GCR block - Tertiary Igneous (TER-IGN)

Tertiary Igneous

Block Description

UK map showing distribution of GCR sites of GCR block TER-IGN
Distribution of GCR sites of GCR block Tertiary Igneous

The GCR sites selected for this GCR Block represent a major episode of intrusive and extrusive igneous activity in north-west Britain that occurred during Palaeocene and early Eocene times (c. 63–52 Ma) that accompanied continental separation and lithospheric attenuation during the early stages of the opening of the North Atlantic Ocean.

For details of stratigraphy see PGN and NEO.

Tertiary Igneous rock distribution

In Great Britain, volcanism was most vigorous in the Inner Hebrides and adjoining north-west Scotland but also extended to southern Scotland, north-east England and the Outer Hebrides. Intrusions of similar age in North Wales and the English Midlands are probably outlying representatives of contemporaneous activity in north-east Ireland, additional activity in the Bristol Channel centred on Lundy. The region encompassing the activity is known as the British Tertiary Volcanic Province (BTVP) or the British Tertiary Igneous Province (BTIP).

The varied igneous rocks, together with associated sediments and metamorphic rocks have responded in different ways to the profound erosion of the last fifty million years. Gabbro and peridotite have given rise to the rugged mountain scenery of St Kilda, the Skye Cuillins and Rum. The considerable, but generally less-rugged, mountains of northern Arran and the Skye Red Hills are composed of granite, while the piles of flat-lying lavas form tabular, ‘trap-featured’ hills in northern Skye, rising sometimes to form high mountains, as at Ben More, Mull.

Tertiary Igneous activity and petrology

The igneous activity took many forms and involved a wide variety of magmas and rock types. The remains of large accumulations of dominantly basaltic lava flows cover extensive areas in Skye and Mull. Laterally extensive swarms of basaltic dykes are most intense near Skye, Rum, Mull, and Arran but extend to the Outer Hebrides, southern Scotland, Cleveland in north Yorkshire and parts of North Wales and central England. Central intrusive complexes consisting of granite, gabbro, peridotite and other rock types occur on a line from Skye to the Bristol Channel and at several places in the north-east Atlantic; these are the deeply-dissected roots of major volcanoes.

Volcanism in the BTVP extended over a period of about eleven million years, largely within the Palaeocene Epoch, apart from the later intrusions within the Eastern Red Hills Centre of Skye, the Sgurr of Eigg pitchstone, the Lundy granite complex and the youngest intrusion in the Mourne Mountains Western Centre, Ireland. The Palaeocene volcanism was preceded by Cretaceous igneous activity in the eastern Atlantic area and it is also possible that the submarine central complex at the Blackstones, south-west of Mull, is Cretaceous in age. Within the BTVP, the life span of individual central complexes was short, of the order of two or three million years (or even less) and the thick lava accumulations built up over even shorter periods.

Volcanic activity in a given area often started with the formation of small amounts of basaltic ash and other volcaniclastic accumulations. These were quickly followed by voluminous subaerial eruptions of basaltic lavas which covered the peneplaned surfaces of older sedimentary and metamorphic rocks ranging in age from the Precambrian to the Cretaceous. Occasionally, the lavas covered landscapes of considerable relief, filling valleys, burying hills and sometimes flowing into shallow lakes, where pillow lavas and hyaloclastites formed. Sedimentary horizons are not common in the lavas, but fluviatile conglomerates, sandstones and fine-grained plant-bearing horizons do occur and provide valuable stratigraphical and palaeogeographical information. The lavas were often subjected to intense weathering between flow extrusion, with the formation of bright red lateritic deposits. With increasing thickness of lavas, heated waters circulating through the flows altered the basalts and deposited distinctive suites of zeolite minerals, for which Skye and Mull are particularly noted.

The lavas were principally fed from fissure eruptions similar to those of present day Iceland. The actual feeders are among the multitude of dykes forming the swarms which extend across the Province; dykes intrude virtually all intrusions and extrusions in the BTVP, so it is likely that lava effusion also occurred throughout the life span of the Province. However, the thick sequences of lavas now preserved in Mull, Skye, and the Small Isles built up between about 63 Ma and 60 Ma, early in the life of the BTVP. Occasionally, lavas must have been erupted as the central complexes developed; there is good evidence from several centres that silicic and intermediate lavas were closely associated with central complexes but there are few substantiated examples of basaltic lavas, with the exception of pillow lavas within the Mull centre. Not all the (predominantly basaltic) magma reached the surface to form flows, some froze in conduits as dykes and plugs and quite large amounts spread laterally through the Mesozoic sediments beneath the lavas to form the prominent dolerite sills of northern Skye and Arran.

