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Earth Science Conservation Review
| Glendarragh Burn, Glenlark | Tyrone |
| Site number: | 139 | ||
| ESCC: | EO | ||
| Locality Type: | Stream section | Status: | ESCR |
| Grid Reference: | H592891 Centroid | ||
| Google maps: | 54.74682,-7.08069 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Precambrian | ||
| Stages: | Dalradian | ||
| Lithostrat: | Glengawna Formation, Southern Highlands Group | ||
| Site Description | |
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| Highlights: | |
This site offers access to exposures of the Glengawna Formation and enables lithological comparisons with the stratigraphically older (Glenelly) and the younger (Mullaghcarn) Formations. | |
| Introduction: | |
The site is located in Glenlark approximately 10kms east-north-east of Gortin. It extends along Glendarragh Burn, one of several north bank tributaries to the Glenlark River to an unnamed south-bank tributary. The Glendarragh Burn flows south-eastwards on the slope of the long interfluve which separates Glenlark from Glenelly. The confluence with the Glenlark River [H592 890] lies 2.25kms upstream of Glenlark Bridge [H576 875]. Access to the site is via the roads/tracks which run along both sides of Glenlark. The strata exposed in Glendarragh Burn are deformed and regionally metamorphosed metasedimentary rocks which lie on the southern, inverted limb of the Sperrin Overfold (F2). All of the rocks have a well-developed composite foliation (S1-S3?) broadly axial planar to this large NE to SW trending, southwards-facing structure. Consequently, throughout this site, bedding and foliation dip consistently towards the north-west and 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) and in the accompanying memoir (Nolan, 1884), the rocks in Glendarragh Burn and Glenlark were represented under the general category of "Chloritic, talcose & micaceous schists" of supposed Lower Silurian age. In the earliest detailed lithostratigraphic study of the Sperrin Mountains (Hartley, 1938) noted the presence of distinctive graphitic schists in Glenlark although on his map, he did not differentiate these from the surrounding "Mullaghcarn Schists". Arthurs (1976) revised the stratigraphy of the Sperrin Mountains and defined the Glengawna Formation as an additional distinctive subdivision lying stratigraphically between the Mullaghcarn and Glenelly Formations. Recent mapping of the area by the Geological Survey of Northern Ireland has confirmed the stratigraphic status of the Glengawna Formation as a distinctive horizon of graphitic mica schists within the Southern Highland Group (Geological Survey of Northern Ireland, 1995), see table below. It should be noted at this point that the stratigraphic and structural model of the southern Sperrin Mountains currently favoured by the Geological Survey of Northern Ireland has recently been challenged by Alsop et al. (1993). They have grouped all of the formations in this area (Glenelly, Glengawna and Mullaghcarn) into a single renamed Sperrin Formation, which they suggest is part of the Argyll Group (Middle Dalradian) not the Southern Highland Group (Upper Dalradian) as earlier workers have suggested. TABLE TO BE ADDED Dalradian stratigraphy and ESCR sites in the southern Sperrin Mountains. This reinterpretation is currently challenged by Arthurs & Johnston & Smith (Arthurs et al., 1994). | |
| Description: | |
In Glendarragh Burn, approximately 300m upstream from Glencullin Road Bridge [H5895 8950] there are good but intermittent exposures of the uppermost beds of the Glenelly Formation. These include units of fine-grained, greyish-green chloritic semipelite interbedded with medium-grained psammite containing micaceous partings, occasional fine to medium-grained quartzitic flags. The stratigraphic top of the Glenelly Formation and base of the younger Glengawna Formation occurs at [H5911 8920] where silvery grey semipelite and quartz-mica schists containing feldspar porphyroblasts (Glenelly Formation) overlie flaggy black graphitic mica schist which represents the lowermost unit in the Glengawna Formation. The remaining outcrops in Glendarragh Burn, down to the Bridge at [H592 891], consist of silvery green semi-pelitic and pale quartz psammitic lithologies typical of the Glengawna Formation. The top of the Glengawna Formation is exposed in an unnamed south-bank tributary [H595 890] to the Glenlark River. This stream is deeply incised in the fluvio-glacial sand and gravel deposits which fill Glenlark at this point. Although the quality of exposure here is poor, the Geological Survey of Northern Ireland has formally designated this as the stratotype for the Glengawna Formation. Despite the indifferent exposure, it is possible to trace the transition between the off-white quartz psammite, quartz-mica schist and grey graphitic schist (Glengawna Formation) and the pale grey, flaggy quartzose psammite and semipelite of the Mullaghcarn Formation. The Glengawna Formation has an estimated total stratigraphic thickness of 400m in Glenlark. | |
