The geology of Anglesey

By Edward Greenly

Greenly, Edward. 1919. The geology of Anglesey. London HMSO [For Geological Survey] Two volumes

Cover photograph

Edward Greenly, author of The geology of Anglesey

2024 notes:

For this web version, chapter numbers and plate numbers have been converted from the original Roman numerals

Where a BGS thin section has been cited, this has been linked to the BGS Britrocks image resource and a Grid Reference added. This enables the reader to quickly view the thin section mentioned. For example (E9533) [SH 491 909]

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Geology of Anglesey Volume 1. Up to page 388

Geology of Anglesey Volume 2. Page 389 onwards

A full list of all Survey photographs taken at the time of the mapping of Anglesey has been appended. It includes a link to view them on the BGS Geoscenic.

Brief contents

Title page and preliminaries

Full Contents

Title page and preliminaries

Part 1 — Preliminary

Chapter 1 — Introduction

General aspects of the Island. Its dimensions. Its climate. Its population. Its names. Its Geological Formations. Tabular Statement of the Formations

Chapter 2 — History of previous research

Introductory. The Mona Complex. Baron Hill and Careg-onen Rocks. The Ordovician Rocks and the Palaeozoic Movements. The Silurian Rocks. The Paleozoic Intrusions. Metasomatism. The Old Red Sandstone. The Carboniferous Limestone. Millstone Grit and Coal Measures. The Red Measures. The Later Dykes. Glacial and Later Geology

The Land Surface. Chemical Analysis. The Present Survey

Bibliography

Methods of reference to publications

Part 2 — The Mona Complex

Chapter 3 — Introduction to the Mona Complex

Regions and Inliers. Composition, Characters, and Sub-divisions. The Designation 'Mona Complex'

Chapter 4 — The petrology of the Mona Complex

Introductory

The Holyhead Quartzite

The South Stack Series: The South Stack Series Proper. The Coeden Beds

The New Harbour Group: The Green-mica-schists. Green Biotite. The Amlwch Beds. The Jaspery Phyllites. The Bedded Jaspers. The Spilitic Lavas

The Skerries Group: The Church Bay Tuffs. The Skerries Grits. Their Conglomerates. The Tyfry Beds

The Gwna Group: The Alternating Grit and Phyllite. Autoclastic Phyllite-and-Grit Mélange. The Gwna Green-schist. The Spilitic Lavas. The Tuffs. The Albite-Diabases. Keratophyre. Basic Schists. The Quartzites. The Limestones. Graphitic Phyllite. The Jaspers. The Jaspery Phyllites

The Fydlyn Group

The Coedana Granite

Hornfels: Cryptocrystalline Hornfels. Crystalline Hornfels. Unity of the Hornfels. Composition. Origin of the Hornfels. Foliated Hornfels

Diorites

The serpentine suite: Serpentine. Pyroxenites. Gabbros. Dolerites. Mutual Relations. Associated Metamorphic Rocks (Talc-schist, Tremolite-schist, Ophicalcite, &c.). Deformation and Metamorphism. Marginal Rocks

The Penmynydd Zone of Metamorphism: The Mica-schists. Quartz-schists. The Limestones. Graphite-schist. Rutiliferous-schists. The Hornblende-schists. The Glaucophane-schist. Its Origin. Banded Marginal Rocks. Origin of the Penmynydd Mica-schists. Conclusions

The Gneisses: Hornblende-gneiss. Biotite-gneiss or Gneiss-proper. Its Granitoid Component. Its Foliated Component. Nature, Origin, and Relations of the Components of the Gneiss. Comparisons and Conclusions

Foliation

General mineral characters of the Mona Complex

Chapter 5 — The palaeontology of the Mona Complex

Chapter 6 — The succession in the Mona Complex

Introductory

Correlations within the complex

Order of succession: Group Sequences and Junctions. The Bedded Succession. The Penmynydd Zone. The Gneisses. Local Successions

Table of the general succession

Chronological order of the succession: Ancient fragments. The Plutonic intrusions and the Penmynydd Zone

