The case of the Billys Beach rock wall

Decorative pebbles adorn the Narooma Chert outcrop at Billys Beach. Photo: Sam Ingram

Decorative pebbles adorn the Narooma Chert outcrop at Billys Beach. Photo: Sam Ingram

This column's quest to uncover the origins of the naturally occurring rock wall decorated with white pebbles at Billys Beach, near Narooma, is finally over. 

After the wondrous wall featured in this column, it captured the imagination of several readers including Emma Ingram of Duffy who despite being aware of the wall for "as long as she can remember", confesses that "it wasn't until I recently visited it again that we realised just how beautiful it is". 

During the week this column tracked down south coast geologists Anne Felton and Stewart Needham who teamed up with Canberra-based rock doctor Doug Finlayson to investigate the wall's geological origins.

The Billys Beach geological pattern. Photo: Alan Nicol

The Billys Beach geological pattern. Photo: Alan Nicol

The trio report that "the rock outcrop is part of a sedimentary sequence that was deposited in deep water off the margin of a much smaller Australian continent about 500 to 450 million years ago".

Further, according to Finlayson and co, "the rock outcrop is part of the Narooma Accretionary Complex and includes black siliceous mudstone transformed into slate and bedded chert, a chemical sedimentary rock largely composed of very fine-grained silica". 

Apparently these sedimentary rocks are among the oldest in eastern Australia and were accreted, buckled and folded onto the continent when major tectonic events drove an oceanic plate against the continental margin. 

"In this process the sediments were dragged down several kilometres causing them to be folded and recrystallised into the slate and chert," report the geologists. "Subsequent tectonic events and the opening of the Tasman Sea have enabled the uplift of the rifted east Australian margin we see today." 

"Chert is very hard and thus resists erosion more than the neighbouring slate layers," says Finlayson "hence the beautiful folded and sculptured layers of black chert and slate in the cliffs at Billys Beach, decorated by beachgoers with white quartz pebbles from the seashore."

Case closed. 

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