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Giant's Grave sectionFermanagh
Giant's Grave, Co. Fermanagh; southernmost waterfall in section formed of 3m of Bin Mountain Sandstone Formation; contact with underlying Termon River Limestone Formation (stratotype) just below hammer head.
Summary Full report
Site Type: Stream section
Site Status: PASSI
District: Fermanagh District Council
Grid Reference: H191731
Rocks
Rock Age: Carboniferous (Chadian, Visean)
Rock Name: Bin Mountain Sandstone Formation, Claragh Sandstone Formation, Termon River Limestone Formation, Tyrone Group
Rock Type: Limestone, Mudstone, Sand, Sandstone, Siltstone
Interest
Fossil Groups: Brachiopod, Coral, Crinoid, Foraminifera, Gastropod, Nautiloid
Other interest: No data, Marine sediments

Summary of site:

At this locality almost the full thickness of the Termon River Limestone Formation can be seen and its contacts with the underlying and overlying formations are clearly exposed. This makes it the ideal reference section for the formation and is therefore the designated stratotype.

The exposures are along both banks of the River Termon (here forming the border with the Republic of Ireland) and in the waterfalls in the stream on the south side of the river. The base of the formation sits on top of the Claragh Sandstone Formation, the top 2m of which are sandstones, siltstones (churned by animals that lived in them or grazed through them) and mudstones. At the confluence of the stream with the river, coarse sandstones and arkoses (sandstones with grains formed of feldspar rather than quartz) typical of the Claragh Sandstone are exposed. The basal bed of the Termon River Limestone Formation is 50cm of dark grey, medium-grained limestone made up of shell fragments and sand grains in contact with each other in a matrix of lime mud. It contains a microscopic fauna of foraminifera and a single genus of microscopic plant. Taken together, these fossils indicate the Chadian stage, an early division of the Lower Carboniferous period. Above this there are scattered outcrops in the stream but further south 24m of beds form a continuous section between two waterfalls. The first 18m of these strata are Termon River Limestones, essentially similar to the basal bed but here incorporating fragments of limestone, spherical calcite grains, lime mud pellets, rounded algal masses and grains coated in lime-fixing algae. Bed thickness is variable, up to 1m maximum and there are thin, lime-rich, silty shales between the limestones. Although few fossils occur in these beds, those present form a diverse fauna of sea snails, nautiloids, crinoids, brachiopods and a small but significant group of corals indicating a late Chadian to early Arundian age. There is also a fauna of foraminifera (microfossils) that further refines the age to the late Chadian. The base of the second waterfall marks the top of the Termon River Limestone and the appearance of 3m of very sandy limestones and lime-rich sandstones mark the base of the Bin Mountain Sandstone. One bed in this sequence contains another microfauna of foraminifera, also of late Chadian age, establishing beyond doubt the Chadian credentials of the entire thickness of the Termon River Limestone Formation. All these rocks bear evidence of the inshore setting in which they formed. The top of the Claragh Sandstone, with its sediment churning fauna, was at the outer fringe of a delta that became submerged by tropical waters to create a shallow, lagoon-type environment. The Termon River Limestone accumulated on the bed of this lagoon which was, for some reason, only lightly colonized by marine invertebrates, although the useful coral (for dating purposes) Dorlodotia pseudovermiculare clung on. The sandy limestones and limey sandstones of the Bin Mountain Sandstone Formation indicate a return to very shallow, inshore conditions with animals again living within the sands. All this happened about 345 million years ago on the southern shoreline of the supercontinent Laurentia as it drifted imperceptibly northwards towards the equator at around a couple of centimetres a year. Since this is a stratotype locality, it is an essential element of the geological history of Northern Ireland and should be protected. There are no evident threats to the site.


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