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Earth Science Conservation Review
| Henry's Bridge, Glensass Burn | Tyrone |
| Site number: | 133 | ||
| ESCC: | EO | ||
| Locality Type: | Stream section | Status: | ESCR |
| Grid Reference: | H515917 Centroid | ||
| Google maps: | 54.77102,-7.19978 | ||
| County: | Tyrone | Council area: | Derry City & Strabane District Council |
| Period: | Precambrian | ||
| Stages: | Dalradian | ||
| Lithostrat: | Dart Formation, Henry's Bridge Member, Southern Highlands Group | ||
| Site Description | |
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| Highlights: | |
This site contains the stratotype (type section) of the Henry's Bridge Member of the Dart Formation. This is a distinctive "green bed" member which can be traced laterally and is a valuable stratigraphic marker within the otherwise mainly psammitic succession of the Dart Formation. | |
| Introduction: | |
The site is located 3.5km east of Plumbridge directly north of the B47 Plumbridge- Draperstown Road. Glensass Burn is a north-bank tributary of the Glenelly River. The area of scientific interest lies in the stream bed and extends for approximately 250m upstream from Henry's Bridge [H515 917]. The strata at this site are regionally metamorphosed (Greenschist Facies) sedimentary and volcanically derived rocks which form part of the Dart Formation of the Southern Highland Group (Upper Dalradian), see table below. The site lies on the southern, inverted limb of a large north-east-south-west trending, south-east-facing F2 fold structure (the Sperrin Overfold). All the rocks have a well developed composite foliation (S0-S3?) which is broadly axial planar to this fold. As a result of folding, the bedding and foliation of the strata at this site and throughout the area, dip consistently towards the north-west and overall, the succession youngs towards the south-west. On the first edition of the one inch to one mile geological map of the area (Geological Survey of Ireland, 1882) and in the accompanying memoir (Nolan, 1884), the rocks north of Henry's Bridge in Glensass Burn were not differentiated from the general category of "Chloritic, talcose & micaceous schists of supposed Lower Silurian Age" which, with the exception of limestones, was applied to all of the metamorphic rocks in the Sperrin Mountains. Hartley (1938) published the first detailed lithostratigraphic map which showed green beds within the "Tourmaline Schists of Glenelly in the vicinity of Henry's Bridge". More recently Arthurs (1976) redefined the main formation boundaries in the Sperrins and included the Glensass Burn within the Dart Formation. These revised boundaries, with minor modifications, form the basis of the Upper Dalradian stratigraphy for the second edition of the geological map of the Draperstown area (Sheet 26) (Geological Survey of Northern Ireland, 1995). TABLE TO BE ADDED Dalradian stratigraphy and ESCR sites in the southern Sperrin Mountains. The stratigraphic and structural model of the southern Sperrin Mountains favoured by Arthurs (1976) and the Geological Survey of Northern Ireland (1995), has recently been challenged by Alsop et al. (1993). They suggest that all the formations of the area, including the Dart Formation, belong to a single Sperrin Formation and form part of the Argyll Group (Middle Dalradian) not the Southern Highland Group (Upper Dalradian). Ongoing work by the Geological Survey of Northern Ireland does not support this reinterpretation which is strenuously challenged (Arthurs et al., 1994). SITE | |
| Description: | |
The strata which are intermittently exposed in Glensass Burn for about 150m above Henry's Bridge consist of greenish quartzose psammite and dark greyish green speckled quartz- feldspar porphyroblastic semipelite with biotite and white mica. The stratigraphic top (structural base) of the Henry's Bridge Member occurs 150m upstream from Henry's Bridge. The Henry's Bridge Member is estimated to be about 50m thick and consists of dark greenish brown, fairly compact massive schist with scattered pale feldspar (albite) porphyroblasts. In thin section [NI 4513] these albite porphyroblasts are both abutted and wrapped by biotite and epidote and the rocks contain subordinate chlorite, clinozoisite and iron ore. Quartz appears to be only a minor constituent and white mica appears to be completely absent from the green beds proper. Within the Henry's Bridge Member there are thin intercalations of more quartzo-feldspathic rocks. These include pale granular albitized semipelite with conspicuous quartz, white mica and small garnets contained within albite porphyroblasts. There are also some thin beds with a more granular gritty psammitic texture. The base of the Henry's Bridge Member occurs about 200m upstream from Henry's Bridge. This is a gradational boundary and the dark green chloritic schists give way upstream to grey and greyish green fine-grained quartzo-feldspathic psammite, psammitic flags and semipelite. | |
| Importance: | |
This site is the stratotype (type section) for the Henry's Bridge Member. This is an important lithological variant within the otherwise largely homogenous turbiditic facies rocks of the Dart Formation. Such horizons have already proved to be of some limited use for establishing the local stratigraphy. It is hoped that research currently being planned into the sedimentological and geochemical characterisation of Dalradian green beds in Northern Ireland and Scotland could result in their more widespread use for establishing correlations on a regional scale. | |
| Interpretation: | |
The Henry's Bridge Member is one of several distinctive chloritic green bed intervals within the Dart Formation. These green beds, which are believed to have originally formed from sediments derived mainly from volcanic sources also locally contain a small amount of siliciclastic sediment. During Upper Dalradian times, the continental crust along the margin of Laurentia became unstable. The crust thinned and fractured under the influence of a tensional tectonic regime, and a series of marine sedimentary basins developed along the continental margin. These basins filled rapidly and subsided to accommodate large thicknesses of mainly siliciclastic turbidite facies sediments (Southern Highlands Group, Dart Formation etc.). Tectonism was accompanied by a major episode of intrusive and extrusive volcanic activity. Although this activity peaked towards the end of Middle Dalradian times (Argyll Group, Dungiven Formation), the presence of "green beds" (Henry's Bridge Member) within the Southern Highland Group is evidence of continued, periodic input of sediments derived from volcanic sources, into Upper Dalradian times. Structurally, the Dart Formation lies on the overturned limb of the Sperrin Nappe. Consequently these strata dip towards the north-west and young towards the south-east. Primary way-up evidence is rare in the Dart Formation, obscured by the advanced state of recrystallization. In this structural zone of the Sperrin Mountains, the rocks have a well developed schistosity that is defined by a penetrative north-west dipping, sometimes crenulating composite planar fabric (S1-S3?). 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 regional metamorphism under Greenschist Facies conditions within the Barrovian Facies Series (Arthurs, 1976). | |
| Conclusions: | |
GlensassHenry's Bridge is a site of undoubted national importance. It is the listed stratotype for the Henry's Bridge Member, a distinctive green bed horizon. Such well defined horizons are comparatively rare in the Dalradian in Northern Ireland and have an important role to play in helping establish local and regional stratigraphic correlations. | |
| 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 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 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 |
| Access | |
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| Approach: | Access is via the parapet of Henry's Bridge or across farmland further up the Glensass Burn. |
| Management: | |
The site lies entirely within Glensass Burn and is currently well fenced from the adjoining pastureland along both banks. Access is via the parapet of Henry's Bridge or across farmland further up the Glensass Burn. The stream is currently clear of overgrowing vegetation, water quality appears to be good and there is no evidence of tipping. There are no buildings in the vicinity of the site and it seems unlikely that the site will be threatened by future building development. Overall, the site is well capable of withstanding reasonable sampling pressures and there is no need to impose special restrictions in this respect. | |
| References (5) | |||
|---|---|---|---|
| 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. Henry's Bridge, Glensass Burn, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=133. Accessed on 2026-11-06. |
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