Relation of the gneisses to the bedded succession

The ancient floor

Chapter 7 — The tectonics of the Mona Complex

Introductory

The maximum primary recumbent structures: Evidence for Inversion. The Maximum Primary Folding as revealed in Holy Isle. The Maximum Primary Structures on the Main Island (with evidence for directions of close and gape). amplitudes, facies, and recapitulation

The major, minor and minimum structures: The Major Secondary Folds and Thrusts. The Minor or Ternary Folding. Types of Minor Folding. Pitch. Minor Thrusting. Principles of Auto-clastic Mélange. The Minimum or Tessary Folding

Structures of the second and third generations: The Valley Thrust-planes. The Foliation of the Western Region. Relations of the Penmynydd Metamorphism

Later structures: Transverse Folds. Final Catamorphic Movements

Folial relations: Relations to Bedding, Folding and Thrusting. Chronology of the Foliation

Chapter 8 — General view of the Mona Complex

Introductory

Developments in the several regions: Holy Isle. The Western Region. The Northern Inliers. The Northern Region. The Middle Region. The Gneissic Inliers. The Deri Inlier. The Pentraeth Ethers, The Aethwy Region

Developments over the island considered as a whole: Probable thickness. Facies and physiography. Geographical distribution of the stratigraphical and tectonic members of the Complex. Arrangement of the Post-Recumbent Foldings. pitch, dip, and strike throughout the island. Distribution of the metamorphism. Laws of the Metamorphism

Recapitulation

Chapter 9 — The age of the Mona Complex

Relations of the Complex to Ordovician Rocks

Relations of the Complex to Cambrian Rocks: The indirect evidence. Pebbles in conglomerates (with table). Horizons and localities of the conglomerates (with table). Pebbles from Llanberis and Bethesda. Pebbles from the Harlech Anticline. Review of the evidence. Conditions of the Cambrian erosion

Conclusion

[NOTE — See also pp.425–6, 559, 906]

Chapter 10 — The detail of the Mona Complex

Holy Isle

Tracts West of the North Stack Fault. The North Stack and the Fault. The Stack Moor. The South Stack. The Stack Moor Syncline. Henborth and Llwyn-y-berth. Rhoscolyn

The Country between the Two Main Faults: The Rhoscolyn Anticline. Rhoscolyn to the Tre-Arddur Gap. The Country about Holyhead. Holyhead Mountain

District North-east of the Namarch Fault: Stanley Gate to Soldier's Point. The Breakwater Coast

The Serpentine-Suite about the Strait of Holy Isle: Mainland and islets: Four-Mile-Bridge to Cymyran. The Lakes, Porth-delisc. The Strait Islets,

Holy Isle: Between the Strait and the Alluvium. West of the Alluvium. The Southern Margin

The Western Region

The New Harbour Group: The Sea-Board. The Interior

The Gwna Group: Llanfaethlu to Brwynog. Llanrhyddlad. Foel High Land

The Church Bay Tuffs

Boundaries

The Northern Inliers

The Garn Inlier

The Mynachdy Inlier

The Fydlyn Inlier: Trwyn-y-crewyn to Fydlyn Beach. Fydlyn Beach to Porth-yr-hwch. Porth-yr-hwch

The Gader Inlier

The Corwas Inlier

The Northern Region

The Llanfairynghornwy Belt: Carmel Head to Gwaen-ydog. The Mynachdy Gneiss

The Coeden Beds

The Amlwch Beds: The Coast. The Interior

The Gwna Beds of Gynfor, Pant-y-gaseg, and Wylfa: General Characters. Wylfa. Gynfor. Pant-y-gaseg

The Skerries Group in the Trwyn Bychan Tract: Hell's Mouth to Porth Wen. Trwyn Bychan to Bull Bay

The Skerries Grits between Llanfechell and Llan-rhwydrys

The Three Mouse Islets

The Skerries

Basic intrusions of the North

The Middle Region and its inliers

Gneiss of the Middle Region: The Southern end. Allor. Llandry-garn and Gwyndy to Llechcynfarwydd. Henblâs to Mynydd-mwyn-mawr. Gneissic Inliers of the Middle Region. Basic Gneisses near Llangwyllog

