By Jesse Zondervan
Where other people see rocks and cliffs, our geologist student blogger Jesse Zondervan sees another world. Join him as he visits Jervis Bay. This blog was originally posted on the ANU science student blog.
A little kangaroo under a beach umbrella sticks his tongue out at us. A small group of beachgoers surround him, but he seems unperturbed as he lies down to relax.
One of our friends has a birthday and has invited us to a weekend in Jervis Bay. This large bay on the east coast of New South Wales is a few kilometres wide. The white beaches and spectacular cliffs are an attraction for international tourists and for Australians enjoying a weekend trip.
Honeymoon Bay is situated on a peninsula used by the Navy. This meant we had to give ID and phone numbers before we were allowed onto the gravel road leading here. It’s a real adventure.
Walking through the bush along the beach, Morgan leads us to a hidden gem: Gosangs Tunnel. We feel a strong wind coming from the entrance of the tunnel through the greyish sandstone. Climbing through it, we enter a different world overlooking the ocean cliffs.
As a geologist, I notice the fracture line along the floor of the tunnel. This tells me the tunnel exploited a weakness in the rocks and is likely to grow over time.
The cliffs are shaped like a stack of drawers, some opened and some closed. A thick, erosion-resistant plateau divides the cliffs in half. Below it, from the sea to the plateau, lies a sequence of dark mud and protruding beige sandstone beds.
In the Permian (250-300 million years ago, before the dinosaurs), Jervis Bay was situated offshore: a few hundred metres to a few kilometres from the coast. During this time, marine sediments accumulated here.
So what created this pattern of mud and sandstone?
Offshore, away from beach sand, current speeds are lower and there’s less wave energy. This is an environment in which mud is able to settle down. Big storms capable of transporting sands further offshore deposit sheets of sand in between.
My friends marvel at the fossils I show them in the mudstone. Dark curved lines define fossilised shells two hundred million years old.
The plateau in the middle is defined by a thick sandstone layer that divides the cliff in two. Both parts show a transition from layers dominated by mud to a dominance of sandstone. Each cycle represents a hundred thousand years of climate change influencing sea levels.
As my friend notices: “If you fall off this cliff, you fall a hundred thousand years.”
I don’t think our little roo mate on the beach will ever realise. But when you visit the Australian beaches and see their cliffs, you’re looking at a piece of natural history.
Jervis bay is closest to Canberra and can be reached in three hours. The Canberra region has got loads more natural gems to explore. If you’d like to know more about the geological history of Jervis Bay check out my blog post (containing diagrams).