Approximately 40% of the island of Ireland, and 50% of the Republic, is underlain by limestone.
With very minor exceptions, Irish limestones belong to two periods of geological history, one known as the Carboniferous (around 300–340 million years ago) and the other known as the Cretaceous (70–120 million years ago). The Carboniferous limestones are normally hard and grey to black in colour, and are found in almost every part of Ireland (every county except Antrim and Wicklow); the Cretaceous limestone (chalk) is somewhat softer and normally white in colour, and is found only in Ulster (Counties Antrim, Armagh, Down, Derry and Tyrone).
Limestone is a sedimentary rock, composed mainly of skeletal fragments of marine organisms such as coral, forams and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3).
Limestones can be formed in several different ways and in different geological situations, usually in the sea. They may be deposited in deep water far from land, in shallow water near the shore, or somewhere 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.
Granite is a common type of felsic intrusive igneous rock that is granular and phaneritic in texture. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Strictly speaking, granite is an igneous rock with between 20% and 60% quartz by volume, and at least 35% of the total feldspar consisting of alkali feldspar, although commonly the term "granite" is used to refer to a wider range of coarse grained igneous rocks containing quartz and feldspar.
Granite is nearly always massive (lacking any internal structures), hard and tough, and therefore it has gained widespread use throughout human history as a construction stone.
In Ireland, granite is to be found in Donegal, Galway, Wicklow and the Mourne Mountains.
The starting coordinates will bring you to the remains of the Babe’s Bridge over the Boyne. The bridge, which was was built about 1210, is the only bridge surviving the great floods of 1330 and is a great example of how the different characteristics of rocks are used in construction and their affects over time.
Look closely at the types of rock at GZ and answer the following questions:
- The bridge is made pre-dominantly from one type of rock, can you:
a) Identify the rock used the most
b) Describe why you chose this including colour, & texture
c) Name where in Ireland you would find this type of rock
- The keystone in the centre of the arch is unusual, can you:
a) Identify any difference between this and the rock named in 1a
b) Describe the shape of the keystone
- What are the main minerals of granite?
- This is the last remaining arch from the bridge, which of the following do you think contributed the most to it's demise and tell me why you think this is:
x) The rock is soft and would erode easily making it not appropriate for construction
y) The bridge wasn't able to handle high powered motor cars
z) The flow of the river is very strong here and over time it would naturally erode
- OPTIONAL: Please feel free to include pictures of your visit, without giving away any spoilers.
Please feel free to log your find as soon as you have sent your answers. I will contact you if there are any issues.