The Hawkesbury Sandstone is a mid-Triassic (290 Ma - 200 million years old) quartz sandstone found in the Sydney Basin that is 200 to 300 meters thick. It is though that the sandstone was deposited in a river delta similar to the area around Bangladesh today. The ancient river meandered across the delta washing out the fines leaving only the rounded quartz sand.
Within the Hawkesbury Sandstone are a series of joints, fractures in the rock caused by tectonic movement or the release of pressure. It is along these weak lines that erosion is the most effective creating linear steep valleys, leaving relatively strong sandstone ridges between.
Chemical weather played a critical role in developing the natural bridge. Water, from the stream or rain, dissolves the quartz cement that holds the individual quartz grains together. This weathering likely preferentially eroded out the middle of one of the narrow linear ridges formed by the fracturing and created the bridge.
This chemical weathering also helps form the many caves found in the Hawkesbury Sandstone.
Please ensure that the following requirements are met otherwise your log may be deleted (after an email).
Logging requirements:
Send me a note with :
- The text "GCZ5HP Hawkesbury Sandstone Natural Bridge" on the first line
- The number of people in your group.
- Send me a note saying if the erosion that created the bridge is continuing and a short explanation as to why
- Post a picture of yourself at the bridge.
Note that since this EarthCache was published the requirement to provide a photograph as proof of your visit is no longer permissable. However, it would be appreciated if you could still provide a photograph although this is now not compulsory.
Please begin your e-mail with the name of the earthcache and make sure your log includes the number of people in your group.
The following sources were used to generate this cache:
- Geology of Sydney Harbor, Australian Museum, 2002, http://www.livingharbour.net/environments/geology.htm
- http://australianmuseum.net.au/The-Sydney-Basin/
- Upper Parramatta River Catchment Education Resource Kit, 2002, © 2001 Upper Parramatta River Catchment Trust, http://www.uprct.nsw.gov.au/HTML/Info%20Sheets/Catchment/C4%20-%20Geomorphology%20Overview.htm
- KEVIN KIERNAN, 5603$$00A5, 03-21-03 14:24:51, Australia Encyclopedia of Cave and Karst Science,. Routeledge, John Gunn, Editor, http://www.routledge-ny.com/ref/caves/australia.pdf