Gneiss of the Nebo Inlier: The Interior. The Coast

The Coedana Granite

Hornfels: The Coast, Llanfaelog, and the Marginal Aureole. The Xenoliths

The Deri Inlier

The Foel Inlier

Mynydd Bodafon: The Quartzite, Hornfels; and Moor Flags. Tectonics

The Penmynydd Zone of the Middle Region: The Coast. The Western Margin. The Rest of the Inland Tract. The Inliers

The Gwna Beds and Tyfry Beds of the Eastern Middle Region: The Coast. South-eastern Margin. Between the Margin and the line through Cerrigceinwen. The Ceinwen Volcanic Zone. The Engan Spilites. The Western Tracts

The Pentraeth Inliers

The Western Inlier

The Eastern Inlier: Dyffryn to Tyfry. Tyfry to the seaward end

The Aethwy Region

The Gwna Beds of the East: The Menai Coast. Cadnant to Red Hill. The Interior. The Eastern Margin. The Llanddona Highlands

The Penmynydd Zone of the Aethwy Region: The Eastern and South-eastern Margins. The North-western Margin. The Centre. Mynydd Llwydiarth

Gneiss of Holland Arms and Gaerwen (with Rhos-cefn-hir)

The Llanddwyn Wedge: Llangaffo to Newborough. The Dunes of Newborough. Llanddwyn Island. Carnarvon

The Order of the minor sub-divisions of The Bedded Succession

Order of the Sub-divisions of the Groups. Horizons of Development of the Penmynydd Metamorphism

Chapter 11 — Recapitulation of the Mona Complex

Part 3 Subsequent formations and phenomena

Chapter 12 — The Baron Hill and Careg-Onen rocks and the Cambrian Period

Introductory

The Baron Hill Rocks: Introduction and petrology. Tectonic relations. Age of the rocks

The Careg-Onen Beds: Introduction and petrology. Palaeontology. Structure and relations. Geological age

Bwlch-Gwyn Felsite and Trefdraeth Conglomerate

Chronology of the Outliers

The Cambrian Period in Anglesey: Relations of the Cambrian and Ordovician Systems in Carnarvonshire. The Cambrian rocks of Anglesey

Chapter 13 — The Ordovician rocks

Introduction

Petrology: Conglomerates and Grits. Shales, Limestone, Cherts. Oolitic Ironstone

The Zonal Succession

Palaeontology: Faunas of the Zones. Upper Hartfell Beds. Zonal Types of Shale

The graptolitic sub-faunas of Anglesey, By Miss Elles

General view of the Ordovician rocks — The Northern Overlap

Thickness of the series

The Composition of the Sub-Ordovician floor (With its bearing on the age of the Mona Complex)

Physiography

Conclusion

Chapter 14 — Detail of the Ordovician rocks

The Menai fragments

The Llangoed area (With the Llanddona Outlier)

The strips along the Berw Faults

The Llangwyllog area

The Principal Area: The South-Western Wing and South-Eastern Margin. The Western Margin. The Country about Llanerchymedd. The Country To the north-east of Llanerchymedd. Llanbabo and the Cors-y-Bol country. The Northern Margin and Parys Mountain

Mynydd-Y-Garn to Carmel Head

Mynydd Eilian

The Northern Wedges: The succession. Fossils of Gynfor. Relation to the Mona Complex. The Gynfor Sections. Mynachdy

Chapter 15 — The Silurian rocks

The Parys Mountain Infold

The Rhos-Mynach Infold

Chapter 16 — The Palaeozoic intrusions

Introductory

Petrology: The Basic Dykes (varieties, selvages, veins, contact-metamorphism). The Basic Sills (diabases, hornblende-picrites, selvages, contact-metamorphism). Dykes and sills of intermediate composition. The Acid Dykes and their contact-metamorphism. The Acid Sills. The Lamprophyres. Scheme of the intrusions. Their unity. General characters and comparisons

