Bedrock
GSI's bedrock map names the rock formation under every part of Ireland, with a description of the rock type. Much of the midlands is limestone; the uplands are largely granite, sandstone, shale or older metamorphic rocks.
What it can tell you about a site:
- Limestone can be karstified: dissolved by water into fissures, swallow holes and caves. That affects drainage, groundwater protection and occasionally foundations. See landslides and karst.
- Rock near the surface makes digging, services trenches and septic tank percolation harder.
- Some rock types are linked to higher radon. See the radon guide.
Subsoil (Quaternary sediments)
GSI's subsoil map shows what lies between the topsoil and the rock. The common types are:
| Subsoil | What it usually means for a site |
|---|---|
| Till (boulder clay), from different rock types | The most common. Firm, often a good bearing stratum, but can be slow-draining if clayey. |
| Sand and gravel | Drains freely. Good for percolation, but offers less protection to groundwater. |
| Peat (blanket or raised bog, fen) | Soft and compressible. Usually needs special foundations, or removal and replacement. |
| Alluvium | River-laid silts and sands on floodplains. Often soft and wet; check flood risk too. |
| Bedrock at or near the surface | Little or no subsoil. Hard digging and limited natural drainage treatment. |
| Made ground | Fill placed by people. Can be variable and unsuitable for foundations without investigation. |
"Subsoil" here is the geologist's meaning. It's different from the soil scientist's soil map, which describes the top metre or so: see the soil guide.
What it can't tell you
- Both maps are regional (bedrock at 1:100,000, subsoil at 1:50,000). Ground conditions can change within a single field.
- They don't give depths, bearing capacity or contamination. A trial pit or site investigation does.
Sources
- Geological Survey Ireland: Bedrock Geology 1:100,000 and Quaternary Sediments 1:50,000, gsi.ie
Last reviewed 2026-10-02.