+++IMPORTANT+++
- No dogs are allowed at GZ as the area is farmland.
- Binoculars or a zoom lens are very helpful answering the questions
- Stay well back from cliff edges — the terrain can be dangerous
- Respect nature, other visitors, and any safety signage.
- Check weather conditions before visiting — the Atlantic coast can be harsh and windy.
Dún Briste
Dún Briste (Irish for "The Broken Fort") is one of Ireland’s most spectacular sea stacks, rising above the Atlantic Ocean. It offers a rare geological snapshot into Earth’s history, revealing well-preserved sedimentary layers from the Carboniferous Period, around 350 million years ago.
The visible horizontal rock layers — limestone, sandstone, and shale — were formed in ancient tropical seas and later shaped by tectonic uplift and relentless erosion. A fierce storm in 1393 finally separated the sea stack from the mainland, leaving behind this striking monument to geological time.
What is a Sea Stack?
A sea stack is a vertical column of rock that has become isolated from the mainland by coastal erosion. The typical formation process includes:
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Cracks in headlands or cliffs are widened by waves through hydraulic action and abrasion.
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These cracks form sea caves, which may enlarge over time and break through the headland to form a natural arch.
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When the arch collapses — often during a storm — it leaves behind a free-standing rock pillar, called a sea stack.
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Continued erosion can eventually wear the stack down to a stump.
Sea stacks are excellent records of coastal evolution, showcasing the power of wave action, weathering, and geological structure.
Dún Briste’s Formation
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The layers of Dún Briste were deposited during the Lower Carboniferous Period.
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They originated as sediments in a shallow tropical sea, rich in marine life.
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Over millions of years, these sediments were compacted into rock, then uplifted and eroded, forming cliffs and eventually the sea stack.
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In 1393, a powerful storm destroyed a natural land bridge that once connected it to the mainland, isolating the stack.
Visible Rock Types
| Rock Type |
Color |
Grain Size / Texture |
Typical Layer Thickness |
Depositional Environment |
Other Features |
| Shale |
Dark grey to black |
Very fine-grained, smooth, sometimes laminated |
Thin to medium (a few cm to ~30 cm) |
Calm, deeper marine waters |
May contain fossils or organic material |
| Sandstone |
Light beige, tan, or grey |
Coarse-grained, gritty texture |
Medium to thick (10 cm to over 1 m) |
High-energy, shallow marine or beach environments |
May show ripple marks or cross-bedding |
| Limestone |
Pale grey to cream |
Fine to medium-grained, may contain visible shell fragments |
Variable, often thick and massive |
Shallow, warm marine environments |
Often fossil-rich; reacts with acid (effervescence) |
These layers reveal alternating shallow and deep marine conditions, as well as possible storm-driven deposits (turbidites).
Logging Tasks
To log this EarthCache, visit the given coordinates and look at the sea stack - it is NOT necessary to climb the sea stack itself - and send me your answers to the following questions via Geocaching Message Center or email. You may log your find immediately after sending your answers — no need to wait for confirmation.
Questions (on-site observation):
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How many visibly distinct layers (based on color or texture changes) can you count in the sea stack?
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Pick two of these layers and describe how they differ in texture, color, or thickness.
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What do the horizontal layers suggest about the original conditions under which they were deposited?
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Estimate the height of the sea stack and compare it to the mainland cliff. What does this suggest about erosion processes?
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Upload a photo of yourself or your GPS device with the sea stack in the background. (No faces required.)