Behaviour on the large scale: The Basic Dykes (dimensions, form, grouping, &c.). The Lamprophyres and Porphyrites. The Acid Dykes (dimensions, grouping, &c.). Composite or multiple dykes. The Basic Sills (relations, &c.). The Acid Sills

The age of the intrusions: Relations to the Mona Complex. Horizons traversed. Pebbles in conglomerates. Effects of earth-movements. The intrusive succession

Recapitulation

Chapter 17 — Detail of the Palaeozoic intrusions

The Basic Dykes. The Lamprophyre Dykes. The Intermediate Dykes. The Acid Dykes. The Basic Sills (diabases and picrites). The Sills of Intermediate Composition. The Acid Sills

Chapter 18 — The tectonics of the Ordovician and Silurian rocks

Introductory

Comparative tectonics of the several in-folds: The Menai Strait to the Llangwyllog area. The principal area and its wings. Ruptured Major Isoclines of the North-West and North-East. The Northern Wedges, Summary

The Carmel Head Thrust-Plane: Evidence of Rupture. Its Inclination. Its Dynamical Effects. Its Magnitude

Petrological effects of the movements: Cleavage. Disruption. Effects upon the Dykes and Sills (catamorphism and anamorphism)

Chronology of the movements: Folding, cleavage, thrusting. faulting, &c.. date of the movements

Comparison with the tectonics of the Mona Complex

General view of the movements

Chapter 19 — Palaeozoic metasomatism

Introduction

Parys Mountain: The Processes and Products. The Lodes. Ferrification. Chronology. Minerals

Sporadic metasomatism: Local Silicification, &c.. Kaolinite. Dolomites. Chronology

Genetics of the metasomatism: Conditions Precedent. Operation and Sources

Recapitulation

Chapter 20 — The Old Red Sandstone

Introductory

Petrology: Conglomerate and Sandstones. Mudstone, Marl, or Dust-rock. Cornstones. Limestones. Staining

Contemporaneous erosion and disturbance

Succession and general view

Tectonics: Molar movements. Internal modifications. Cleavage and movement

Thickness

Age, and relations to the Carboniferous

Physiography and Recapitulation

Chapter 21 — Detail of the Old Red Sandstone

The Coast. The Coed-y-gell Escarpment. The Penrhoslligwy Plateau. The Base around Mynydd Bodafon. Bodafon to Llangefni. Trwyn-cwmrwd and the Staining

Chapter 22 — The Carboniferous System and the Carboniferous Limestone

1. The Carboniferous Series as a whole

2. The Carboniferous Limestone Series

Introductory

Petrology: Sandstones. Conglomerates. Shale and Chert. The Limestones. Dolomitisation

Palaeontology: The Zones and Sub-Zones. Faunas of the Sub-Zones. Characters of the Fauna, by Dr. Ivor Thomas

Contemporaneous erosion and disturbance: Limestone Conglomerates. Erosive Junctions. Behaviour of Sandstones. Sandstone Pipes. Local Discordances, Local Disturbances. Discussion

General view of the Carboniferous Limestone

Thicknesses, overlap, and unconformity

Composition of the Sub-Carboniferous floor

Physiography

Conclusion

Chapter 23 — Detail of the Carboniferous Limestone

The Principal area: Lligwy, The Coast, and Pentraeth, 629- The Escarpment or Western Margin. The Interior. The Cefni to Bodorgan. The Esgeifiog Strip

The Straitside area: The Shore Section. The Escarpment. The Dip-Slope

The Small Outliers

The Penmon area: The Escarpment and Eastern Coast. The Northern Coast. Puffin Island

