Geological excursions in the Harlech Dome: classical areas of British geology

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Allen, P. M., Jackson, Audrey A. 1985 Geological excursions in the Harlech Dome: classical areas of British geology, Institute of Geological Sciences. (London: Her Majesty's Stationery Office.)

Geological excursions in the Harlech Dome: classical areas of British geology[edit]

Chapter 1 Introduction


Geological history

Geological structure



Chapter 2 Geological excursions

1 Ffridd-bryn-coch
2 Barmouth
3 Clogau goldmine
4 Capel Hermon
5 Bryn-llin-fawr
6 Allt Lwyd
7 Upper Afon Melau valley
8 Llandanwg

Chapter 3 Geological notes on popular walks

9 Barmouth to Hafotty mines
10 Panorama Walk
11 Roman Steps (Bwlch Tyddiad)
12 Drovers' Road, Llanbedr to Bontddu
13 New Precipice Walk
14 Precipice Walk
15 Ty'n-ygroes to Gwynfynydd
16 Pont Dolgefeiliau to Gwynfynydd
17 Torrent Walk

Chapter 4 The Harlech dome road circuit






  • National Grid references, given in the form [SH 7140 2863] throughout, all lie within the 100-km square SH.
  • Numbers preceded by E refer to thin sections in the collections of the British Geological Survey.


Every year hundreds of students of geology visit the Harlech dome. The only guide to this classical area of Cambrian geology has hitherto been a geological map at one inch to one mile published by C. A. Marley and T. S. Wilson in 1946. To meet the needs of these visitors, this book which supplements the recently published 1:50 000 Harlech geological sheet, provides detailed geological notes and large-scale maps of carefully selected excursions in the Harlech dome. In addition, geological notes are provided (and a glossary of terms) for some of the more popular walks in the area, with he hope that they can be used by anyone with an interest in geology.

G. M. Brown Director. British Geological Survey Keyworth, Nottingham NG12 5GG. 4 January 1985.


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ALLEN, P. M., EASTERBROOK, G. D. 1978. Mineralised breccia pipe and other intrusion breccias in Harlech Dome, north Wales. Trans. Inst. MM. Metall., Vol. 87, pp. B157–161.

ALLEN, P. M. JACKSON, A. A. 1978. Bryn-teg Borehole, North Wales. Bull. Geol. Surv. G. B. , No. 61, 51 pp.

ALLEN, P. M. JACKSON, A. A. RUSHTON, A. W. A. 1981. The stratigraphy of the Mawddach Group in the Cambrian succession of North Wales. Proc. Yorkshire Geol. Soc. , Vol. 43, pp. 295–329.

ANDREW, A. R. 1910. The geology of the Dolgelly Gold Belt, North Wales. Geol. Mag. , Dec. 5, Vol. 7, pp. 159–171, 201–211, 261–271.

COX, A. H. and WELLS, A. K. 1927. The geology of the Dolgelly District, Merionethshire. Proc. Geol. Assoc. , Vol. 38, pp. 265–331.

CRIMES, T. P. 1970. A facies analysis of the Cambrian of Wales. Palaeogeogr. , Palaeoclimatol., Palaeoecol. , Vol. 7, pp. 113–170.

DZULINSKI, S. and WALTON, E. K. 1965. Sedimentary features of flysch and greywacke. In Developments in Sedimentology, Vol. 7. (Amsterdam: Elsevier Press.)

FEARNSIDES, W. G. 1905. On the geology of Arenig Fawr and Moel Llyfnant. Q. J. Geol. Soc. London, Vol. 61, pp. 608–640.

FOSTER, H. D. 1968. The glaciation of the Harlech Dome. Unpublished PhD thesis, University of London.

GLASBY, G. P. 1974. A geochemical study of the manganese ore deposits of the Harlech Dome, North Wales. J. Earth Sci., Vol. 8, pp. 445–450.

HALL, G. W. 1975. The gold mines of Merioneth. (Gloucester: Griffin Publications.)

KOKELAAR, B. P. 1979. Tremadoc to Llanvirn Volcanism on the southeast side of the Harlech dome (Rhobell Fawr), N. Wales. Pp. 591–596 in The Caledonides of the British Isles reviewed. HARRIS, A. L., HOLLAND, C. H., LEAKE, B. E. (Editors). Spec. Pub. Geol. Soc. London, No. 8.

