Boundary Fault. The southerly Lanark Basin more locally in the Southern Uplands, where formed along the Southern Upland Fault. Large granitic intrusions such as the Cheviot were volumes of arc-related volcanic rocks were emplaced. extruded along the central axis of the Midland During the transcurrent and transtensional Valley, on lines weakened by the transtensional phases, much of the orogen-parallel, sinistral stresses (e.g. Bluck, 2000). The late Devonian movement appears to have been taken up by the Scottish Border Basin formed after Acadian Great Glen Fault and its north-east continuation, inversion in Mid-Devonian times and extended with at least 700 km of displacement (Dewey into the Midland Valley, Northumberland and and Strachan, 2003). The formation of the Late the Solway Firth. Silurian–Early Devonian Old Red Sandstone The Anglo-Welsh Basin was formerly basins is also attributed to sinistral transtension interpreted primarily as the product of load- (Dewey and Strachan, 2003; Soper and generated flexural subsidence of the Caledonian Woodcock, 2003). The Acadian Orogeny ended foreland (James, 1987; King, 1994; Friend et al., this phase of basin formation and caused 2000). Dewey and Strachan (2003) and Soper transpressive shortening of the early Palaeozoic and Woodcock (2003) prefer a transtensional basins flanking the Midland Microcraton, as mechanism for its formation. Transtensional well as the inversion and erosion of the Old movement on faults produced variations in the Red Sandstone rocks not underlain by the micro- basin-fill in Pembrokeshire (e.g. Marshall, craton (Soper and Woodcock, 2003). The cause 2000a,b) and introduced coarse, clastic, detritus of the Acadian event was probably the collision farther north (Tunbridge, 1980a). The isolated of a Gondwana-derived continental fragment succession in Anglesey was probably deposited (Soper and Woodcock, 2003) with the Midland contiguously with the PÍídolí–Pragian sequences Microcraton segment of the amalgamated to the south, with which there are marked Laurussian continent. The evidence for the similarities, although the initial coarse terrane boundary in the vicinity of the Bristol conglomerates are unique and of local Channel is now confined to the Lizard mafic– derivation, and lacustrine deposits suggest ultramafic complex, interpreted as an ophiolite internal or impeded drainage. and a fragment of the Rheic suture (e.g. Soper and Woodcock, 2003). The Orcadian Basin was a large Mid-Devonian STRATIGRAPHICAL FRAMEWORK intramontane lake basin, totally unconnected FOR THE OLD RED SANDSTONE to the open sea, apart perhaps from a brief period. Its formation was probably due to a Stratigraphical classification of the rocks in the combination of both gravitational extension, and geological record has traditionally fallen transtensional movements on basin-margin into two broad categories – lithostratigraphical faults. The Midland Valley of Scotland was not a and chronostratigraphical. Lithostratigraphical single discrete basin in the Devonian Period. classification is based on the physical Weakened by a long history of igneous activity, characteristics of a rock body, such as colour, internal, transtensional fault movements opened rock type (lithofacies) and mode of formation. pull-apart basins and transpressive movements Chronostratigraphical classification is based on subsequently inverted them, resulting in the the relative age of a rock body, determined by its recycling of the basin-fills and providing weak fossil content (biostratigraphy) as correlated with

16 Stratigraphical framework for the Old Red Sandstone

standard, defined and internationally agreed and Givetian) are defined in Germany, the Upper geological marker horizons (the ‘golden spikes’), Devonian stages (Frasnian and Famennian) are and in the case of igneous rocks, by radiometric named from the carbonate-bearing marine age dating. Biostratigraphical classification is succession of southern Belgium. achieved by the study of component fossil and The problems of classification and correlation microfossil groups, with subdivisions based on of the Old Red Sandstone of Great Britain are marker species or assemblages of species. Thus, inherent in its terrestrial origins and the patchy in the Devonian System, there are biostrati- preservation of its non-marine fossils. The graphical zonal schemes for graptolites, fossils that are present indicate that the ammonoids, brachiopods, fish, conodonts, Devonian Period was a time of profound microvertebrates and miospores. changes in the evolutionary record, with the first The chronostratigraphical subdivisions of the significant colonization of terrestrial habitats by Upper Silurian and Devonian (Figure 1.4) are vascular plants (Edwards, 1979a), the rapid internationally agreed and defined in fossili- expansion of the first aquatic vertebrates, and ferous marine strata in continental Europe. The their emergence onto land. However, no direct Upper Silurian PÍídolí Series (not yet divided correlations can be made with the European into stages) and Lower Devonian Lochkovian marine successions and the internationally and Pragian stages are defined in the deep-water, agreed stages. Because of this, a series of loosely graptolite-bearing succession of the Prague defined local stages (Downtonian, Dittonian, Basin in the Czech Republic. The last two Breconian and Farlovian) were erected for the replace the previously used, but not completely Anglo-Welsh Basin, but now have been largely equivalent Gedinnian and Siegenian stages subsumed into the international stages as a defined in the nearshore succession of the result of increasing refinement in correlation. German Rhenish Basin. The highest Lower Figure 1.4 (based on House et al., 1977; and Devonian stage is the Emsian, defined in Marshall and House, 2000) shows the stages and Belgium. The Middle Devonian stages (Eifelian their correlation.

Figure 1.4 Major subdivisions of the Old Red Sandstone and its chronostratigraphical classification. Ages from Williams et al. (2000).

17 Introduction to the Old Red Sandstone of Great Britain

The principal macrofossils are fish fragments. rare, remains. Progress is, however, being A biozonal scheme was erected for the Old Red made by chains of correlation involving Sandstone in the Anglo-Welsh Basin (see Figure miospores that are common to the Old Red 5.3, Chapter 5) and was extended to continental Sandstone and the Rhenish marine succession, Europe. Refinement of the scheme continues the latter then being correlated with the (Blieck and Janvier, 1989; Blieck and Cloutier, Bohemian stages. For example, the recognition 2000), but the occurrence of fish remains is of the Breconensis-zavallatus Zone in the patchy and of limited use in high-resolution Ardennes allows correlation of the Anglo-Welsh correlation. Miospore classifications (e.g. and Rhenish Gedinnian–Siegenian successions Richardson et al., 2000; Streel et al., 2000) and (Richardson et al., 2000) (Figure 5.3, Chapter microvertebrate classification (Vergoossen, 5). A widespread volcanic ash deposit (the 2000) also aid correlation and stratigraphical Townsend Tuff Bed) and a basin-wide calcrete resolution. However, the problem of detailed (the Psammosteus Limestone) are valuable correlation of the terrestrial Old Red Sandstone marker horizons in the Anglo-Welsh Basin, succession with the Bohemian, German and providing lithostratigraphical correlation of the Belgian marine stages, in which miospores are succession.