Chapter 24 — The Millstone Grit and Coal Measures

Introductory

The Millstone Grit: Petrology. Kaolinite. Nature of the Base. Palaeontology

The Coal Measures: Petrology. Palaeontology

Summary and correlation

Chapter 25 — The Red Measures

Introduction

Petrology: General. Heavy Minerals, &c.. Pebbles, their nature and condition

Unity Of The Red Measures

Succession and thickness

Relations, age, and conclusion

Chapter 26 — Detail of the Red Measures

Mainland and Foel Ferry. Holland Arms and Llangaffo. Berw and Morfa-mawr Shafts Material. The Shaft and Boring Records

Chapter 27 — The tectonics of the Carboniferous rocks

Types of Structure. The Internal and External Systems. The Berw Fault. Relations and Age of the Movements. Comparison with the Tectonics of the Older Systems. Recapitulation

Chapter 28 — The Later Dykes

Introductory

Petrology (general characters, veins and selvages, contact-metamorphism)

Comparison With The Palaeozoic Dykes

Behaviour on the large scale

Relations to the Palaeozoic dykes

Relations to sedimentary groups

Geological age

The Late Faults

Chapter 29 — Detail of the Later Dykes

Table. Those that outcrop in Carboniferous Rocks. Those that outcrop in the Mona Complex

Chapter 30 — Drifts and glaciation

Introductory

Modifications of the surface

Striation (deflections, &c.). Modelling, Shattering, &c.. Condition of the Pre-Glacial Surface

The Drifts: Introductory. Boulder-clays. Sands and Gravels. Moraines. Special Physical Features of the Drifts (eskers, bluffs, drumlins). Composition of the Drifts. Erratics (insular trains, extra-insular, shells, &c.). Insular and Extra-Insular Drifts. Uplifts. Carboniferous Limestone of Bryn-gwallen. Chronology of the Drifts

Theoretical summary: The Ice-sheet. Overflow-notches on the Bethesda Mountains (Depths of Ice upon the Mountain-land and Anglesey). Movements of the Ice. The Episode of Recession. The Decay of the Ice

Chapter 31 — The detail of the Drifts And Glaciation

Arrangement Of The Subject

Holy Isle: Surface and Striation. The Drifts: The Coast. The Interior (drumlins, boulders)

The North Central Country: Surface and Striation. The Drifts: The Coast. The Interior (drumlins, boulders)

The South Central Country: Surface and Striation. The Drifts: The Coast. Railway Sections. The Rest of the Interior. Boulders

The Northern Country: Surface and Striation. The Drifts: The Coast. The Interior (drumlins, &c.)

The Eastern Margin: Surface and Striation. The Drifts:-The Coast (red and blue clays). The Interior. Gravels and Eskers

The Penmon country: Surface and Striation. The Drifts:-Llanddona to Penmon. Penmon to Beaumaris (gravels and red clay)

The Menai country: Surface and Striation. The Drifts:-Ty'n-y-caeau Gravels, Menai shore, and Llanddwyn. The Inland Margins. The Railway Sections. The Rest of the Interior (boulders, &c.). Summary of the District

Chapter 32 — The later Superficial Deposits

Introductory. Raised Beach. Submerged Forest. Marine Alluvia. Fresh-water Alluvia. Blown Sand (Desert Scenery, Sand-blast, &c.). Miscellaneous Phenomena

Chapter 33 — The sea-floor

Sources of Evidence. The Menai Strait. The Sea-Basin. The Tertiary Anticline

Chapter 34 — The development of the land-surface

The platforms: The Menaian Platform. The Monadnocks. The Tregarth Shelf. Age and Origin of the Platforms. Tertiary Decay

The erosion of the platforms: Watershed and Valley Systems (Longitudinal and Transverse Valleys, the Northern Barrier, &c.)

