ESCR - Little River and Slate Quarry Bridge

Earth Science Conservation Review

Summary Full report
Little River and Slate Quarry Bridge	Tyrone

Site number:	31	File Number:	H77
ESCC:	EO
Locality Type:	Stream section	Status:	ASSI
Grid Reference:	H72977268,H727727 Centroid, Not Entered
Google maps:	54.59683,-6.8721
County:	Tyrone	Council area:	Mid Ulster District Council
Period:	Silurian, Ordovician
Stages:	Ashgill, Cautleyan, Hirnantian, Llandovery
Lithostrat:	Killey Bridge Formation, Tirnaskea Formation

Site Description
Highlights:
Around Slate Quarry Bridge, the Little River, and its tributary the Tirnaskea Stream, expose fossiliferous Upper Ordovician and Lower Silurian sediments. Within this relatively small area the structural complexity of the Pomeroy Lower Palaeozoic inlier is uniquely well exhibited. The site includes the type locality for the Tirnaskea Formation and an historically important fossil locality of Lower Ashgill (Cautleyan) age.
Introduction:
The inlier of Lower Palaeozoic sediments covers an area of some 13km2 to the west of Pomeroy town. The central area of the inlier consists of Upper Ordovician sediments, but exposure is sparse, and it is unclear what structural relationship exists between the small scattered exposures. At Slate Quarry Bridge fault-bounded blocks expose sediments which range from Ashgill (Cautleyan) to Lower Silurian (Llandovery) in age according to their fossil assemblages. Folding, thrusting, and normal faulting are all present. Most previous work has concentrated on the palaeontology of this area, particularly the pioneering work of Portlock (1843), and that of Fearnsides et al. (1907), Reed (1952), Mitchell (1977), Tunnicliff (1982), and Evans (1994). An overgrown quarry site (Irish Grid Ref. H72977268), and a river cliff exposure a few metres downstream from this, constitute the type locality for over 60 fossil species described from the Killey Bridge Formation.
Description:
The oldest rocks exposed in this area are grey calcareous mudstones of the Killey Bridge Formation (KBF). These lie in a fault-bounded block at the eastern end of the site, and on faunal evidence are of Lower Cautleyan age. The historically important quarry locality lies within this block, and it is from here that the great majority of Killey Bridge Formation fossils have been collected. In 1992 the cliff exposure adjacent to the quarry locality was all but removed by new quarrying, but this created additional exposure and collecting opportunities in the process. The mudstones are generally massively bedded, micaceous, and slightly calcareous when fresh, with a weak cleavage fabric. Fossil assemblages from this locality are very diverse, but small brachiopods, bivalves and trinucleid trilobites predominate. Beds with a similar fauna occur on a sharp bend in the river 120m west of the old quarry, and also in the Trinaskea Stream section. There is no faunal evidence for younger (Rawthyan) portions of the KBF in this area, although thin thrust slices may be present in the Tirnaskea stream. The Tirnaskea Formation (TF) consists predominantly of dark grey mudstones with a very sparse low diversity fauna (Mitchell, 1977). The upper beds are pale green cleaved mudstones poorly exposed 20m downstream from Slate Quarry Bridge, where they grade into blue-grey lower Llandovery graptolitic shales in a comformable Ordovician-Silurian boundary section. Traces of the TF can be seen in the bed and banks of the Little River as far west as the junction with the Tirnaskea stream. The section downstream from the bridge is probably that described by Portlock (1843, p.230), and may be regarded as the type locality for the formation. The Tirnaskea stream section consists of numerous small outcrops in the bed and banks of the stream. From the confluence with the Little River, there is no exposure for around 40m upstream. The first outcrop seen consists of KBF shales with a Lower Cautleyan fauna, so this must be thrust over the TF and Llandovery shales which outcrop in the Little River. About 20m of fairly continuous exposure in the lower KBF is present from 50m above the confluence, and at the top of this section there is a prominent grey sandstone horizon which forms a low waterfall. Dips between 50-60 deg. to the north and south have been recorded from this section, indicating that the beds are folded with fold axes nearly perpendicular to the direction of thrusting (NNW). The next outcrop seen a few metres upstream is of the Tirnaskea Formation, followed by an outcrop of Llandovery shales. Dips are to the north, and the absence of the upper KBF here suggests that this is a narrow thrust slice in which the sequence is overturned. 5m above the Llandovery outcrop lower KBF shales, with another fine-medium grey sandstone horizon, are thrust over the Llandovery shales, although as usual the fault is not exposed, and must be inferred from the disjointed sequence.
Importance:
National Geology: This site uniquely reveals the structural complexity of the Palaeozoic Pomeroy Inlier. It has some of the best exposed Upper Ordovician shelf/platformal sediments, showing late Caledonian deformation, and contains the type locality of the Tirnaskea Formation. There is a good section through the Ordovican-Silurian boundary, and the site has been intensively studied since the historically important work of Portlock (1843). International Geology: The old quarry locality at the eastern end of the site is the type locality for no fewer than 63 fossil species, including two important genotype species.
Interpretation:
The Killey Bridge Formation was deposited in a relatively deep water, outer platformal setting. However, there is strong evidence for regular downslope mass movement of sediment, which carried with it faunas from more upslope benthic assemblages (Evans, 1994). The grey sandstone horizons appear to have such an origin as turbidites. In this area all of the KBF shales contain Cautleyan faunas, and so represent only the lower portion of the formation. Thrusting can be detected because it has juxtaposed faunally and lithogically distinctive units here, but elsewhere in the inlier, poor exposure means that thrusting would be very difficult to demonstrate. The Tirnaskea Formation contains a typical low diversity Hirnantian shelly fauna with graptolites, which can be attributed to further deepening, combined with global cooling in the Hirnantian stage which was associated with the end-Ordovician glaciation. The overlying Silurian shales contain only graptolites in this area, which suggests that by this time deepening had reached the stage at which slope facies sedimentation occurred.
Conclusions:
The site exposes fossiliferous sediments ranging from the Cautleyan stage of the Ashgill series to the M. acuminatus zone of the Llandovery series. Diverse faunas of the Killey Bridge Formation indicate a deep water platformal setting in the Cautleyan, with further deepening to upper slope facies by the Llandovery. At least four late Caledonian thrust faults are present, offset by subsequent (? Upper Carboniferous) normal faults. The fossils have been intensively studied, and the old quarry site is internationally important as the type locality for over 60 fossil species.
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) Within the site boundaries, which are drawn 5m from the current river banks on either side of the river, the Little River has wooded banks adjoined by pasture. Upstream from Slate Quarry Bridge the banks are unwooded, but overgrown by scrub. To the east of the bridge, the southern bank of the Little River is abutted by improved pasture. The Tirnaskea stream section runs through rough, poorly-drained, pasture. In fact the term 'stream' in this context is somewhat optimistic, as the locality consists of little more than a drainage ditch. It is very heavily overgrown by bushes and almost inaccessible. There has been some recent quarrying activity (1992) 10m east of the old quarry locality on the south bank of the Little River.

