The distinct hexagonal columns found at Kiama’s Bombo Headland Quarry on the New South Wales south coast, like those of the Giant’s Causeway in Ireland, are a natural phenomenon offering geologists insights into the Earth’s core mantle.
But they also tell a different story, one of hard-working men doing loud and dangerous mining work as old Sydney town established itself on the back of basalt.
Aboriginal and Torres Strait Islander readers are advised this story has a photo of people who have died.
First mined in the 1880s, basalt came to be known as Kiama’s blue diamond.
“Kiama’s blue diamond is an analogy to the black diamond as coal was called during the 19th century,” said local historian Malcolm Bedford.
“It also refers to the hardness, the bluey gray color and the monetary value of the finished product be it rocks, stones or gravel.”
Basalt in, sandstone out
In 1838, farmer James Holt bought 320 hectares of land at Kiama, before selling it to the Railways Commission of NSW 50 years later.
Columns of rock were discovered on the site, basalt mining began, and the Bombo Headland Quarry was established.
The blue diamond coast exploded with the possibility that basalt replaced sandstone blocks to construct public buildings on the NSW south coast.
Later, the rock was hand crushed for shingle metal used for roads, tracks, railway ballast and concrete.
“The locals dug this out to build the local buildings and then they realized crushing it they could sell it to Sydney, and the Sydney people were delighted,” Mr Bedford said.
“Kiama’s first name was actually Blue Haven, before it went back to the Aboriginal name of Kiama.”
‘Bumbo’ is considered rude
Mr. Bedford knows a lot about the history of the Bombo Headland Quarry.
He is affiliated with the Pilots Cottage Museum in Kiama, an 1881 heritage building constructed with basalt mined from the region’s seven quarries.
“Aboriginal people [Wodi Wodi of the language group Dharawal] lived here [for] 50,000 maybe 60,000 years, and they had a word for thunder, which is bumbo,” he said speaking from the quarry, abandoned now except for the occasional walkers, photographers and bird watchers.
During the day the coastal quarry echoes birdsong but during a storm the rocks, moving under the weight of the sea, are really loud.
“It’s a rolling thundery-type noise and I wonder if that’s why the Aboriginal name for this area was bumbo, their word for thunder,” he said.
“It was called Bumbo Beach and Bumbo Quarry and it was the Bumbo railway station in the 1880s until a local minister of religion objected and said, ‘That’s a rude name. We have to change the u to an o’.
“So then it got changed to Bombo, and this area is now Bombo Quarry.”
Today, the heritage-listed former quarry in Kiama is a drawcard for geologists.
University lecturer Solomon Buckman, who regularly visits the site, identifies geological highlights tracing back 270 million years.
“Geologists come from all over the place to look at these fantastic volcanic rocks,” Dr Buckman said.
“It’s a great example of columnar jointed basalt and there are all kinds of hidden gems and information that geologists extract from these rocks.”
Dr Buckman, a senior lecturer in geology at the University of Wollongong, frequently takes students to Bombo Headland Quarry on field-based studies of internal rocks.
The headland rocks they study erupted as hot lava, at a temperature of about 1,200 degrees Celsius, onto the Earth’s surface about 270 million years ago.
“As they cool, they contract and fracture, forming very distinct hexagonal columns,” Dr Buckman said.
“It’s about 20 or 30 meters thick in this location.”
A geologist’s dream
Several aspects of the site are important to geologists.
A number of igneous dykes, which are formed when magma fills a fracture in older rocks, exhibit natural processes that bring xenoliths (bits of foreign rock) to the surface.
“We can only drill a couple of kilometers down into the Earth’s crust, but these dykes bring samples for us from hundreds of kilometers down, up to the surface,” Dr Buckman said.
“We study those and get an idea of what the Earth is composed of.
“These are the kind of processes that bring diamonds to the surface.
“It’s an interesting area because we have what’s called the Kiama Magnetic Reversal recorded in the rocks here,” Dr Buckman said.
“It’s a big flip in the Earth’s magnetic field about 260 million years ago.”
Bombo Quarry also consists of conglomerate, mudstone and volcaniclastic sandstone, a sedimentary rock that formed the Sydney Basin in Permian times, about 250 to 300 million years ago.
“Back when Australia was still attached to Antarctica, it was part of the super continent Gondwana,” Dr Buckman said.
The sandstone sediments eroded from the surrounding mountains and deposited into the Sydney Basin, a shallow marine environment.
“The sandstone’s quite a soft rock. It’s composed of lots of little grains of quartz and feldspar and other little fragments of rock that have been cemented together,” Dr Buckman said.
“They don’t use that in terms of quarrying, they are after this hard basalt rock.”