The iconic coastal features of Port Campbell National Park on Victoria’s Great Ocean Road are among Australia’s most visited natural attractions. The Twelve Apostles are visited by millions of visitors each year.
While many come to take photos and witness the scale and beauty of this coast very few are aware of the deep and near time geological secrets that are preserved within every layer.
The clay and limestone layers that form the foundation of the modern seascapes belong to a geological unit called Port Campbell limestone. They were deposited in subtropical to cool marine conditions from 14 to 7 million years ago when sea levels were much higher than today.
The coastline during this time (the Miocene epoch) reached 70km to the north of today’s coastline. The lower grey layers were formed under 100m of water perhaps when the shoreline would have been near this northern extremity. The younger yellow layers above them were formed in shallower conditions under 50m of water.
During the last ice age (18,000 years ago) sea levels were 125m lower than today and the shoreline of the coast would have been 70km Southwest of today’s shore.
The sea stacks and coastal features we view today are much younger than you might think. They have been sculpted into form by wind, wave and rain driven erosion helped along by a rising sea level after the last ice age.
Currently the ocean is actively eroding a cliff and sea stack base that is softer and more slippery than the brittle limestone at the top. Wave action undercuts the softer base encouraging the formation of overhangs, sea caves, arches, and ensuring a vertical cliff face.
A look through binoculars from any lookout on this coast will reveal a fragile coastline that will continue to change its shape as arches collapse, overhangs drop and sea stacks crumble.
The layers that make up each layer of Port Campbell limestone are made up of billions of marine seashells and microscopic fossils. These microfossils can be broadly grouped as planktonic (floaters) and benthic (deeper water bottom dwellers).
The beauty and spectacular shapes of these tiny fossils can be viewed under a microscope at Port Campbell Visitor Information Centre.
The layers you view along the coast are not actually flat! They are tilted from 1° to 3° from horizontal. If you travel from Gibson Steps to London Bridge the layers are warped into a series of “wobbles” called folds a little like flattened corrugated iron.
The folds are formed by tectonic activity caused when the New Zealand (Pacific Plate) bumps against the Australasian plate.
Some of the horizontal lines in the limestone layers of Port Campbell National Park are broken by fractures. A fracture is defined as a joint or crack where the layers show no displacement across the fracture and faults where layers have split and been displaced. The faults are fossilised evidence of ancient earthquakes from 7 to 5 million years ago.
Few visitors realise that you can actually view a fossilised earthquake fault from the main viewing area at the 12 Apostles. Focus just to the right (directly above the little penguin colony.) You are looking for a diagonal fault along which the horizontal layers don’t line up. Check for this fault next time you view the 12 Apostles and now you know what you are looking for you just might find other joints and faults as you travel along the coast.
Along the cliff line and near the top of the steps at Gibson Steps you will notice a few metres of red brown 3 million year old soil known as the Hesse clay. Here the Hesse clay sits directly above yellow marine limestones which are 11 million years old. This means 8 million years of geological time are missing at this point. How does that happen?
This break in time was caused by tectonic forces from the Pacific plate bumping into the Australasian plate. These forces uplifted the limestone layers out of the sea over millions of years and eroded them.
At the 12 Apostles, a little over 1km to the west of Gibson Steps, the Hesse Clay sits directly above yellow Port Campbell limestones that are 10 million years old. This means that only 7 million years of geological time and limestone formation are missing. How can we make sense of that?
Well, the answer to this lies in the “wobbles and folds” that have caused the layers to tilt upwards by 1° – 3° from between Loch Ard Gorge and Gibson Steps, in fact by the time you get to Loch Ard Gorge the Hesse clay sits directly above yellow Port Campbell limestone that is only 8 million years old. This means that only 5 million years of geological time is missing.
People regularly complain they don’t have enough time to enjoy Port Campbell National Park……..maybe it’s to do with all that missing time.
The distinctive yellow colour found in the upper layers of Port Campbell limestone is due to its very pure calcium carbonate content that has been stained with minute amounts of iron oxide or rust. The grey colour in the lower limey clay layers is due to a higher concentration of clay minerals and organic material.
Rusty facts: it does not take much to stain the limestone yellow, merely <<1% rust/iron oxide will turn layers like the those in the 12 Apostles the golden yellow colour you see today!
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Great Ocean Road Regional Tourism acknowledges the Traditional Custodians of the Great Ocean Road region the Wadawurrung, Eastern Maar & Gunditjmara. We pay our respects to their Elders, past, present and emerging. We recognise and respect their unique cultural heritage and the connection to their traditional lands. We commit to building genuine and lasting partnerships that recognise, embrace and support the spirit of reconciliation, working towards self-determination, equity of outcomes and an equal voice for Australia’s first people.