Limestone
Around 40% of Ireland is underlain by limestone. With only small exceptions, Irish limestones belong to two geological periods, namely the Carboniferous (around 300–340 million years ago) and the Cretaceous (70–120 million years ago). The Carboniferous limestones are usually hard and grey to black in colour, and are found in almost every part of Ireland (every county except Antrim and Wicklow). The hard nature of the carboniferous limestone makes it ideal for use in building stone. The Cretaceous limestone (chalk) is somewhat softer and normally white in colour, and is found solely in Ulster (Counties Antrim, Armagh, Down, Derry and Tyrone). Cretaceous limestone is too soft to be used in building, but is often used in cement-making.
Limestone is predominantly or entirely composed of calcite. Some limestones are almost pure carbonate; others contain significant proportions of other material – most commonly sand, clay (mud or shale) and chert (very fine grained silica). The non-carbonate material may be distributed throughout the rock, may occur as small nodules (especially of chert) or may be concentrated in distinct beds (most commonly, beds of shale) inter-bedded with the limestone. The limestone may also contain small amounts of metallic minerals such as pyrite, marcasite, or galena, and in some places these may be concentrated in distinct veins or masses; where these mineral deposits are sufficiently large and concentrated to be exploited economically, they are termed orebodies, as found at Tara Mines (Navan), Galmoy (Co. Kilkenny) and Lisheen (Co. Tipperary).
Sample of Fossiliferous Irish Limestone
Limestone is formed in various different ways and in different geological situations, normally in the sea. It may be deposited in deep water far from land, in shallow water near the shore, or anywhere in between. Many limestones are predominantly composed of the calcareous shells or skeletons of marine organisms, but others are formed chemically by precipitation of carbonate from shallow waters. Some form in extensive horizontal layers (beds) which may be as thin as a few millimetres or as thick as several metres. Others form as massive unbedded banks or mounds of fine-grained calcareous debris (mud mounds), which can be many metres thick in the centre, thinning out towards the edges.
Fossils
The limestone here could also be categorised as Fossiliferous Limestone, as you can see it contains an abundance of fossils. Fossils are the remains of prehistoric organisms preserved in petrified form or as a mold or cast in rock.
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Please send me answers to the below questions via my profile. Some at-home research will be required, in addition to your visit to the site.
1. Based on the above information regarding carboniferous and cretaceous limestone, to what period can you conclude the limestone here originates?
2. How did you come to the above conclusion?
3. Is limestone a sedimentary, metamorphic or igneous rock?
4. Describe the type of fossils you can see here. Please name at least one type. English or Latin name is acceptable.
5. Estimate how many separate fossil samples are visible in the second vertical slab right from the corner pillar.
6. OPTIONAL: feel free to upload pictures from your visit, but please do not include pictures of the specific slab mentioned.
Sources:
www.gsi.ie
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