LYNAS, B. D. T. 1973. The Cambrian and Ordovician rocks of the Migneint area, North Wales. J. Geol. Soc. London, Vol. 129, pp. 481–503.

MATLEY, C. A. and WILSON, T. S. 1946. The Harlech Dome, north of the Barmouth estuary. Q. J. Geol. Soc. London, Vol. 102, pp. 1–40.

MOHR, P. A. 1964. Genesis of the Cambrian manganese rocks of North Wales. J. Sediment. Petrol., Vol. 34, pp. 819–829. MORRISON, T. A. 1975. Goldmining in Western Merioneth. 

(Llandysul: Merioneth Historical and Records Society.) 98 pp. RAMSAY, A. C. 1866. The geology of North Wales. Mem. Geol. Surv. G. B. , Vol. 3, 381 pp.

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RIDGWAY, J. 1975. The stratigraphy of Ordovician volcanic rocks on the southern and eastern flanks of the Harlech Dome in Merionethshire. Geol. J. , Vol. 10, pp. 87–106.

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Acidic Relating to igneous rocks containing over 63 per cent silica
Altiplanation terrace Hillside bench cut in solid rock and formed in periglacial conditions by processes involving solifluction and mass movement
Argillaceous Relating to sediment composed of clay sized particles
Autoclastic breccia A rock formed by mechanical crushing in situ
Basalt Fine-grained, dark coloured igneous rock, usually extrusive, composed mainly of plagioclase in the labradorite to bytownite range and pyroxene
Base metal Common chemically active metal; for example, lead, copper, zinc
Basic Relating to igneous rocks with less than 52 per cent of silica
Benthonic Relating to bottom-dwelling marine life
Bornite Sulphide of copper and iron (Cu5FeS4) with iridescent tarnish (Peacock Ore)
Boulder Clay Glacial deposit consisting of unsorted boulders and pebbles in clay matrix. Synonymous with till
Brachiopod Solitary marine invertebrate characterised by having two symmetrical but dissimilar shells or valves
Breccia A coarse-grained elastic rock composed of angular rock fragments
Breccia pipe Roughly cylindrical body of intrusive breccia
Caledonian orogeny Name used for the orogenic belt (the Caledonides) stretching from Ireland through Britain to Scandinavia. Dated as end-Silurian but also used to include earlier pulses
Chalcopyrite Bright brass-yellow ore mineral of copper (CuFeS2)
Chronostratigraphy Branch of stratigraphy which relates strata to time
Cleavage Fissile structure in a rock produced by deformation or metamorphism which enables it to split into thin laminae along secondary aligned fractures
Comagmatic Related to igneous rocks interpreted as having been derived from a common parent-magma
Contourite Sedimentary rock deposited from a contour-following bottom current. Usually recognised as a layer of relatively coarse-grained sediment in marine muds or silts
Dolerite Medium-grained, igneous intrusive rock of similar composition to basalt
Drumlin Low, oval hill or ridge of glacial till built under the margin of the ice and shaped by its flow so that the longer axis is parallel with the direction of movement of the ice
Epiclastic Related to a rock formed by the breakdown and consolidation of preexisting rocks
Fluvioglacial Related to the deposits produced by meltwater streams flowing from a glacier
Fold Bend of a planar structure in rocks, for example, bedding planes or cleavage
Fold axis Line on a map which traces the crest or trough of a fold
Galena Bluish grey ore mineral of lead (PbS)
Geosyncline Large-scale downwarp in the surface of the earth in which thousands of metres of sedimentary and volcanic rocks accumulate
Graptolites Extinct group of colonial marine animals with a corneous skeleton. In Dicoonema the colony formed a conical net
Greenschist facies Low grade regional metamorphism corresponding to a temperature range of 300 to 500°C
Greywacke An impure sandstone with more than 15 per cent interstitial matrix consisting of mica, chlorite and quartz. Grains include quartz, feldspar and lithic rock fragments
Head Deposit consisting of locally derived unsorted material formed by solifluction usually under periglacial conditions
Hiatus Break in continuity of the stratigraphic record either by erosion or non-deposition, and the time-value associated with this period
Hyaloclastite A deposit formed by the rapid chilling of a lava or magma where it flows into water or saturated sediment causing it to shatter into small angular fragments
