Amazing views from Square Rock!

By Evan

Earlier this week week (yes, I was a bit slow to the draw in getting to this post scheduled), a bunch of fellow students at RSES decided to do a Sunday hike. Yes, I know that Australians likely call this a “bushwalk”, but I guess I have to display my Canadian pride somewhere. 😉 We went to Namadgi National Park, located a short hour drive south of the ANU. The hike of interest was this location known as Square Rock, a simple 9 km hike that takes about 3 hours, depending on how long you stay at the viewpoint. Needless to say, Square Rock gives you an amazing view, though it is not for those who get vertigo (it is a pretty steep cliff!). Us geologists are an outdoorsy lot, and I have to say it was wonderful to get out of the city and enjoy the wilderness.

Square Rock is composed of granite. It is part of the Murrumbidgee Batholith, which covers much of the southern Australian Capital Territory. The batholith was emplaced between 424 and 414 million years ago, which places its formation during the late Silurian and early Devonian periods. Granites are generally considered to be one of the most boring types of rocks, due to the fact that they usually do not contain economic minerals, and because they are one of the most common rocks you will find. However, while doing hiking geology, you can find some interesting things, even in granite.

Most of the granite is massive and coarse grained, full of quartz (the grey stuff), k-feldspar and plagioclase (the white stuff) and horneblende (the black stuff).
The black thing here is what is known as a xenolith. These are basically chunks  existing rocks that surrounded the batholith when the granite formed.
This is the granite that makes up Square Rock. It looks different than the previous granite because it has phenocrysts of k-feldspar (the pink stuff). Phenocrysts are large crystals in a groundmass of finer crystals.
No trip to Square Rock would be complete without seeing a Black Rock Skink!
It is also worth going to see Gibraltar Falls, which is accessible just before reaching Namadgi Park.