ESCR - Lime Hill Farm

Earth Science Conservation Review

Summary Full report
Lime Hill Farm	Tyrone

Site number:	33	File Number:	H67
ESCC:	EO
Locality Type:	Stream section	Status:	ASSI
Grid Reference:	H69407395 Centroid
Google maps:	54.609,-6.92594
County:	Tyrone	Council area:	Mid Ulster District Council
Period:	Quaternary, Silurian
Stages:	Llandovery, Midlandian Undifferentiated, Pleistocene
Lithostrat:	Limehill Beds, Little River Group

Site Description
Highlights:
At Lime Hill Farm Lower Silurian (Llandovery) shales are exposed in a small stream. The shales contain a graptolite fauna, but deviate from typical Llandovery graptolitic facies because here a low diversity shelly fauna occurs with graptolites. This is the type locality for the zonal index graptolite Monograptus sedgewickii (Portlock, 1843).
Introduction:
Lime Hill is located 1.7km north of Pomeroy on the western edge of the Lower Palaeozoic inlier. A small exposure of graptolitic Llandovery mudstones occurs in a stream bed and banks. The locality was first recorded by Portlock (1843, p.231), who described a number of new fossils. Subsequently, material collected by the Geological Survey of Ireland was listed by Bailey (1878). The informal name 'Lime Hill beds' was applied to these shales by Fearnsides et al. (1907), forming part of the Little River Group of those authors. The significance of this site derives from its unusual association of graptolites with a restricted shelly fauna, and its status as a type locality.
Description:
Overall, about 6m of shales and mudstones are exposed here, although there is some uncertainty due to minor faulting and folding in the section. The oldest exposed beds occur at the downstream limit of exposure in the base of a small waterfall and in a 1.5m high rock bluff above it. These are blue-black micaceous mudstones and shales, faintly banded with a weak cleavage, which yield a sparse graptolite fauna. The upper limit of these beds is seen 0.4m above stream level in the rock bluff. Overlying them are 1.45m of dark grey shaley mudstones which contain a more abundant fauna of monograptids, petalograptids, orthocone nautiloid fragments, and rarer nowakiid molluscs and brachiopods. Fossils tend to be concentrated towards the base of this unit in more massive dark horizons. Harder upper beds are paler in colour, with faint banding of green and yellow siltstone. These are sparsely graptolitic. On the upstream side of the bluff the mudstones become contorted by minor folding, and have a characteristic 'pencil' weathering style due to more strongly developed cleavage in the folds. The contorted shales can be traced into the river bed, but are truncated approximately 2m upstream from the bluff by a normal fault, apparently with a small downthrow on the upstream side. The next beds seen are dark grey and black micaceous mudstones and siltstones. They are at present only poorly exposed in the bed of the stream and the base of the river cliff. The hard silty unfossiliferous beds of this unit can be readily examined, whilst the softer black micaceous mudstones, which are richly graptolitic, are largely obscured by river gravel. It is these darker mudstones which provided Portlock (1843) with most of his material. About 4m of this upper unit is exposed, and on average the whole sequence dips 40 deg. to the south west. The Llandovery rocks are overlain here by a good section (1.5- 2.5m thick) of till, presumably of Midlandian age. Above the till up to 0.5m of outwash sands are exposed in places. Palaeontology: The following graptolites were recorded from this site by Fearnsides et al. (1907): Climacograptus hughesi Nicholson Climacograptus scalaris Hisinger Petalograptus palmeus Barrande Petalgraptus palmeus var. latus Barrande Petalograptus minor Elles Monograptus discretus Nicolson Monograptus hisinger var. Monograptus involutus Lapworth Monograptus jaculum Lapworth Monograptus sedgwickii (Portlock) TYPE LOCALITY Monograptus sedgewickii var. distans (Portlock) TYPE LOCALITY Lagarograptus tenuis (Portlock) TYPE LOCALITY Although some of these species have subsequently been revised, the fauna was referred to the Monograptus sedgewickii zone by Fearnsides et al. (1907), and placed in the Fronian stage (C1) by Cocks et al. (1971). The shelly fauna includes orthocone nautiloids, Nowakia brevis Tunnicliff, 1983 (Type Locality), the bivalves Guerichia sp. ?, Mytilarca sp. ? and an indeterminate lunularcidiid, a probable atrypoid brachiopod, and crustacean carapace fragments (Tunnicliff, 1983).
Importance:
National geology: This is one of only two sites in Northern Ireland where shelly fossils of Silurian age occur. The Lime Hill beds deviate significantly from coeval graptolitic shales elsewhere, and probably represent a more up-slope depositional setting. International geology: This is the type locality for the internationally recognised zonal graptolite, Monograptus sedgewickii (Portlock, 1843). It is also the type locality for Lagarograptus tenuis (Portlock, 1843) and Nowakia brevis (Tunnicliff, 1983). Nowakia is unknown in pre-Devonian rocks elsewhere, and the bivalve-brachiopod fauna is a particularly good early example of a benthic association more common in the Upper Palaeozoic.
Interpretation:
The Lime Hill beds comprise one of the youngest Lower Palaeozoic sections exposed in the inlier. Graptolite facies of the type present here are usually interpreted as deep water slope deposits, but the occurrence of a shelly fauna may indicate either a sediment starved outer shelf setting, or an upper slope setting. Whilst the graptolites and nowakiid were planktonic, and the orthocone nautiloids and crustaceans nektonic, the mode of life of the brachiopods and bivalves is more problematic. There is no evidence of current action which might have transported shells into the depositional area, and the lack of bioturbation suggests that bottom conditions were strongly dysaerobic or anaerobic. Palmer (1991, p.47) suggests that such associations include genuinely epibenthic shelly fossils, and are characteristic of shelf margin or upper slope facies poor in oxygen or nutrients. The brachiopods and bivalves in this interpretation are physiologically tolerant of extreme bottom conditions. An alternative hypothesis expressed by Tunnicliff (1983) views the bivalves and brachiopods as epiplanktonic, living attached to floating algal masses in the surface waters, and dropping into deep water on death. In either case the assemblage as a whole is distinctive and at Lime Hill a particularly good example is present. Contemporaneous Llandovery shales in the main Longford- Down outcrop to the south appear to have been deposited in deeper water typical graptolitic facies, and lack the shelly component. Tunnicliff also noted the similarity between this fauna and examples known from the Devonian. Nowakia makes its earliest appearance at Lime Hill, and some of the bivalves have a morphological similarity to unrelated Devonian species occurring in a similar facies.
Conclusions:
The Lower Silurian (Llandovery) mudstones exposed near Lime Hill, Co. Tyrone, contain graptolites characteristic of the M. sedgewickii biozone, including the zonal index fossil for which this is the type locality. Deposition occurred in relatively deep water where normal benthic communities were excluded by oxygen deficient bottom waters. Unlike typical Silurian graptolitic facies, however, the Lime Hill beds have a low diversity shelly fauna consisting of elements which either lived in the surface waters (orthocones, nowakiids), or were tolerant of dysaerobic bottom waters (brachiopods, bivalves).
Notes:
The full range of ESCR site relating to the Lower Palaeozoic are as follows: Outer Ards Stratigraphic, Palaeontological and Structural sites Coalpit Bay – Lower Palaeozoic (record no. 1350) Helen’s Bay – Grey Point – Lower Palaeozoic (record no. 1351) Kearney Point – Knockinelder Bay – Lower Palaeozoic (record no. 1352) Orlock Bridge – Lower Palaeozoic (record no. 1353) Tieveshilly – Carrstown – Lower Palaeozoic (record no. 1354) Orlock Bridge - Structural (record no. 652) Whiskin Rocks - Structural (record no. 653) White House Port, Cloghy - Structural (record no. 654) Knockinelder Bay and Kearney Point - Structural (record no. 655) Millin Bay - Structural (record no. 656) Pomeroy area Craigbardahessiagh and Bardahessiagh River – Palaeontology (record no. 29) Lime Hill Farm – Palaeontology (record no. 33) Little River and Slate Quarry Bridge – Palaeontology (record no. 31)

