September 8, 2015
Posted by Evelyn Mervine
Back in March and April 2015 my husband and I travelled to Australia for a couple of weeks for vacation. We went to visit some good friends who live in Sydney. We spent most of our time in Sydney and some of the surrounding areas, such as the Blue Mountains. We also spent a few days up in the Port Douglas area, including a couple of days on a boat out on the Great Barrier Reef.
While we were in Sydney, we were very impressed by all of the sandstone, which can be seen in outcrop at many places in Sydney, including along many of the beaches. We observed many interesting features in these sandstone outcrops, such as impressive cross-bedding structures, trace fossils, interesting erosional forms, and oxidation/coloring of various types and shades. We also observed that many buildings, walls, bridges, and other structures in Sydney are constructed out of sandstone blocks, which we assume were locally derived.
I’ve already shared some pictures of Sydney sandstone outcrops here, here, and here and of some sandstone building stones here. Today, I want to share some more pictures of Sydney sandstone and also briefly discuss its geology.
The sandstone that we observed around Sydney is known as the Syndey Sandstone or the Hawkesbury Sandstone. A nice scientific paper about the sandstone can be found here. The sandstone is Triassic in age and is interpreted to have been deposited in a fluvial environment by the ancient Hawkesbury River. Based on the scale and nature of features in the sandstone, such as cross-bedding and abandoned filled channels, this ancient river was likely a very large, sand-rich, braided river with deep main channels. Some of the abandoned channel fills are up to 18 m deep. The Hawkesbury Sandstone is extensively exposed in the Sydney area, outcropping over ~20,000 square kilometers in the Sydney Basin. The sandstone unit has a maximum thickness of ~290 m. The sandstone is a beautiful rock and has been used extensively as a building stone in Sydney, from the 1700s through to the present day.
Here’s a map showing the extent of the Hawkesbury Sandstone in the vicinity of Sydney:
Here are some more pictures of this remarkable sandstone that we encountered pretty much everywhere in Sydney. The pictures below were all taken during a walk along the ocean, near Bondi Beach.
That’s all for today… stay tuned for some more pictures from our recent Australian travels!
I live in Sydney and am a geologist although I have not worked in that field for years.
The Sydney Basin is quite large and the deposits in are thick. There is a great deal of variation in the characteristics of the sandstone with some units of highly competent stone into which vertical sided cuttings of thirty feet or can be made with perfect safety.
Other units are impermeable.
Others soft and fret easily.
When I get a moment I will send you some photographs of interesting structures. (Including a columnar sandstone!)
I would love to see your pictures of Sydney sandstone. If you email me some pictures I’ll be happy to share some here on Georneys.