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Earth Science Conservation Review
| Bonnety Bush, Altihaskey | Londonderry |
| Site number: | 586 | ||
| ESCC: | EO | ||
| Locality Type: | Inland exposure | Status: | ESCR |
| Grid Reference: | H726888 Centroid | ||
| Google maps: | 54.74237,-6.87272 | ||
| County: | Londonderry | Council area: | Mid Ulster District Council |
| Period: | Ordovician | ||
| Lithostrat: | Tyrone Volcanic Group | ||
| Site Description | |
|---|---|
| Highlights: | |
Bonnety Bush is a remote moorland location comprising a number of outcrops of coarse volcanic tuffs of the Tyrone Volcanic Group. The site is particularly noteworthy for the occurrence of red chert (jasper). | |
| Introduction: | |
Bonnety Bush is located at Altihaskey in the Townland of Tullybrick. The site comprises several isolated outcrops and boulders which form a low rise on the otherwise flat glacial-outwash plain south of Lough Fea. Access is via a track which runs southwest from [27380 38930] on the Tullybrick Road and then across open moorland, a total distance of about 1km. Alternative access is via a farm track south from the Davagh Road. On Portlock's early geological map of the area, (1843) the rocks in the vicinity of Bonnety Bush were represented as "mica schists" and grouped with the metamorphic rocks of the Sperrin Mountains "of hornblendic type". In the text, Portlock referred to "micaceous iron ore at Tullybrick ... disposed on iron flint and quartz; in the latter case in veins in the mica-slate, associated with iron jasper". The first edition of the one inch to one mile scale geological map (Sheet 26, Draperstown, Geological Survey of Ireland, 1882) represented rocks at Bonnety Bush as "chloritic, Talcose & Micaceous Schists of Lower Silurian age" and did not differentiate them from the mica schists of the Sperrin Mountains to the north of the site. Hartley (1933) published the first detailed lithological map and description of the central Tyrone Ordovician volcanicplutonic terrane. He included the rocks at Bonnety Bush in a division which he referred to as the "Phyllites and Chloritic Schists of Tullybrick" part of the volcanic tuff and lava (volcanic) series into which they grade. Hartley also noted that elsewhere in the vicinity the ferruginous chert was found in association with haematite for example in Glenscallip Burn and on the south side of Slieve Gallion. On the second edition (1:50,000 scale) of the geological map (Sheet 26, Draperstown, Geological Survey of Northern Ireland, 1995), the rocks at Bonnety Bush are included within the Tyrone Volcanic Group. The Tyrone Volcanic Group is an arc-volcanic type geological sequence, which formed along the Laurentian foreland at the northern margin of the Iapetus Ocean during closure of the Iapetus Ocean during Ordovician times. In a regional tectono-stratigraphic context, the Tyrone Volcanic Group forms part of the Tyrone-Girvan Sub-Terrane of the Midland Valley Terrane as designated by Bluck et al. (1992). | |
| Description: | |
Bonnety Bush contains a variety of lithologies but is perhaps best known for the occurrence of red chert (jasper) and banded chert. The outcrops on the eastern side of the site consist predominantly of pale green to grey coloured, well foliated, sometimes sheared, chloritic tuff containing lapillae of pale acid felsic material up to 1cm and occasional lenticular lenses of purple to maroon jasperine chert. The basic tuff contains fragments of basic lava replaced by green and purple chlorite with inclusions of epidote grains. The matrix of the tuff consists of pale-grey intergrowths of quartz, feldspar and white mica. The best examples of red chert occur in outcrops at the southwestern corner of the site and are most easily accessible from the Davagh Road. Here, massive red and purplish red cryptocrystalline chert is interbedded with pale green and grey tuff. There is a crude large scale bedding strike 035° with a dip of 025° towards the southeast the chert appears to pass upwards and downwards into variable green schistose tuff. | |
| Importance: | |
Bonnety Bush is a site of national importance. It contains exposures of a distinctive suite of strongly deformed Ordovician tuff and ferruginous chert (jasper) and as a location has featured prominently on local geological itineraries. Ferruginous chert localities are becoming rare elsewhere in the Tyrone Volcanic Group, thus adding pressure and importance to this site. | |
| Interpretation: | |