Keywords
Rocks:	Mudstone, Sand, Sandstone, Shale
Fossil Groups:	Bivalve, Brachiopod, Graptolite, Trilobite
Structures:	Caledonian, fault

Access
Management:
Recent quarrying in the KBF shales at H72977268 has enhanced the accessibility and visibility of these richly fossiliferous beds. At present it is the best Lower Palaeozoic fossil collecting site in Northern Ireland. The locality can withstand sustained pressure from collecting, but rock debris generated by collecting activities and natural weathering will need to be cleared away on a 3 - 5 yearly cycle to prevent outcrops becoming obscured and overgrown. Collecting from heaps of loose material rather than from in situ should be encouraged. With good management and the co operation of the landowner this site could become a much needed educational resource. Other outcrops along the Little River require only monitoring at the present time. Casual collection from these should be discouraged to prevent excessive erosion and bank collapse. The value of the Tirnaskea stream section could be greatly enhanced by selective vegetation clearance, removal of slumped bank debris, and by 'overdeepening' the stream to expose more continuous sections of bedrock, particularly where structural features are currently obscured. At the moment this section cannot sustain regular collecting pressure due to problems of bank undermining and debris generation. However, ease of access from the Pomeroy-Slate Quarry Bridge road means that this section could also become a valuable educational resource, if sensitively managed.
Threats:	River bank under- mining due to collecting pressure.
Uses
Uses:	Pasturing; some quarrying in recent years (1992).
Educ. Level:	Secondary / Advanced level / Undergraduate / Postgraduate

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. Little River and Slate Quarry Bridge, County Tyrone, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=31. Accessed on 2026-11-06.