Hydrothermal alteration The alteration of rocks or minerals by the action of hot water circulating underground
Hyolithid Extinct group of molluscs with a narrowly conical lidded shell
Intermediate Relating to igneous rocks containing 52 to 63 per cent silica
Intrusive breccia Heterogeneous mixture of angular fragments which has been mobilised and intruded into its present position
Lapilli Fragments between 2 and 64 mm in diameter ejected by a volcanic eruption
Lava breccia Autoclastic breccia produced by the fragmentation of the upper and lower crusts of a lava during now
Lithostratigraphy That part of stratigraphy which deals with the nature and composition of strata
Load cast A sole mark or depression on the base of a bed caused by unequal settling and compaction of the overlying material
Magnetite Ore mineral of iron (Fe3O4)
Mass-flow deposits (mud flow) Deposit formed by mass-movement of soil and rock debris down a slope. May be triggered by earthquake or by oversaturation
Mass wasting Dislodgement and downslope movement of soil and rock material under gravity. Includes processes such as solifluction, rock slides, soil creep and mass-flow
Microdiorite Medium-grained intermediate igneous rock consisting of plagioclase in the oligoclase to andesine range. Other minerals may include augite, hornblende, biotite, hypersthene. Primary quartz is normally less than 5 per cent
Microgranite Medium-grained acid igneous rock containing quartz and feldspar with some dark minerals which may include hornblende and biotite
Microtonalite Medium-grained intermediate igneous rock of similar composition to microdiorite but containing more than 10 per cent primary quartz
Molybdenite Lead-grey flaky molybdenum mineral (MoS2)
Moraine Debris eroded and redeposited by a glacier consisting of unsorted unstratified till. Lateral and terminal moraines are the deposits at the side and end of a glacier respectively
Oolith Round or oval accretionary particle in a sedimentary rock (0.25 to 2 mm in diameter)
Orogeny Process of formation of mountains
Pericline A fold in the form of a dome or basin
Periglacial Relating to the conditions and processes existing around the margins of a glacier or ice-sheet
Phenocryst Large conspicuous crystal in a porphyritic rock
Porphyry An igneous rock of any composition containing phenocrysts in a fine-grained groundmass
Porphyry copper An ore deposit composed of a large body of rock containing small quantities of disseminated chalcopyrite and other sulphide mineral
Pyrite Brass-yellow ore of iron known as 'fools gold' (FeS2)
Pyrrhotite Silver-yellow magnetic form of iron sulphide
Quartz-microdiorite A form of microdiorite with between 5 and 10 per cent quartz
Quartz wacke Impure sandstone containing more than 15 per cent detrital matrix (sericite and chlorite). Grains are mainly of quartz with less than 10 per cent feldspar and less than 10 per cent of rock fragments
Rhodochrosite Pink or reddish ore of manganese (MnCO3)
Solifluction Downslope flow of waterlogged soil and other unsorted material
Spessartine Red-brown to yellowish manganese aluminium silicate (variety of garnet)
Sphalerite Yellowish brown to dark brown zinc ore (ZnS)
Subarkose Sandstone with little detrital matrix (less than about 15 per cent) with grains predominantly of feldspar with less abundant lithic clasts and quartz
Subgreywacke Sandstone with little detrital matrix (as in subarkose). Grains consist of quartz, rock fragments and feldspar. The proportion of rock fragments exceeds that of feldspar
Subduction Process whereby one part of the earth's crust descends beneath another
Subvolcanic Relating to intrusions and other phenomena in the basement beneath a volcano
Till Synonymous with boulder clay
Trilobite An extinct group of marine anthropods (class Trilobita) in which the exoskeleton was divided longitudinally into three lobes (Figure 20)
Tuff A rock formed by the consolidation of volcanic ash
Tuffite A mixed rock consisting of >25 per cent pyroclastic and >25 per cent epiclastic or detrital material
Turbidite The consolidated deposit of a turbidity current. These rocks are characterised by graded bedding, moderate sorting and well developed sequence of bedding structures (Bouma cycle)
Xenolith A foreign inclusion in an igneous rock