St Michael's Cave, North Avalon
A couple of days ago I posted an image looking at the cliffs at the southern end of Avalon Beach, taken during a floatplane flight up the Northern Beaches in late 2009. The image above is in North Avalon. At the time I took it, I thought some of the houses looked precariously close to the cliff edge and worth a shot. It was only when I researched it I discovered the story below.
Although the peninsula that you see is composed primarily of Hawkesbury sandstone and the underlying Narrabeen group of sandstones and shales from the Triassic period (250-200 million years ago), tectonic movements over the eons have created vertical cracks through the layered sedimentary rocks.
In the succeeding Jurassic period (200-145 million years ago), molten rock forced its way through these cracks, forming dolerite dykes that rise from the depths and form thin vertical sheets punctuating the otherwise sedimentary geology.
Dolerite dykes weather more slowly on sea-level rock platforms, as can be seen at Long Reef (off to the south - left), but dykes exposed above sea level tend to erode more quickly, as is the case with St Michael’s Cave.
Much of the metre-wide dyke here is mineral feldspar which, when decomposed by weathering turns to a light clay. As the dyke has been weathered over the eons, so too have the immediately-adjacent shales and sandstones, and a cave has formed, penetrating nearly 70m into the headland.
Not only have the sides of the cave weathered but also the roof, from which material has continued to fall to the floor of the cave.
In earlier times, especially during those brief periods when the sea level was higher than it is now, the fallen material would have been washed away, leaving the floor of the cave clean. But when the sea level stabilised at its current level, the fallen material simply lay where it fell, slowly building up the floor level.
Today, the lip of the cave is now tens of metres above sea level. The mouth of the cave is now only about 8m wide and 4m high but the cave heightens as the floor near the lip falls away until a point is reached where the cave is about 17m high, although in the centre is a large pile of debris that has fallen from the roof and remained where it fell.
The dyke still traverses the full length of the roof of the cave, at the rear of which high-pitched squeaks and the smell of guano reveal a breeding site for common bent-wing bats and large-eared pied bats.
It would be interesting to know what significance was attached to the cave by the local Aborigines. This wondrous natural feature did not long escape the attention of arriving Europeans, although it is not known which of them was the first to enter the cave.
For all its beauty and wonder, this window into the geological past is a dangerous place to be - rocks continually fall from the roof and could easily kill a person - so the mouth of the cave is now fenced off for public safety.
And as for that trampoline in the grounds of the cliff-edge property on the left of the image, it is just landward of a fence - on the largest version of the image you can see that most of the properties have fence lines or walls that prevent direct access to the cliff...
St Michael's Cave, North Avalon
A couple of days ago I posted an image looking at the cliffs at the southern end of Avalon Beach, taken during a floatplane flight up the Northern Beaches in late 2009. The image above is in North Avalon. At the time I took it, I thought some of the houses looked precariously close to the cliff edge and worth a shot. It was only when I researched it I discovered the story below.
Although the peninsula that you see is composed primarily of Hawkesbury sandstone and the underlying Narrabeen group of sandstones and shales from the Triassic period (250-200 million years ago), tectonic movements over the eons have created vertical cracks through the layered sedimentary rocks.
In the succeeding Jurassic period (200-145 million years ago), molten rock forced its way through these cracks, forming dolerite dykes that rise from the depths and form thin vertical sheets punctuating the otherwise sedimentary geology.
Dolerite dykes weather more slowly on sea-level rock platforms, as can be seen at Long Reef (off to the south - left), but dykes exposed above sea level tend to erode more quickly, as is the case with St Michael’s Cave.
Much of the metre-wide dyke here is mineral feldspar which, when decomposed by weathering turns to a light clay. As the dyke has been weathered over the eons, so too have the immediately-adjacent shales and sandstones, and a cave has formed, penetrating nearly 70m into the headland.
Not only have the sides of the cave weathered but also the roof, from which material has continued to fall to the floor of the cave.
In earlier times, especially during those brief periods when the sea level was higher than it is now, the fallen material would have been washed away, leaving the floor of the cave clean. But when the sea level stabilised at its current level, the fallen material simply lay where it fell, slowly building up the floor level.
Today, the lip of the cave is now tens of metres above sea level. The mouth of the cave is now only about 8m wide and 4m high but the cave heightens as the floor near the lip falls away until a point is reached where the cave is about 17m high, although in the centre is a large pile of debris that has fallen from the roof and remained where it fell.
The dyke still traverses the full length of the roof of the cave, at the rear of which high-pitched squeaks and the smell of guano reveal a breeding site for common bent-wing bats and large-eared pied bats.
It would be interesting to know what significance was attached to the cave by the local Aborigines. This wondrous natural feature did not long escape the attention of arriving Europeans, although it is not known which of them was the first to enter the cave.
For all its beauty and wonder, this window into the geological past is a dangerous place to be - rocks continually fall from the roof and could easily kill a person - so the mouth of the cave is now fenced off for public safety.
And as for that trampoline in the grounds of the cliff-edge property on the left of the image, it is just landward of a fence - on the largest version of the image you can see that most of the properties have fence lines or walls that prevent direct access to the cliff...