Rocks of Gibraltar
Mount Gibraltar, or “The Gib” as it is locally known, is home to some very unique and valuable rock that has been used in the construction of some significant buildings around New South Wales. It is thought that there are only two areas in the world- The Gib and parts of Russia, where this type of rock has such quality.
The rock is very strong, dense and fine grained and was quarried from the site for 100 years for building blocks, kerb stones, railway ballast and road surface from several quarry sites.
Geology
This area forms part of the Sydney Basin and a predominate feature of the basin is a thick layer of Hawkesbury Sandstone. Hawkesbury Sandstone is a Triassic period sedimentary quartzose sandstone that contains some elements of feldspar, iron and clay compounds. It was formed when sands from vast river systems were deposited over time and compressed to become the sandstone we see today throughout the region. Associated with the Hawkesbury Sandstone is Wianamatta Shale, which also forms layers of rock, this was again laid down over millions of years.
Mount Gibraltar is the exposed remnant of a volcanic intrusion that pushed up the overlying Hawkesbury Sandstone and Wianamatta Shale. These have eroded away over time leaving the volcanic rocks exposed. The rock is known as microsyenite, or Bowral Trachyte.
A diagram showing the formation is below:
The Earthcache
In order to log this Earthcache, you'll need to answer the following questions and email your answers. Feel free to log your find in anticipation- I'll let you know if there are any issues. Logs without the emailed answers may be deleted. It is also preferable that you upload a picture of you/ your group and your GPSr at the quarry with your log.
At GZ you'll find a sign. There is a section entitled 'Geology?'. Read this section and answer the following question:
1. Over how many millions of years of erosion have occurred to leave what we see today?
Further on there is another section entitled 'Quarries'. Read this section and answer the following question:
2. Name three of the structures that have been built containing Bowral Trachyte?
Venture up to the main quarry at around S 34 28.327 E 150 25.800.
At this point you'll find a sign.
3. Complete the sentence "The rock is an .....".
Go further into the disused quarry to the piles of rocks. Look in the piles for a sample to examine. Note that the larger pieces have some weathering and a darker colour however small fragments that are laying around and a are a lighter colour are probably best used for examination.
Examine your sample and answer the following questions:
4. What size are the grains of the rock? a) around 1mm? b) 1-2mm, c) 2-5mm, d) 5mm plus?
5. What colours can you see in your sample?
References:
Wingecarribee Shire Council
http://www.adderley.net.au/geology