The central complexes generally post-date the adjoining lavas, but they were intruded by later members of the dyke swarms. Within the complexes the magmatic sequences were rarely straightforward: in Mull early granite intrusions were followed by numerous basaltic, intermediate and acid cone-sheets, by gabbros and peridotites and by further granites; in Skye, the sequence was apparently simpler, the gabbros and peridotites of the Cuillins were cut by numerous basaltic cone-sheets and subsequently by granites of the Red Hills. Thus, the central complexes record varied intrusive sequences in which basaltic and granitic magmas have been intimately associated. Occasionally, it may be shown that contrasted magmas must have coexisted, forming, for example, the composite basalt–quartz–porphyry sheets and dykes of the Province and the complicated intrusion breccias found in Ardnamurchan, Skye and other central complexes.

Within the central complexes, individual centres of activity are defined by arcuate intrusions – cone-sheets, ring-dykes and stocks – which have a common focus. In most central complexes, and in particular Ardnamurchan, Skye and Mull, the intrusions of one centre may cut those of an earlier one, recording movements in the focus of magmatic activity with time. Ardnamurchan provides an exceptionally clear and often cited example of this phenomenon.

GCR site selection

The Tertiary igneous activity occurred in geographically well-defined areas which usually include a central volcano and surrounding lavas. These areas have been made the basis of the ‘networks’ for site selection.

• Isle of Skye

• Rum, Eigg, Muck, Canna–Sanday

• Ardnamurchan

• Isle of Mull

• Isle of Arran

• Rockall

• St Kilda

• Langbaurgh Ridge and Cliff Ridge

Volume Introduction

Tertiary Igneous

Site List

Your selection found 52 GCR sites. Sites are sorted alphabetically by country, local authority and then by site name -
CodeNameCountryLocal AuthorityGrid RefGCR Site Account
2266Cliff RidgeEnglandNorth Yorkshire CCNZ575116Not available
730Langbaurgh RidgeEnglandNorth Yorkshire CCNZ560121Site Account
1235Ard BheinnScotlandEast Ayrshire and North Ayrshire MainlandNR943335Site Account
1240Corrygills ShoreScotlandEast Ayrshire and North Ayrshire MainlandNS042353Site Account
1238Dippin HeadScotlandEast Ayrshire and North Ayrshire MainlandNS052224Site Account
1237Drumadoon - TormoreScotlandEast Ayrshire and North Ayrshire MainlandNR894287Site Account
1236Glen CatacolScotlandEast Ayrshire and North Ayrshire MainlandNR905450Site Account
1239South Coast of ArranScotlandEast Ayrshire and North Ayrshire MainlandNR952208Site Account
48RockallScotlandEilean Siar (Western Isles)xxSite Account
49Shiant IslandsScotlandEilean Siar (Western Isles)NG415980Site Account
50St KildaScotlandEilean Siar (Western Isles)NA100000Site Account
75Allt Geodh' a' GhamhnaScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG369197Site Account
25Allt Molach - Beinn ChaisgidleScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM622313Site Account
2Allt nam Ba - Beinn nan StacScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM405943Site Account
76An CleireachScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG335444Site Account
14Ardnamurchan Point to SannaScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM418670Site Account
18ArdtunScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM378248Site Account
17BearraichScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM417274Site Account
81Beinn an DubhaichScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG598192Site Account
13Beinn na Seilg to Beinn nan OrdScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM450627Site Account
10Ben HiantScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM547623Site Account
8Camas Mor, Isle of MuckScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM403789Site Account
22Carsaig BayScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM533219Site Account
15Centre 3, ArdnamurchanScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM467702Site Account
5Cnapan Breaca to Long LochScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM383987Site Account
80Coire UaigneichScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG535212Site Account
24Cruach ChoireadailScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM595305Site Account
83Cuillin HillsScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG490210Site Account
6Dibidil to Southern MountainsScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM367948Not available
9East Canna and SandayScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG288037Site Account
1FionchraScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG340000Site Account
70Fiurnean to Rubha na h-Airde GlaiseScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG510475Site Account
12Glas Bheinn to Glebe HillScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM479644Site Account
11Glas Eilean to Mingary PierScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM493626Site Account
3Hallival - AskivalScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM396963Site Account
4Harris BayScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM340955Site Account
82KilchristScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG600215Site Account
20Laggan BayScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM451415Site Account
26Loch Ba to Ben MoreScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM529368Site Account
19Loch SguabainScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM633305Site Account
23Loch Spelve to AuchnacraigScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM711297Site Account
78MarscoScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG508252Site Account
79Meall a' Mhaoil - Maol na GainmhichScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG555312Not available
72RoinevalScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG417347Site Account
74Ros a' MheallainScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG365415Site Account
2074Rubh' an EireannaichScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG646247Site Account
77Rubha HunishScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG410739Site Account
21'S Airde BeinnScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM472540Site Account
7South-west EiggScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNM442876Site Account
73TaliskerScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG318333Site Account
71The StorrScotlandLochaber, Skye and Lochalsh and Argyll and the IslandsNG495541Site Account
2775Ailsa CraigScotlandSouth AyrshireNX019997Not available
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