These are two examples at the site:
1 → Rocks that formed deep underground have been exposed to oxygen when they reached the surface. Some of the rocks have changed colour and started to crumble. This is related to the iron compounds reacting with oxygen in the air (called oxidation).
This Oxidation is a reaction between the minerals in the rock and oxygen, over time this process changes the mineral composition of the rock. As time passes and the rocks oxidise more and more, they become less resistant to weathering (such as rain and wind), which is why they start to crumble.
2 → Carbon dioxide from the air/soil combines mixes with water. This produces carbonic acid which reacts with the minerals in the rocks, this disrupts their atomic structures, dissolving parts of them. This can look like un-uniform erosion. The process is called Carbonation.
The mineral calcite, which is common in limestone, is particularly vulnerable to carbonation. This mineral dissolves easily in the carbonic acid and gets washed away, this can cause anything from the small holes in the rock you can see at this site to large caves.
To log this cache, visit the site and send your answers to the following questions to the cache owner.
Q1 → The processes described above can only be seen on some rocks. Looking at different rocks at GZ, what would be different about the different type of rocks for this to be the case?
Q2 → What do you think (historically during the creation of Brisbane city) has caused these processes to move more quickly than they might have otherwise?