| Importance: | |
Glendarragh Burn offers a rare opportunity to see outcrops across the otherwise indifferently exposed Glengawna Formation and to study stratigraphic relationships with the overlying and underlying formations. The site is important as it exposes the top of the formation and the base of the succeeding Mullaghcarn Formation. The Glengawna Formation is also an important lithological variant and marker horizon within the otherwise homogeneous turbidite facies sequence which constitutes the Southern Highland Group. Further work is required to understand more fully the paragenesis of graphite (organic carbon?) within the Glengawna Formation. | |
| Interpretation: | |
The metasedimentary rocks exposed in Glendarragh Burn and in Glenlark formed as a result of regional (Greenschist Facies) metamorphism, deformation and south-east-directed thrusting of turbidite facies sediments. These sediments were originally deposited in subsiding marine basins on the continental margin of Laurentia during late Precambrian times (Upper Dalradian). The Glengawna Formation represents a significant change in depositional environment during part of the Upper Dalradian. The presence of a significant component of apparently strata-bound graphite in the schists of the Glengawna Formation points perhaps to a change of sediment provenance, to a source with a higher organic carbon content. Perhaps, more plausibly, the Glengawna Formation represents a facies change with establishment of depositional conditions where organic carbon within the sediment had an opportunity to rain out and survive in muds on the sea bed in a generally low-energy environment. During late-Caledonian times, 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 Glengawna Formation lies on the overturned limb of the Sperrin Overfold. Consequently these strata dip towards the north-west and young towards the south-east. Primary way-up evidence is rarely seen in the Glengawna and Glenelly formations 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 of a penetrative, northwest-dipping, sometimes crenulating, composite planar fabric (S1-S3?). | |
| Conclusions: | |
Glendarragh/Glenlark is a site of undoubted national significance. It is the stratotype for the top of the Glengawna Formation and will continue to play an important role in stratigraphic studies in the Dalradian of Northern Ireland. | |
| 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 135 - Garvagh Burn Key Site 136 - Barnes Gap Key Site 137 - Barnes Burn Key Site 138 - Golan Burn 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 This area is also important for Mineralogy and Metallogenesis. See Key Site 558 - Glenlark. | |
| Keywords | |
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| Rocks: | Pelitic rock, Psammite, Semi-pelitic rock |
| Access | |
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| Approach: | At present the Glendarragh Burn is readily accessible via a gate beside Glencullin Road Bridge [H5895 8950]. |
| Management: | |
The areas of interest at this site lie entirely within the confines of the Glendarragh Burn and the small, deeply-incised south bank tributary to Glenlark Burn. At present the Glendarragh Burn is readily accessible via a gate beside Glencullin Road Bridge [H5895 8950]. The Burn is well fenced from adjoining pastureland along both banks and does not appear to be under any imminent threat from development or tipping. Currently the water quality in this stream appears to be good. The stream on the southern side of Glenlark is less well fenced and lies directly south of a large privately owned commercial forest which now extends across much of the upper reaches of Glenlark. It is conceivable that this southern part of the site could, in the future come under some pressure and become less accessible if afforestation extends further towards the south-west along Glenlark. Overall the site is capable of withstanding reasonable sampling pressures and there is no need to impose special restrictions in this respect. | |
| 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. Glendarragh Burn, Glenlark, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=139. Accessed on 2026-11-06. |
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