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Earth Science Conservation Review
| Garvagh Burn | Tyrone |
| Site number: | 135 | ||
| ESCC: | EO | ||
| Locality Type: | Stream section | Status: | ESCR |
| Grid Reference: | H621937 Centroid | ||
| Google maps: | 54.78779,-7.03468 | ||
| County: | Tyrone | Council area: | Derry City & Strabane District Council |
| Period: | Precambrian | ||
| Stages: | Dalradian | ||
| Lithostrat: | Garvagh Bridge Member, Glenelly Formation, Southern Highlands Group | ||
| Site Description | |
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| Highlights: | |
Garvagh Burn contains the stratotype (type section) of the Garvagh Bridge Member, a mixed unit consisting of calcareous schist, semipelite and impure meta-limestone within the Glenelly Formation. The site also contains turbidite facies rocks which are typical and constitute the bulk of the Glenelly Formation. | |
| Introduction: | |
Garvagh Burn is a north-bank tributary of the Glenelly River. It drains southwards from Dart Mountain on the main Sperrin Ridge and passes beneath Garvagh Bridge [H620 937], approximately 3km east of Cranagh on the B47 Glenelly Road. The site lies directly downstream from Garvagh Bridge with access via disused ground directly east of the bridge. The strata in Garvagh Burn are metasedimentary and metavolcanic rocks which form part of the Glenelly Formation of the Southern Highland Group (Upper Dalradian). These rocks lie on the southern, inverted limb of a large NE-SW trending, SE facing F2 fold structure (the Sperrin Overfold). All rocks in this area have a well developed composite (S1-S3?) foliation broadly axial planar to the Sperrin Overfold and consequently bedding and foliation dip consistently at about 45 degrees towards the north-west while overall, the succession youngs to the south-east. On the first edition of the one inch to one mile geological map of the area (Sheet 26, Draperstown) (Geological Survey of Ireland, 1882) limestone was shown in the Garvagh Burn and differentiated from the, "Chloritic, talcose & micaceous schists" classification which was, with a few exceptions applied to all other metamorphic rocks in the Sperrin Mountains. Manuscript notes on Nolan's field map state that a quarry was opened in the "limestone schist" for agricultural use but the quality proved too poor to warrant further extraction. In the memoir which accompanied Sheet 26, (Nolan, 1884) it was suggested that the limestone at Garvagh Bridge may be a continuation of the limestone found in Golan Burn. When Hartley (1938) produced the first detailed lithostratigraphic map of the Sperrin Mountains, all rocks in Glenelly, including Garvagh Burn, were grouped within the division he named "Tourmaline Schists of Glenelly South". Hartley described the green beds at Garvagh Burn (and Oughtboy Burn) but did not accord them any formal stratigraphic recognition. Arthurs (1976) redefined the main Dalradian formation boundaries in the Sperrin Mountains and incorporated all rocks in Glenelly within a revised Glenelly Formation. These revised boundaries, with minor modifications, formed the basis of the Upper Dalradian stratigraphy for the second edition of the geological map Sheet 26 (Draperstown) (Geological Survey of Northern Ireland, 1995), see table below. TABLE TO BE ADDED Dalradian stratigraphy and ESCR sites in the southern Sperrin Mountains. Recently the stratigraphic and structural model of the southern Sperrin Mountains proposed by Arthurs (1976) and currently favoured by the Geological Survey of Northern Ireland (1995), has been challenged by Alsop et al., (1993). They propose that all formations in the South Sperrins, including the Glenelly Formation, form part of one unit, Sperrin Formation, which belongs to the Argyll Group (Middle Dalradian) not the Southern Highland Group (Upper Dalradian). Recent and ongoing resurvey work by the Geological Survey of Northern Ireland does not support this reinterpretation which has since been challenged by Arthurs and by Johnston & Smith (Arthurs et al., 1994). | |
| Description: | |
