The Skellig Islands are made mostly of Devonian Old Red Sandstone, laid down roughly 360 to 385 million years ago. It is the reason the islands look so angular from the sea: jagged peaks, sharp ridges, and stacked rock ledges, often with red and purple hues where iron-rich layers are exposed.
If you are visiting on a boat trip, this geology is not just background detail. It directly affects what you can see from the water: steep, tilted rock layers, sea caves at the base of the cliffs, and large fallen blocks where the Atlantic has bitten into weaker zones over time.
Devonian Old Red Sandstone
During the Devonian Period, Ireland sat south of the equator. This region had a warmer, drier climate than today, and the Skellig area was part of a large sedimentary basin fed by rivers coming off nearby high ground.
The Old Red Sandstone formed like this:
- Ancient mountains broke down into sand and silt
- Rivers carried the sediment into low-lying basins
- Layers built up over millions of years
- Pressure cemented the layers into stone
The red and purple tones visible across the islands’ rock faces come largely from iron oxide in the sediment.
Tectonic folding (Variscan Orogeny)
Later, during the Variscan Orogeny, a massive mountain-building event about 290 to 270 million years ago, caused by colliding tectonic plates, tectonic compression folded and tilted the sandstone beds on both Skellig Michael and Little Skellig. This is why the rock layers can look steep and stacked rather than flat.
What to look for on the Skellig boat tour:
- Spot diagonal rock layers that look like the island has been tipped on its side
- Follow stacked ledges wrapping around the cliffs, especially where different layers stick out at different angles
- Pick out sharp ridges and narrow “steps” where the sandstone breaks along bedding planes
- Look along the base for sea caves and dark cut-ins where waves exploit cracks and weaker layers
- Notice fallen blocks and fresh-looking scars that hint at ongoing rock fall after storms
Part of Ireland's highest mountain range
The Skelligs are part of the same broader geological system as the MacGillycuddy’s Reeks, including Carrauntoohil, Ireland’s highest peak.
These mountains and islands were shaped during the same Variscan mountain-building period and are composed largely of the same Devonian Old Red Sandstone. The Skelligs can be thought of as the offshore continuation of this ancient mountain landscape, with the peaks that remain rising above the Atlantic today.
Atlantic erosion and the modern landscape
The sandstone gives the Skelligs their structure, but the Atlantic shapes the islands and keeps changing the cliffs.
Key processes include:
- Wave erosion: Swell and breaking waves hammer the base of the cliffs, widening cracks and cutting caves and notches.
- Salt weathering: Sea spray leaves salt behind in small cracks; as it crystallises it can pry the rock apart over time.
- Storm damage: Big winter seas do a lot of the heavy work, especially at the waterline where energy is concentrated.
- Rock falls: Once a crack opens, blocks can detach, leaving sharp faces and piles of broken rock on ledges and at the base.
This is why the islands look so jagged from the water: hard edges, narrow ridges, and abrupt ledges rather than rounded slopes. Sea conditions on the day also change what you can see, especially the caves and lower rock details near the waterline.
Why seabirds love this geology
All those narrow ledges and steep faces are not just dramatic to look at. They are ideal nesting and roosting spaces, with birds using almost every usable ledge and rock surface in season, especially on Little Skellig.
If you want a closer look at how the cliffs are used, see our guide to the Northern gannet colony of Little Skellig.
Seeing the islands from the water
Little Skellig is a protected nature reserve and landing is not permitted, so you view it from the sea. Skellig Michael can be seen clearly from the boat on both landing tours and eco tours, and you can also pick out many of the same tilted sandstone layers and sharp ridge lines from the water.
If you are comparing options, the Eco Tour typically gives you more time close to the cliffs and rock faces.