Between 290 and 200 million years ago (Permian-Triassic periods) sand that had eroded mountains in western and central New South Wales were deposited by huge river systems to form the Hawkesbury Sandstone, which make up most of the rock seen in the Sydney Basin.
These layers of sandstone were eventually pushed upwards, and finer sediments (clays) were deposited in depressions, or lakes, on the surface. Those clays became the claystones and shales that are seen throughout parts of the Sydney region, in particular between Parramatta and the Nepean River - an region known as the Cumberland Plain. One outcrop of shale can be seen at the Dundas Dig EarthCache (GC1HGD5). Another appears along the banks of the Parramatta River.
This two-stage EarthCache starts at a location on the Parramatta River where a stone wall was built between the 1790s to the 1840s. It is thought the wall was constructed to provide off stream mooring for vessels delivering goods between Old Sydney Town and Parramatta. Take a close look at the rock used to build the retaining wall.
Now walk upstream to the Parramatta Rivercat Wharf (Waypoint 2), and then cross the river at the weir. Here you will find an outcrop of Ashfield shale. Note: The weir and river foreshore are subject to regular flooding.
Logging requirements
To complete this EarthCache answer the following questions:
- What type of rock was used to build the stone wall?
- Describe the Ashfield shale (i.e. colour, texture, hardness)?
- What is the relationship between the shale found along the banks of the river and the soil found in the mangroves downstream?
You can log your visit along with your find immediately, however, you must email me with the answers to the questions above. Failure to comply with these instructions will result in your log being deleted.
Your log should indicate how many people were in your party and any additional information or experiences you gained from this Earthcache. Although not required, a photo of you at one of the locations is appreciated.
Sources
- A wall of history discovered, Parramatta City Council press release, February 2008
- Field geology of New South Wales, David F Branagan and Gordon H Packham ; edited and produced by Ross Stewart, New South Wales Department of Mineral Resources, 2000.
Geocaching NSW aims to enhance and improve the activity of geocaching in New South Wales. Visit the association website at www.geocachingnsw.asn.au.