Bell Rock
Named for the shape of the rock before you, it was previously known as Sebastopol Rock and is located on top of Sebastopol Hill. These days people come to Bell Rock to enjoy the view, and being out and about with nature. It wasn’t always this way though.
In the late 1800s the allure wasn’t the view, but something a lot shinier!
Around that time St. Arnaud at that time was a hub for mining (silver and gold) and the area around here was where a number of the larger mines were sited.
Prior to that though the scene was a whole lot different. In fact 540 million years ago this was the bottom of the ocean.
Geology
During the Cambrian period (541 to 485.4 million years ago), Victoria was covered with an ocean that extended far to the east and was deep in most places. St. Arnaud was at the bottom of this ocean.
The geology of this area consists of rocks from the St. Arnaud Group. These are a sequence of quartz rich marine turbidites (sedimentary rocks that are created from underwater avalanches or landslides of mud or sand). While there is some conjecture to the exact age of the St. Arnaud Group, rocks in surrounding areas have evidence of graptolite fossils from the Ordovician period (484 to 443 million years ago). The St. Arnaud Group is unfossilised, therefore indicating that these rocks are older than Ordovician. These are some of the oldest rocks in Victoria.
The St. Arnaud Group consists mainly of three distinct formations; Warrek, Beaufort and Pyrenees Formations. The majority of the rocks around this area are of the Pyrenees Formation which is made up of thick-bedded sandstone, minor siltstone and shale. The majority sandstone has a colour ranging from light yellow to medium grey and has fine to medium grain sizes.
The small outcrop of rocks in front of Bell Rock (right in front of where you are standing) is a good example of the Pyrenees Formation.
Chasing gold
The miners initially began to chase alluvial gold but soon turned to the deep, quartz reefs. Primary gold mineralisation in the St. Arnaud Group is hosted by quartz and sulphides within the fault zones and brittle fracture lines. In the St. Arnaud Group it is the Pyrenees Formation rock that hosts the gold. A high sand to mud ratio of the Pyrenees Formation makes is more brittle and prone to fracture, allowing quartz veins to form within it.
How to claim
To claim this earthcache, please supply me (via my profile) with the answers to the following questions
- How would you describe the grain size (fine, medium or coarse) of the Pyrenees Formation on (or near) Bell Rock?
- As well as rock of Pyrenees Formation there is also quartz located within Bell Rock. What causes the milky quartz to obtain its colour?
- Provide the reason as to how Bell Rock obtained its shape?
- In which direction does Bell Rock run?
A photograph of you or your GPS in front of Bell Rock could also be attached, but this is optional.
Feel free to log your find without waiting for my answer, but note, I will delete your log if no contact is made within 7 days of your log.
References
Krokowski de Vickerod, J., Moore, D.H., & Cayley, R.A. (1997) St Arnaud - 1:100000 Map Area Geological Report
[Author unknown], Geological History of Victoria, accessed on 4/10/2014 at http://vic.gsa.org.au/Publications/IVG2PDF/IVG2Chapt004.pdf