During the Permian-Triassic periods (290-200 million years ago) sands were deposited by huge river systems to form the Hawkesbury Sandstone. This rock is the oldest visible throughout much of the Sydney Basin.
These layers of sandstone were eventually pushed upwards, and finer sediments (clays) deposited in depressions, or lakes, on the surface. Those clays became the claystones and shales now known as the Ashfield Shale and Bringelly Shale – the main rocks in the Dundas–Carlingford area.
During the Jurassic period (less than 175 million years ago), cracks formed in the crust and volcanoes appeared throughout the region.
At this location a small narrow volcanic pipe known as a diatreme formed. The core of this forms the hill that continues in a south-easterly direction towards Alexander Street.
The rocks that formed in the core of the diatreme are a volcanic breccia (mixture of shattered fragments) of basalt. Other materials in the diatreme include pyroxene chromite xenoliths brought up from the Earth's mantle deep below the surface.
A sample of basalt.
Some of the shale close to the breccia was hardened by the heat of the magma intrusion. This rock looks darker and is harder than the shale nearby and is known as hornfels. It can still be broken easily, but not necessarily along its bedding lines.
In the 1830s, this site was identified by Surveyor-General Sir Thomas Mitchell as a suitable source of rock for surfacing roads. A quarry was set up and provided rock for Sydney's roads until 1902.
After the quarry closed in the 1940s, the area became a landfill, and in the 1960s was converted to a recreation area and playing field.
Most of the volcanic rock was excavated from this site or is below the surface, but some of it remains scattered across the ground and in parts of the cliff face.
The quarry site.
To log your find at this EarthCache you must send us a message with the answers to questions 1 and 2.
- What was the rock type quarried from this location until 1902, and what is its common name?
- Describe (colour, structure and type of rock) the three main layers of sedimentary rock to the south of the waypoint.
Include in your found log the number of people in your party and if possible a photo of you holding a sample of hornfels (shale hardened by the magma intrusion). Please do not take the sample with you - replace it where you found it.
Logs not accompanied by a message with the answers to the question above will be deleted in accordance with EarthCache guidelines.
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