Earth Science Conservation Review
| Sruhanleanantawey | Londonderry |
| Site Type: | Stream section |
| Site Status: | ASSI |
| Grid Reference: | H788886 |
| Google maps: | 54.73964,-6.77653 |
| Rocks | |
|---|---|
| Rock Age: | Ordovician (Arenig, Llanvirn) |
| Rock Name: | Tyrone Volcanic Group |
| Rock Type: | Chert, Mudstone, Porphyry |
| Interest | |
| Fossil Groups: | Graptolite |
| Other interest: | Caledonian |
Introduction:
Sruhanleanantawey Burn rises on Glenarudda Mountain at a height of about 500m above sea level. The stream flows northwards to become a tributary of the White Water which in turn is part of the Moyola catchment. Various lithologies of the Tyrone Volcanic Group are exposed in the bed and banks of Sruhanleanantawey Burn above the 380m contour. Most significantly, isolated outcrops of chert, black siltstone and mudstone are interbedded, apparently conformably, with the volcanic sequence. The black mudstones contain a fragmentary but identifiable graptolite fauna indicating an Arenig to Llanvirn age for the mudstone and by implication the Tyrone Volcanic Group at this locality.
On Portlock's early geological map of the area, (1843) all rocks in the vicinity of Sruhanleanantawey were classified together as "metamorphic rocks of hornblendic type". There was no specific reference to the black graptolitic shales on either the map or in the text.
On the first edition of the one-inch to the mile scale geological map (Sheet 27, Cookstown, Geological Survey of Ireland, 1880) rocks on the northern slope of Glenarudda Mountain were subdivided into "pyroxenic rocks" and granitoid porphyry (elvanite) with no indication of any other lithologies. The memoir which accompanied the map (Egan, 1881) contained no specific reference to the site.
Cole (1887) identified a volcanic series including andesites and tuff within the hornblendic and pyroxenic rock as shown on the Geological Survey map but did not specifically describe the graptolitic sediments at Sruhanleanantawey.
Hartley (1933) published the first detailed lithological map and description of the central Tyrone Ordovician volcanicplutonic terrane. He included the rocks on the north slope of Glenarudda Hill within a broad belt of tuffs and andesites which he referred to as the Tyrone Igneous Series, drawing a lithological analogy with the Highland Border rocks of Scotland. Hartley (1936) later discovered cleaved and pyritized graptolitic shales in Sruhanleanantawey Burn and, on the basis of the presence of Dicranograptus, proposed an Upper Llandeilo to Lower Caradoc (Ordovician) age for these strata.
On the second edition (1:50,000 scale) of Sheet 27, Cookstown, Geological Survey of Northern Ireland, 1983), the northern slopes of Glenarudda Mountain were mapped as undifferentiated volcanic rocks of Upper Ordovician age. Other lithologies present include quartz porphyry and metadiorite (Cameron et al., 1997).
In 1984, Hutton resampled Hartley's original graptolite locality and recovered a more extensive fauna that included fragments of Tetragraptus serra (Brongniart) and Sigmagraptus s.l. This led to a reappraisal of the age of the graptolitic beds, and by inference the Tyrone Volcanic Group. On the basis of the new graptolite fauna, Hutton et al. (1992) proposed an Arenig to Llanvirn age (Lower Ordovician) for the Tyrone Volcanic Group as opposed to the Upper Llandeilo-Lower Caradoc age originally proposed by Hartley (1936).
The Tyrone Volcanic Group forms part of the Tyrone-Girvan Sub-Terrane of the Midland Valley Terrane as designated by Bluck et al., (1992). This terrane comprises ophiolitic and arc-volcanic type geological sequences which formed along the Laurentian foreland at the northern margin of the Iapetus Ocean during closure of the Iapetus Ocean during Ordovician times (Hutton et al., 1985).
On Portlock's early geological map of the area, (1843) all rocks in the vicinity of Sruhanleanantawey were classified together as "metamorphic rocks of hornblendic type". There was no specific reference to the black graptolitic shales on either the map or in the text.
On the first edition of the one-inch to the mile scale geological map (Sheet 27, Cookstown, Geological Survey of Ireland, 1880) rocks on the northern slope of Glenarudda Mountain were subdivided into "pyroxenic rocks" and granitoid porphyry (elvanite) with no indication of any other lithologies. The memoir which accompanied the map (Egan, 1881) contained no specific reference to the site.
Cole (1887) identified a volcanic series including andesites and tuff within the hornblendic and pyroxenic rock as shown on the Geological Survey map but did not specifically describe the graptolitic sediments at Sruhanleanantawey.
Hartley (1933) published the first detailed lithological map and description of the central Tyrone Ordovician volcanicplutonic terrane. He included the rocks on the north slope of Glenarudda Hill within a broad belt of tuffs and andesites which he referred to as the Tyrone Igneous Series, drawing a lithological analogy with the Highland Border rocks of Scotland. Hartley (1936) later discovered cleaved and pyritized graptolitic shales in Sruhanleanantawey Burn and, on the basis of the presence of Dicranograptus, proposed an Upper Llandeilo to Lower Caradoc (Ordovician) age for these strata.
On the second edition (1:50,000 scale) of Sheet 27, Cookstown, Geological Survey of Northern Ireland, 1983), the northern slopes of Glenarudda Mountain were mapped as undifferentiated volcanic rocks of Upper Ordovician age. Other lithologies present include quartz porphyry and metadiorite (Cameron et al., 1997).
In 1984, Hutton resampled Hartley's original graptolite locality and recovered a more extensive fauna that included fragments of Tetragraptus serra (Brongniart) and Sigmagraptus s.l. This led to a reappraisal of the age of the graptolitic beds, and by inference the Tyrone Volcanic Group. On the basis of the new graptolite fauna, Hutton et al. (1992) proposed an Arenig to Llanvirn age (Lower Ordovician) for the Tyrone Volcanic Group as opposed to the Upper Llandeilo-Lower Caradoc age originally proposed by Hartley (1936).
The Tyrone Volcanic Group forms part of the Tyrone-Girvan Sub-Terrane of the Midland Valley Terrane as designated by Bluck et al., (1992). This terrane comprises ophiolitic and arc-volcanic type geological sequences which formed along the Laurentian foreland at the northern margin of the Iapetus Ocean during closure of the Iapetus Ocean during Ordovician times (Hutton et al., 1985).
| Enlander, I., Dempster, M. & Doughty, P., 2024. Sruhanleanantawey, County Londonderry, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=588. Accessed on 2024-12-26 |
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