Studies of special features: The Rivers Cefni and Braint. The Menai Strait and its Development. The Strait of Holy Isle. Fydlyn. The Lakes. The Forms of the Coast-line. Types of Scenery. Nature, Origin, and Age of the Boss-Lands

Post-Glacial erosion: Inland (diversion of rivers, limestone-escarpments). The Coast (Llangwyfan Islet, &c.). Holyhead Breakwater

Chronological sketch of the development of the land-surface

Part 4 — Economics

Introductory

>Chapter 35 — Coal mining

Its History. Output and Quality. Details of the Mines and Seams. Tabulation of the Seams. Structure of the Coal-field. The Relations of the Red Measures. Concluding Considerations

Chapter 36 — The metalliferous mines of Parys Mountain

Introductory. History of the Mines. Extent of the Mine. The Ores. The Lodes. Primary Ore-Zones. Gossan and its date. Anglesite. Treatment, in early and modern times. Output. Conclusion

Chapter 37 — Other metalliferous deposits

Sulphides and their Associates. Miscellaneous. Ironstones. Note on Gold and Silver

Chapter 38 — Various mineral industries and resources

Silica-Rocks. Limestone for Burning. Brickworks. Building-Stone. Sites for Houses. Road-stones. Railway

Ballast. Asphalt. Paving-Flags, Curbs, and Setts. Minor Industries and Resources. Sand. Peat. Marl. Millstones and Rollers. Ochre and Umber. Asbestos. Rocks suitable for Decoration. Churchyard and Cemetery Monuments

Chapter 39 — Water-supply

Introductory, Difficulties of the Subject. The Mona Complex (its deceptive structures, &c.). The Ordovician and Associated Rocks. The Old Red Series. The Carboniferous Rocks. The Glacial and Later Superficial Deposits. Borings near Boundaries. Pollution. General Remarks

Chapter 40 — Agriculture

Introductory. Soils of the principal formations. The distribution of the Drifts. Zones of Glacial Overlap. Reclamations of Land. The Woodlands. Grass-weaving. Summary

Part 5 — Conclusion

Chapter 41 — Additional studies

Miscellaneous studies: The Hereford Earthquake of 1896. The Ice-worn Slab in the Dolmen of Henblâs. Land-surface and Earth-movement. The Depth of Red Measure Erosion. The Age of the Palaeozoic Intrusions. Palaeozoic and Tertiary Vulcanism. A Persistent Petrological Tendency. The Age of the Careg-onen Beds. The Determination and Development of the Coast-line. The Comparative Heights of Anglesey and of the Mountain-land

Additional studies on the Mona Complex: The Mona Complex outside Anglesey. Vulcanism in the Mona Complex. Another Analysis of the Holyhead Quartzite, Some Improvements in Nomenclature, The Plâs Newydd Boring. The Age of the North Stack and Namarch Faults. The Penmynydd Zone. Folded. Mono-planic Schists. The Age. of the Gneisses. The 'Mutual Relations of the Basic and Acid Gneisses. The Occurrence of Soldier's Point Beds to the West of the Namarch Fault. The Fydlyn-Gwna Junction. The Gwna-Skerries Junction. The Age of the Mona Complex. Folding and Metamorphism. Reconciliation of the Metamorphic witli the Tectonic Succession, Ulterior Bearings of the Study of the Complex

Chapter 42 — General chronological recapitulation

Appendices

Appendix 1 Particulars of the geological maps

Appendix 2 Statistics of fossils, rock-specimens and slides, chemical analyses, and photographs

Appendix 3 Particulars of the microphotographs

Appendix 4 Particulars of the folding-plates

Cancellation of certain appendices

Supplement

Metamorphism in the Mona Complex

Introductory

Comparative view of the metamorphic products

Ætiology of the metamorphism: Metasomatic, Thermal, and Catamorphic Agencies. Dynamical Origin of the Regional Anamorphism. Foliation and Folding. Ætiology of the Textures. The Two Paths of Intensification. Tectonics and Anamorphism: a Correlation

The metamorphism in space and time

Recapitulation

General and regional ætiology

Figures and plates

BGS Photographs

The geology of Anglesey

2024 notes:

Brief contents

Part 1 Preliminary

Part 2 — The Mona Complex

Part 3 Sunsequent formations and phenomena

Part 4 — Economics

Part 5 Conclusion

Appendices

Full Contents

Part 1 — Preliminary

Part 2 — The Mona Complex

Part 3 Subsequent formations and phenomena

Part 4 — Economics

Part 5 — Conclusion

Appendices