Keywords
Rocks:	Mudstone, Shale, Siltstone
Fossil Groups:	Bivalve, Brachiopod, Graptolite, Nautiloid
Fossil List:	Lagarograptus tenuis , Monograptus discretus , Monograptus hisinger var. , Monograptus involutus , Monograptus jaculum , Monograptus sedgewickii , Monograptus sedgewickii var. distans , Nowakia brevis, Petalograptus palmeus
Geomorph:	till

Measurements
Length:		Width:		Height:	6 m
Depth:		Area:

Access
Approach:	1.7km north of Pomeroy.
Management:
The area of outcrop is relatively small and would be degraded by sustained geological sampling. This applies particularly to the well exposed section in the river bluff. Some graptolitic horizons, now obscured by gravel in the stream bed, may well have been 'quarried out' by Portlock's collectors as early as the 1830's. A larger (and drier) exposure could be created by removing approx. 6m2 of cattle trampled mud and rock debris on the north side of the river. Fencing-off the stream around the exposure is also recommended. The cliff of glacial till appears to be very stable, and has not slipped over any critical parts of the sequence, but higher beds could be exposed by cutting back slipped till at the eastern end of the site.
Threats:	Over-sampling.
Uses
Uses:	Land adjoining the site area, as defined on the accompanying map, is currently being used for hay-cropping/cattle grazing.

