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.
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.
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
- geoscience: the earth The Sydney Basin,
Australian Museum, 2002,
http://www.amonline.net.au/geoscience/earth/sydbasin.htm
- 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
