There are more than 1,200 geological Formations and Members defined within the Republic. Their delineation is based on a variety of factors, many of which are of no relevance to the hydrogeological properties of the aquifer (for example, type of fossil). GSI’s Groundwater Section have, therefore, evaluated these Formations and Members and reduced them to 27 'Rock Unit Groups' that have hydrogeological significance.

The Rock Unit Groups are defined within a stratigraphic framework on the basis of what is understand to be important differences between rock units/ rock unit groups in terms of groundwater flow properties. For example,

• limestone purity and susceptibility to karstification;

• bedding presence or absence and its influence on the prevalence of jointing;

• degree of deformation and its impact on flow properties (e.g., older rocks have been deformed many times since their formation, so lack pore spaces and connected fracture networks).

The way in which the aquifer classification has been undertaken is on the basis of ‘Rock Unit Groups’rather than the individual ‘rock units’(Formations). Note that a particular Rock Unit Group can, and often does have, a different aquifer classification in different parts of the country (for example, the Dinantian Pure Unbedded Limestones aquifer classification ranges from Ll to Rkc and Rkd, depending upon location).

The 27 rock unit groups are as follows:

1. Permo-Triassic Sandstones

2. Permo-Triassic Mudstones and Gypsum

3. Westphalian Sandstones

4. Westphalian Shales

5. Namurian Shales

6. Namurian Sandstones

7. Namurian Undifferentiated

8. Dinantian Shales and Limestones

9. Dinantian Mixed Sandstones, Shales and Limestones

10. Dinantian Sandstones

11. Dinantian Pure Bedded Limestones

12. Dinantian Upper Impure Limestones

13. Dinantian Dolomitised Limestones

14. Dinantian Pure Unbedded Limestones

15. Dinantian Lower Impure Limestones

16. Dinantian (early) Sandstones, Shales and Limestones

17. Dinantian Mudstones and Sandstones (Cork Group)

18. Devonian Kiltorcan-type Sandstones

19. Devonian Old Red Sandstones

20. Silurian Metasediments and Volcanics

21. Ordovician Metasediments

22. Ordovician Volcanics

23. Cambrian Metasediments

24. Precambrian Quartzites, Gneisses & Schists

25. Precambrian Marbles

26. Granites & other Igneous Intrusive rocks

27. Basalts & other Volcanic rocks

There is also an ‘unclassified’class for areas of the map where the geology is undefined. Sands and Gravels are defined separately.

Scale: 1:100,000

There are more than 1,200 geological Formations and Members defined within the Republic. Their delineation is based on a variety of factors, many of which are of no relevance to the hydrogeological properties of the aquifer (for example, type of fossil). GSI’s Groundwater Section have, therefore, evaluated these Formations and Members and reduced them to 27 'Rock Unit Groups' that have hydrogeological significance.

The Rock Unit Groups are defined within a stratigraphic framework on the basis of what is understand to be important differences between rock units/ rock unit groups in terms of groundwater flow properties. For example,

• limestone purity and susceptibility to karstification;

• bedding presence or absence and its influence on the prevalence of jointing;

• degree of deformation and its impact on flow properties (e.g., older rocks have been deformed many times since their formation, so lack pore spaces and connected fracture networks).

The way in which the aquifer classification has been undertaken is on the basis of ‘Rock Unit Groups’rather than the individual ‘rock units’(Formations). Note that a particular Rock Unit Group can, and often does have, a different aquifer classification in different parts of the country (for example, the Dinantian Pure Unbedded Limestones aquifer classification ranges from Ll to Rkc and Rkd, depending upon location).

The 27 rock unit groups are as follows:

1. Permo-Triassic Sandstones

2. Permo-Triassic Mudstones and Gypsum

3. Westphalian Sandstones

4. Westphalian Shales

5. Namurian Shales

6. Namurian Sandstones

7. Namurian Undifferentiated

8. Dinantian Shales and Limestones

9. Dinantian Mixed Sandstones, Shales and Limestones

10. Dinantian Sandstones

11. Dinantian Pure Bedded Limestones

12. Dinantian Upper Impure Limestones

13. Dinantian Dolomitised Limestones

14. Dinantian Pure Unbedded Limestones

15. Dinantian Lower Impure Limestones

16. Dinantian (early) Sandstones, Shales and Limestones

17. Dinantian Mudstones and Sandstones (Cork Group)

18. Devonian Kiltorcan-type Sandstones

19. Devonian Old Red Sandstones

20. Silurian Metasediments and Volcanics

21. Ordovician Metasediments

22. Ordovician Volcanics

23. Cambrian Metasediments

24. Precambrian Quartzites, Gneisses & Schists

25. Precambrian Marbles

26. Granites & other Igneous Intrusive rocks

27. Basalts & other Volcanic rocks

There is also an ‘unclassified’class for areas of the map where the geology is undefined. Sands and Gravels are defined separately.

Scale: 1:100,000