Earth Science Conservation Review

Summary Full report
Largy Quarry
Site number:243File Number:H24
ESCC:ED
Locality Type:Quarry (disused) Status: ASSI
Grid Reference: H299476 Approximate
County: FermanaghCouncil area:Fermanagh & Omagh District Council
Period:Carboniferous
Stages:Visean
Lithostrat:Ballyreagh Conglomerate Formation, Kilskeery Group, Topped Mountain Sandstone Formation
Site Description

Largy Quarry, Co. Fermanagh, showing large scale, low angle cross-laminations in coarse-grained sandstones with pebble beds, of the Topped Mountain Sandstone Formation.
Highlights:
Largy Quarry offers one of the best sections through the upper part of the Topped Mountain Sandstone Formation. Interbedded with the sandstones are conglomerates and pebbly sandstones that are virtually indistinguishable from lithologies normally associated with the succeeding Ballyreagh Conglomerate Formation.
Description:
Largy Quarry is located on the east face of Largy Mountain, 5km west of the village of Tempo and quarrying operations continue intermittently. The quarry exposes about 20m of strata inclined at about 20 deg. to the NW. Coarse-grained pale grey, green and dull fawn arkosic sandstones dominate the sequence but are interbedded with thin to thick pebbly sandstones and fine to coarse, matrix-supported conglomerates.
Loose block of Topped Mountain Sandstone Formation showing interbedded nature of conglomerates in the parallel laminated sandstones.

Loose block of sandstone showing variable colour of rock and common occurrence of purple clasts of vesicular rhyodacite (ruler 0.14m).
Large scale examples of very low angle planar cross-laminations are evident in the west wall of the quarry overlain and underlain by parallel laminated beds. Bedding thickness varies from thin to very thick and at some levels, particularly near the top of the south face, laminae of green or purple mudstone intervene between sandstones. These mudstones do not contain miospores but are frequently associated with deep, sand-filled desiccation cracks and crude, non-directional ripples.
Loose blocks of Topped Mountain Sandstone Formation showing large scale dessication cracks and crude ripple marks with laminae of green and purple mudstone respectively (ruler 0.14m)
The range of clasts that occur in conglomerates and pebbly sandstones is relatively small but is nevertheless distinctive in that they contain considerable quantities of Lower Palaeozoic greywacke and in this are quite different from both the Westphalian Drumlish Conglomerate and the Devonian Gortfinbar Conglomerate. Other clast types include abundant fawn to purple- grey trachyandesite, rare quartz and very rare pink coarse- grained granite and numerous clasts of reworked contemporaneous purple-brown mudstone, siltstone and sandstone. These are usually contained in a matrix consisting of very coarse sand mixed with angular lithic fragments. Clasts rarely exceed a diameter of 10cm in Largy Quarry although boulders over 0.3m diameter are recorded in the Ballyreagh Conglomerate.
Interpretation:
The sedimentary rocks that constitute the three formations of the Kilskeery Group record the history of punctuated subsidence of a non-marine basin. Gradual and sustained subsidence of this basin in late Asbian times accommodated the 1000m of sand-dominated sediments of the Topped Mountain Formation. Accelerated uplift in the early Brigantian, of the southern margin of this northern land area led to increased erosion and southward progradation of alluvial fans, in the process depositing largely coarse-grained and conglomeratic debris of the Ballyreagh Formation. A gradual diminution in the rate of uplift of the metamorphic and Lower Palaeozic massif in the late Brigantian led to its eventual burial by the distal alluvial plain and playa-lake sediments of the Ballinamallard Mudstone Formation in the south, and possibly by the Greenan Sandstone in the north (Mitchell & Owens, 1990). This situation persisted into Pendleian times at which point the sedimentary record ends.
Notes:

For information on other sites in Subarea-4 see;
Key Site 1177 - Subarea-4; Fintona Block.
This area is also important for Mineralogy and Metallogenesis. See Key Site 560 - Largy Hill.

Keywords
Rocks:Conglomerate, Mudstone, Sand, Sandstone
Structures:cross-bedding, dessication cracks, ripple marks
Access
Approach:5km west of the village of Tempo.
References
Blieck, A. 1991: Reappraisal of the heterostracans (agnathan vertebrates ) of Northern Ireland. Irish Journal of Earth Sciences, vol. 11 pt. 1, pp.65-69
Map(s): See 1:50,000 O.S. Sheet 18 (Enniskillen); / GSNI (1982) 1:50,000 Solid Geology of Sheet 45 (Enniskillen).
Rec Type ESCR report    
Enterer: Doughty, P.J.
Updates: 28 Apr 2003 / 05 FEB 01 / 28 MAR 97 / 27 MAR 97
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