The rocks at this site are deformed volcanic tuff and ferruginous chert which are part of the Tyrone Volcanic Group. These rocks originally formed eruptions of lavas and as explosive pyroclastic flows in a volcanic island arc environment. The presence of significant quantities of chert (jasper) and fine mm-scale rhythmic banding and intercalations of black shale (elsewhere in the Tyrone Volcanic Group) within the chert indicates marine depositional conditions. According to Hartley (op. cit.) these cherts are inorganic chemical precipitates rather than organically derived chert. The Tyrone Volcanic Group and the closely related Tyrone Plutonic Group are important components of the Caledonian-Appalachian Orogen in Ireland. They latter represent fragments of ancient oceanic crust obducted onto the continental margin of Laurentia at the northern margin of the Iapetus Ocean during closure of Iapetus sometime during the Ordovician (Hutton et al., 1985). During the Caledonian Orogeny, these volcanic rocks were deformed and metamorphosed in the low-grade Greenschist Facies. This led to the obliteration and modification of most of the original depositional structures and textural features. The widespread presence of mylonitic textures in rocks in the surrounding area (Key Site 587 - Tullybrick) with the development of very strong horizontal stretching lineations is evidence of intense cataclastic deformation. This horizontal lineation indicates that, despite proximity to the Omagh Fault which separates the Tyrone Volcanic Group from the Dalradian (Southern Highland Group), the rocks at Tullybrick were deformed largely as a result of transform movements most probably related to transpressive accretion of the Tyrone-Girvan sub-terrane. | |
| Conclusions: | |
Bonnety Bush is considered to be a site of national importance which affords an opportunity to study a distinctive suite of volcanic rocks including ferruginous chert within the Tyrone Volcanic Group. | |
| Notes: | |
For information and references on sites within the Tyrone Basic Plutonic Complex see the following sites: Key Site 580 - Black Rock Key Site 581 - Scalp Hill For information and references on other sites within the Tyrone Volcanic Group see the following sites: Key Site 582 - Mveela More Key Site 583 - Creggan Key Site 584 - Copney Hill Key Site 585 - Beaghbeg Key Site 587 - Tullybrick Key Site 588 - Stuhanleanantawey Burn For information and references on Granitoid sites within the Caledonian Complexes of N. Ireland see the following sites: Key Site 589 - Cashel Rock Key Site 590 - Craigballyharky and Craigbardahessiagh Key Site 591 - Craiganawork, Pomeroy Key Site 592 - Tintagh, Slieve Gallion Key Site 593 - Loughmacrory Key Site 594 - Cushendun For general information and references on the Caledonian Igneous Complexes of N. Ireland, and for directions to other regional sites, see the following site: Key Site 595 - Introduction to the Caledonian Igneous Complexes of N. Ireland | |
| Keywords | |
|---|---|
| Rocks: | Chert, Jasper |
| Structures: | Caledonian |
| Access | |
|---|---|
| Approach: | Access is via a track which runs southwest from [27380 38930] on the Tullybrick Road and then across open moorland, a total distance of about 1km. Alternative access is via a farm track south from the Davagh Road. |
| Management: | |
The site consists of a number of scattered outcrops in an area of peat and rough pasture. No direct threat to the integrity of the site was evident at the time of inspection. The chert outcrop at the southwestern corner of the site is undoubtedly the most important part of this site and should be protected from further degradation by restricting hammering and sampling. The rocks of the Tyrone Volcanic Group are considered to be prospective for a number of minerals including base metals and gold. Licensed mineral exploration has previously been carried out throughout central Tyrone. The area which includes this site is likely to attract further exploration attention sometime in the future. | |
| Threats: | Hammering and sampling should be restricted. |
| Uses | |
| Uses: | Rough pasture |
| Map(s): | GEOLOGICAL SURVEY OF IRELAND, 1882. 1:63,360 Geological Sheet 26 (Draperstown). (Dublin, Ordnance Survey for the Geological Survey of Ireland). GEOLOGICAL SURVEY OF NORTHERN IRELAND, 1995. Draperstown. Northern Ireland Sheet 26. Solid and Drift Geology. 1:50,000. (Keyworth, Nottingham: British Geological Survey). | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Last updated: | 04 MAR 2001 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Bonnety Bush, Altihaskey, County Londonderry, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=586. Accessed on 2026-11-05. |
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