Before looking at The Blowhole, here’s a primer on how water forms caves. Imagine the sea as it crashes against the base of cliffs. The waves carry sand and rock debris, which act like a big horizontal piece of sandpaper, cutting a notch into the vertical rockface until a crack or joint is reached. Compressed water and air wedge in the crack, forcing the rock apart. Continued erosion makes a cave longer and wider.
The Blowhole was a sea cave and tunnel until the roof at the back of the tunnel collapsed. The result was a broad arch with a blowhole behind it.
The second formation, Tasman Arch, was once a large sea cave like The Blowhole. Wave action over thousands of years made the cave wider and longer until most of the roof collapsed.
Devils Kitchen was likely a sea cave and then a tunnel. The waves over many thousands of years have eroded the rock, causing the roof to completely collapse. The water follows its inexorable path, cracking joints to create new caves in the rockface.
“Water is patient, Adelaide. Water just waits. Wears down the cliff tops, the mountains. The whole of the world. Water always wins.”
- From “The Waters of Mars,” Doctor Who