Exposures in the Garvagh Burn, directly downstream from Garvagh Bridge, consist mainly of flaggy and massive grey-green speckled semipelite with feldspar porphyroblasts. These are interbedded with subordinate pale psammite and quartz mica schist containing scattered tourmaline. Downstream, this succession passes into approximately 10m of chloritic green beds, mainly albite porphyroblastic chlorite-biotite-epidote schist with minor garnet and up to 5% calcite. There is also some green speckled semipelite with conspicuous tourmaline needles on foliation surfaces towards the junction with the stratigraphic base of the Garvagh Bridge Member. The Garvagh Bridge Member is about 18m thick and is best observed in the disused, partially infilled quarry [H6207 9359] directly east of the Burn.  | |
| Importance: | |
Garvagh Burn is the officially designated stratotype (type section) for the Garvaghy Bridge Member. This distinctive limestone unit represents an important facies variant within the otherwise uniform turbidite facies rocks of the Glenelly Formation. Although Nolan (1884), proposed a correlation between the limestones at Garvagh Bridge and Golan Burn [H523 872] there is no evidence that this is a persistent horizon and it has not yet been possible to trace the Garvagh Bridge Member for any distance along strike. It is anticipated that the Garvagh Burn site could be a significant site in future research proposed by the Geological Survey of Northern Ireland into the sedimentology and provenance of limestones within the Argyll and Southern Highland Groups of the Dalradian in Northern Ireland and Scotland. | |
| Interpretation: | |
During Upper Dalradian times, the crust along the margin of Laurentia became unstable. The continental crust thinned and fractured under the influence of a tensional tectonic regime. A series of marine sedimentary basins developed along the continental margin. These basins filled rapidly during Upper Dalradian times and subsided to accommodate large thicknesses of mainly siliciclastic turbidite facies sediments (Southern Highlands Group, Glenelly Formation etc.). The Garvagh Bridge Member is the uppermost (youngest) of three significant calcareous horizons interbedded with the predominantly turbiditic rocks of the Glenelly Formation. These calcareous members include limestone and calcareous schist and are taken to represent original facies variations within the Glenelly Formation. The Garvagh Bridge Member indicates that intermittently, in parts of the depositional basin, turbidite facies deposition co-existed with or gave way to shallow water conditions. In these lower energy environments, established perhaps along the edges of the depositional basins, limestones were deposited in shallow water, well-oxygenated environments. During the late-Caledonian Orogeny, these sediments were deformed, folded and subjected to regional metamorphism. Metamorphic mineral assemblages typically include chlorite, biotite, muscovite and albite-oligoclase with tourmaline present as a characteristic accessory in semi-pelitic lithologies. Such mineral assemblages are consistent with the rocks having experienced Greenschist Facies conditions within the Barrovian Facies Series (Arthurs, 1976). Structurally, the Glenelly Formation lies on the overturned limb of the Sperrin Overfold. Consequently these strata dip north-west and young towards the south-east. Primary way-up evidence is however rare in the Glenelly Formation because of the advanced state of recrystallization in these rocks. In this structural zone of the Sperrin Mountains, the rocks generally have a well developed schistosity defined by a penetrative northwest-dipping sometimes crenulating composite planar fabric (S1-S3?). | |
| Conclusions: | |
Garvagh Burn is a site of national geological importance. It contains the listed stratotype (type section) for the Garvagh Burn Member, an important stratigraphic marker horizon in the Glenelly Formation. The Garvagh Bridge Member also has a role to play in planned studies aimed at understanding facies variations within the Upper Dalradian and extending stratigraphic correlations on a national or international scale. | |
| Notes: | |
