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