References (21)
Baily, W.H., 1878. Explanatory memoir to accompany sheet 34 of the maps of the Geological Survey of Ireland. Memoirs of the Geological Survey of Great Britain, Dublin & London.
Blake, J.F., 1882. A Monograph of the British fossil Cephalopoda, Pt. 1, Introduction and Silurian species. Palaeontographical Society Monographs, pp.248, plates 31.
Cocks, L.R.M., Holland, C.H., Rickards, R.B. & Strachen, I., 1971. A correlation of Silurian rocks in the British Isles. Journal of the Geological Society of London, vol. 127, pp.103-136.
Cripps, A.P., 1988. A new species of stem-group Chordate from the Upper Ordovician of Northern Ireland. Palaeontology, vol. 31, pp.1053-1078.
Elles, G.L., 1942. The type of Monograptus sedgwickii (Portlock). Geological Magazine, vol. 79, pp.31-32.
Evans, D.H., 1993. The cephalopod fauna of the Killey Bridge Formation (Ordovician, Ashgill), Pomeroy, Co. Tyrone . Irish Journal of Earth Sciences, vol. 12, pp.155-189.
Fearnsides, W.G., Elles, G.L. & Smith, B., 1907. The Lower Palaeozoic rocks of Pomeroy. Proceedings of the Royal Irish Academy, vol. 26(B), pp.97-128.
Harper, J.C., 1948. The Ordovician and Silurian rocks of Ireland. Proceedings of the Liverpool Geological Society, vol. 20, pp.48-67.
Hutt, J.E., 1968. A redescription of the Llandoverian monograptid 'Graptolithus' tenuis Portlock, 1843. Geological Magazine, vol. 105, pp.251-255.
Jaekel, O.N., 1899. Stammersgeschichte der Pelmatozoen. 1, thecoidea and cystoidea., pp.442, plates 18 Julius Springer, Berlin.
McCoy, F., 1846. A synopsis of the Silurian fossils of Ireland., pp.72, plates 5, Dublin.
Mitchell, W.I., 1977. The Ordovician Brachiopoda from Pomeroy, Co. Tyrone. Palaeontographical Society Monographs, pp.138, plates 28.
Palmer, D. & Rickards, R.B., 1981. Graptolites., pp.182 Boydell Press, Woodridge.
Portlock, J.E., 1843. Report on the Geology of the County of Londonderry and of parts of Tyrone and Fermanagh. HMSO, Dublin.
Reed, F.R.C., 1920-1921. A monograph of the British Ordovician and Silurian Bellerphonetacea. Palaeontographical Society Monographs, pp.92, plates 13.
Reed, F.R.C., 1952. Revision of certain Ordovician fossils from County Tyrone. Proceedings of the Royal Irish Academy, vol. 55,section B, pp.31-136.
Tripp, R.P., 1957. The trilobite Encrinurus multisegmentatus (Portlock) and allied Middle and Upper Ordovician species. Palaeontology, vol. 1, pp.60-72.
Tunnicliff, S.P., 1978. Types of the Ordovician trilobites Celtencrinurus multisegmentatus (Portlock) and Cryptolithus latus (Portlock). Palaeontology, vol. 21, pp.455-458.
Tunnicliff, S.P., 1980. A catalogue of the Lower Palaeozoic fossils in the collection of Major J. E. Portlock., pp.111 Ulster Museum, Belfast.
Tunnicliff, S.P., 1982. A revision of late Ordovician bivalves from Pomeroy, Co. Tyrone, Ireland. Palaeontology, vol. 25, pp.43-88.
Tunnicliff, S.P., 1983. The oldest known nowakiid (Tentaculitoidae). Palaeontology, vol. 26, pp.851-854.

Map(s):	See the 1:50,000 O.S. Sheet 13 (The Sperrins).
Record type:	Environment Serv. GIS, ESCR report
Entered by:	Crowther, P.R.

Enlander, I., Dempster, M. & Doughty, P., 2026. Lime Hill Farm, County Tyrone, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=33. Accessed on 2026-11-06.