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 a marine turbiditic sandstone with minor mudstone, minor carbonaceous siltstone and local carbonate concretions.
Weathering
Rocks gradually wear away. This process is called weathering. There are two main categories of weathering:
- mechanical weathering
- chemical weathering
Mechanical Weathering
Mechanical weathering (or physical weathering) is caused by physical changes such as changes in temperature, freezing and thawing, the effects of wind, rain and waves and biological means. It is a process which requires the rock to be physically moved or manipulated in order for the weathering to take place, but if does not change the chemical composition of the rock.
Temperature changes
Rocks, like many other materials, expand as they heat up and contract as they cool down. If a rock is heated and cooled many times, cracks form and pieces of rock fall away. This type of physical weathering happens a lot in deserts, because it is very hot during the day but very cold at night.
Wind, rain and waves
The physical process of rain or waves colliding with the rock can also cause weathering. Over a long period of time these collisions will eventually break down the rock.
Wind causes weathering by blowing tiny grains of sand and other particles against the rock; literally sandblasting the rock. Eventually this process will wear the rock away.
Freeze-thaw
Water expands slightly when it freezes into ice. This is why water pipes sometimes burst in the winter. The formation of ice can also break rocks.
If water gets into a crack in a rock and then freezes, it expands and pushes the crack further apart. When the ice melts later, water can get further into the crack. When the rock freezes again, it expands and makes the crack even bigger.
This process of freezing and thawing can continue until the crack becomes so big that a piece of rock falls off.
Biological
Biological weathering takes place when rocks are worn away by living organisms. Trees and other plants can grow within the cracks in a rock formation. As the roots grow bigger they push open cracks in the rocks making them wider and deeper. Over time the growing tree eventually prizes the rock apart.
Tiny organisms like bacteria, algae and moss can grow on rocks and produce chemicals which can break down the surface layer of the rock. Burrowing animals such as rabbits can accelerate the formation of cracks.
Chemical weathering
Chemical weathering occurs when rocks are broken down by a chemical change. Rainwater can become slightly acidic by absorbing carbon dioxide in the atmosphere and this reacts with the mineral grains in the rock giving rise to new minerals and salts.
The degree of chemical weathering depends on the type of rock for example limestone is more readily chemically weathered than granite. When the now acidic rainwater falls on limestone or chalk, a chemical reaction happens. New soluble substances are formed in the reaction. These are washed away and the rock is weathered.
Some types of rock are not easily weathered by chemicals. For example, granite is a hard rock that is weathered only slowly. Still some of its minerals do react with the acids in rainwater to form new, weaker substances that crumble and fall away.
How to claim
To claim this Earthcache, please supply me (via my profile) with the answers to the following questions
- The processes of weathering and erosion are often confused with each other. What is the difference between these two processes?
- Looking at the Eagle's Beak, describe the average size of the grain. Small size (less than 1mm), medium size (between 1 and 3 mm) or coarse (greater than 3mm).
- What type of weathering do you believe has formed the Eagle's Beak?
- Have a look around near the Eagle's Beak. List another type of weathering that is present within the area. What is your evidence to back up your assertion?
A photograph of you or your GPS in front of GZ 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.
References
Bibby, L.M. & Moore, D.H., 1998. Charlton 1:100 000 geological map. Geological Survey of Victoria.
[Author unknown], Geological History of Victoria, accessed on 4/10/2014 at http://vic.gsa.org.au/Publications/IVG2PDF/IVG2Chapt004.pdf
BBC KSC Bitesize, accessed on 14/8/2015 at http://www.bbc.co.uk/bitesize/ks3/science/environment_earth_universe/rock_cycle/revision/6