For site specific information and references on other Dalradian sites in the Sperrin Mountains, Londonderry and Tyrone see the following sites: For Central and Southern Sperrin Mountains: Key Site 130 - Butterlope Glen Key Site 131 - Craig Key Site 132 - Mullaghcarbatagh Key Site 133 - Henry's Bridge, Glensass Burn Key Site 134 - Oughtboy Burn Key Site 136 - Barnes Gap Key Site 137 - Barnes Burn Key Site 138 - Golan Burn Key Site 139 - Glendarragh Burn, Glenlark Key Site 140 - Mountfield Old Quarry Key Site 141 - McNally's Burn, Broughderg For North Sperrin Mountains and Londonderry: Key Site 142 - Letterbrat Quarry Key Site 143 - Banagher Glen Quarry Key Site 144 - Strabane Quarry Key Site 145 - Glenmornan River, Artigarvan Key Site 146 - Balix Hill Key Site 147 - Drain Quarry Key Site 148 - Kildoag Quarry Key Site 149 - Loughermore Mountain Key Site 150 - Burntollet and Tamnymore Wood Key Site 151 - Dog Leap, Limavady Key Site 152 - Kittybane Quarry Key Site 153 - Prehen Quarry Key Site 154 - Creevagh Hill Old Quarry For site specific information and references on other Dalradian sites in North-east Antrim see the following sites: Key Site 155 - Benvan Key Site 156 - Escort Port Key Site 157 - Torr Head Key Site 158 - Altmore Burn Key Site 159 - Leckpatrick Point Key Site 160 - Carnaneigh Key Site 161 - Loughan Bay Key Site 162 - Loughareema Channel Key Site 163 - Knocknacarry Bridge Key Site 164 - Cushendun Bay and Rock Port For general information on the Dalradian rocks of Northern Ireland see the following site: Key Site 124 - The Dalradian Rocks of Northern Ireland For general and site specific information on the Pre-Dalradian rocks of Northern Ireland see the following site: Key Site 123 - The Pre-Dalradian Rocks of Northern Ireland For general information and an extensive reference list on the Precambrian of Northern Ireland see the following site: Key Site 122 - An Introduction to the Precambrian of Northern Ireland | |
| Keywords | |
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| Rocks: | Green beds, Limestone |
| Access | |
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| Approach: | The site lies directly downstream from Garvagh Bridge with access via disused ground directly east of the bridge. |
| Management: | |
This is a comparatively compact site including part of Garvagh Burn and an adjacent disused quarry pit. Access to the site is via a locked gate and through derelict ground directly east of Garvagh Bridge. Outcrops within the Burn, though overgrown and moss covered, do not appear to be under any imminent threat. There is however a potential problem in the disused quarry pit, where the main lithologies of the Garvagh Bridge Member are best exposed. At present this quarry is partially filled with a mixture of farm and household waste. Drainage in the quarry floor is poor and the combination of waste and water already makes access to some of the outcrop difficult if not dangerous. In the longer term, if dumping continues in an uncontrolled manner, waste will eventually obscure some of the most informative outcrops at the site. Outcrops in the stream and quarry are relatively abundant and the site should be capable of withstanding reasonable sampling pressures. There is therefore is no need to impose special restrictions on sampling. | |
| Threats: | Dumping of farm and household waste. |
| References (5) | |||
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| Alsop, G.I. & Hutton, D.H.W., 1993. Major southeast-directed Caledonian thrusting and folding in the Dalradian rocks of mid-Ulster: implications for Caledonian tectonics and mid-crustal shear zones Geological Magazine, vol. 130 pt. 2, pp.233-244. | |||
| Arthurs, J.W., 1976a. The geology and metalliferous mineral potential of the Sperrin Mountains area Geological Survey of Northern Ireland Special Report. | |||
| Arthurs, J.W., Johnston, T.P., Smith, R.A., Alsop, G.I. & Hutton, D.H.W., 1994. Discussion on major southeast-directed Caledonian thrusting and folding in the Dalradian rocks of mid-Ulster: Implications for Caledonian tectonics and mid-crustal shear zones Geological Magazine, vol. 131 pt. 3, pp.417-421. | |||
| Hartley, J.J., 1938. The Dalradian rocks of the Sperrin Mountains and adjacent areas in Northern Ireland Proceedings of the Royal Irish Academy, vol. 44 pt. B. | |||
| Nolan, J., 1884. Explanatory memoir to accompany Sheet 26 of the maps of the Geological Survey of Ireland Memoirs of the Geological Survey of Ireland. | |||
| Map(s): | GSI (1882). 1:63,360 Geological Sheet 26 (Draperstown), Dublin, OS for GSI GSNI (1995). 1:50,000 Draperstown, Northern Ireland Sheet 26, Solid, BGS, Keyworth, Nottingham | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Last updated: | 6 Jun 2005 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Garvagh Burn, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=135. Accessed on 2026-11-06. |
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