🗺️ Geological Background
Port Campbell sits on the Shipwreck Coast of western Victoria, famous for its limestone sea cliffs and iconic formations like the Twelve Apostles. Beneath and within these limestones lies a distinct marl layer—a softer, darker, clay-rich sedimentary rock.
Marl is a mixture of calcium carbonate (CaCO₃) and clay minerals. It typically forms in shallow marine environments where both fine clays (delivered by rivers) and calcareous muds (from marine organisms) accumulate. Over millions of years, this material compacts and hardens into marl rock layers.
At Port Campbell, the Port Campbell Limestone (Miocene age, 23 to 5 million years ago) is interbedded with these marl seams. These marl layers are less resistant to erosion than the limestone above and below them, which leads to undercutting, slumping, and formation of the cliffs and sea stacks we see today.
🔎 What to Look For
When you visit the cliffs, look carefully at the rock faces:
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Colour & Texture: The marl is usually grey to blue-grey, softer, and more crumbly compared to the creamy-white limestone.
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Layering: Marl often occurs as distinct seams between limestone beds.
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Erosional Effects: Because marl is weaker, it weathers out more quickly, creating ledges, notches, or undercuts in the cliff face.
📚 Educational Notes
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Formation: Marl indicates periods of increased clay input into a marine system — often linked to changes in climate, sea level, or river activity.
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Geotechnical Importance: These weaker layers strongly influence the coastal erosion patterns along the Shipwreck Coast.
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Fossils: Both marl and limestone here can contain marine fossils — mostly microfossils — indicating a Miocene-age shallow sea environment.
📝 Logging Tasks
To log this EarthCache, you must visit the coordinates and answer the following questions. (Please send answers via the message center — do not post them in your log.)
Look for signs of erosion or undercutting at the marl horizon. Describe one feature you can see that shows the marl’s weaker resistance.
Take a photo of yourself or your GPS with the cliffs in the background — please do not include close-ups of the marl itself to preserve the site.
⚠️ Safety Notes
📖 References & Further Reading
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Geological Survey of Victoria (2016): Geology of the Port Campbell Embayment.
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Parks Victoria – Port Campbell National Park Geology Factsheet.
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BMR Bulletin 141: Miocene Stratigraphy of Western Victoria.
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Geological Society of Australia – Earth Science Heritage Sites.