Eagle Rock is an hour and a half hour drive from Melbourne CBD and a perfect place to visit as part of a road trip along the Great Ocean Road or even a day trip to the beach! The area is also one of two Marine National Parks in the Central Victoria bioregion.
The cliffs (pictured) and surrounding geology is interesting because of the Oligocene (23+ Ma) marginal marine Anglesea Sand and sandstone, conglomerate, claystone, basalt and pyro-clastics of the Angahook Formation. It has been suggested that the basalt represents a sub-aerial (underwater) flow – basalt (lava) erupting underwater!
Fossils which can be found in the Point Addis Limestone (above the basalt) include Oysters, calcerous algae, echinoid fragments (sea urchins), foraminifera (really tiny shells) and gastropods (like snails) – see if you can pick them out in the cliff! Do you think the basalt was already cool when they started living on it? Or maybe did the basalt flow over and around them?
The animal fossils can tell you how deep the eruptions were. The fossils tell you it was a pretty shallow environment, and probably a bit rough. There’s cross-stratification of the sand layers (sand building up over time in layers) which also indicates shallow waters. Over time, tuff cones might have grown higher than the sea level!
Look at the pillow basalts closely, why does it look so fine grained? What minerals make it black? It would have erupted and cooled quickly, so the crystals wouldn’t have been able to grow large. The minerals are themselves dark (olivine, pyroxene). Can you see the ‘bread-crust’ fractures? They look like the surface of freshly baked bread and occur when the outside cools quicker than the inside and ‘cracks’ as it expands.
Can you also see the red/orange Iron staining? The iron in the basalt weathers out and forms Iron-Oxides (i.e. rust).
Just around the corner are pyroclastic deposits, including hyaloclastites (broken up volcanic rock), pyroclastic lapilli tuffs (ash and volcanic rock that falls from the sky after an eruption), tuffs and reworked volcanic sediments. All of the volcanic rocks along this cliff are from tuff cones, a type of volcano where the magma hits the water and reacts.
For more detailed (geological) information on the area, this paper or this website are two good places to start. Although some of the fossils are cool, please don’t knock them out of the cliff to take home. If you do that, no one else will be able to see them! And in the end they usually end up in a shoe box on a shelf in the shed anyway.
Make sure you get there at low tide, so you can explore Eagle Rock safely. Access is via stairs. Parking Federal Street, Aireys Inlet VIC 3231
AusGeol has created a virtual field trip with narration of the entire outcrop! Explore this fantastic resources by clicking on the image below.