Education, Geology, Other

Hawkesbury sandstone: a journey

Geologist Greg Thomson takes a step back in time to examine the history of what we know today as yellow block sandstone, or Hawkesbury Sandstone.

Geologist Greg Thomson takes a step back in time to examine the history of what we know today as yellow block sandstone, or Hawkesbury Sandstone.

Our story starts 240 million years ago (early Triassic) in NSW. We stand knee-deep in fresh water flowing north-east with sand between our collective toes. The water is warm, which is surprising considering proximity to the south pole.

Looking across this flat vast flood plain, there are occasional vegetated billabong basins, which fill with mud and fish.

Not that we would notice, but this river flat is slowly sinking, allowing sediments to stack over each other during floods and quieter periods, which will ultimately create a sediment pile up to 250m thick.

Fast forward 239,940,000 years. We now stand with the Gadigal people of the Eora Nation atop the resulting sandstone out-crop, the result of around 200 million years of deposition, heat, pressure, uplift, and erosion.

The Gadigal people have thrived here for millennia. They show us how the sandstone is used for tool-sharpening, and water-eroded caves for ceremonies, art, and shelter.

Not long after, we see white sails: European people arrive with flags. They settle here and colonise Warrane (also spelt as Warran, Warrang and Weerong), renaming it Sydney Cove.

The sandstone in Eora territory was (re)named Hawkesbury Sandstone in 1844 by Charles Darwin – naming it after the Hawkesbury River where it was most common.

The Hawkesbury River was named by Governor Philip in June 1789 after the First Earl of Liverpool, Charles Jenkins, who was titled Baron Hawkesbury, as he came from Hawkesbury Upton in England.

Now we jump to present-day Sydney, where many buildings around the city have been built from the yellow block sandstone.

A stroll from Cadman House (built in 1816) near Circular Quay up Argyle Street, past the Orient Hotel, and through the bustling markets on the weekend reveals many sandstone buildings.

Yellow block sandstone is made up of around 65 per cent quartz, 30 per cent clay and five per cent siderite (iron carbonate). It would have been quarried in the Pyrmont, Ultimo and the CBD of Sydney.

Let’s look at the Sydney Art Gallery, which has recently had its 150th birthday. This would have been quarried from Pyrmont and Ultimo.

English architect Walter Liberty Vernon migrated to Australia when he was 37 years old with his wife and two sons. He sought to escape the dirty London air and relieve his asthma.

Seven years later, in 1890, he was appointed the NSW Government architect. In 1895 he was commissioned to design and supervise the construction of the Art Gallery of NSW.

The pillars and blocks found at the Art Gallery are all Hawkesbury Sandstone, which can provide the strength for large structures as well as the weakness to be sculptured into beautiful shapes.

The combination of quartz, siderite and clay when excavated is lifeless grey. But over the course of a couple of weeks the siderite does its magic and turns the stone a golden yellow hue.

This feature first appeared in the June issue of Quarry.

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