To get to Buchan from Melbourne follow the Princes Highway to Bairnsdale and proceed to Nowa Nowa via Bruthen and take the turn off to Buchan on the left. If approaching from the east follow the Princes Highway to Nowa Nowa and proceed to Bairnsdale via Bruthen and take the turn off to Buchan on the right. Once there continue north up the Buchan – Gelantipy Road. Until you pass Seldom Seen. Take the right fork in the road to Little River Falls. To find the cache continue past Little River Falls until you encounter the Little River Gorge lookout turn off. A reasonable gravel road takes you to a car park that places you about 400m from the cache via an easy down hill walk along a well maintained track. Note: There is no mobile phone coverage in this area. Also, it may not be so easy walking back up the hill!
This site is inside Snowy River National Park and is managed by local rangers. National Parks are special protected places so please obey all signs and requests from rangers and be sure not to take specimens.
Little River Gorge is the deepest gorge in Victoria and is a fine example of an erosional landscape. It also provides the best exposure of the Snowy River Volcanics, the rocks that make up much of this region, but big outcrops are only visible in the distance. Other features of the gorge include spectacular waterfalls and excellent examples of 'rock rivers'. These are places where the valley wall has over steepened and started mass wasting. 'Rivers' of rock-fall and landslide material can be distinctly seen. The gorge has such steep walls because of the type of rocks present and the fracture pattern they have developed. Further down stream in this valley the rock type changes to a more easily eroded granite and the gorge opens up into a less steeply walled valley as a result.
The Snowy River Volcanics are rocks that formed from outpourings of very violent volcanic eruptions beginning approximately 410 million years ago as the Buchan Rift opened. The resultant layers of volcanic rocks are several thousand metres thick. They consist of very thick layers of pyroclastic rhyodactites, some minor lava flows, conglomerates and pyroclastic ash-fall deposits called tuffs.
The word pyroclastic literally means hot fragment and is a very apt description. Eruptions of the Snowy River Volcanics would have been very explosive; much larger than any eruptions seen anywhere in modern times. Mt Pinatubo and Mt St Helens were just nothing compared to these eruptions! Massive amounts of hot rock, fragmented magma and pumice are blasted into the air and then rain down on the landscape. This material is often so hot it 'welds' itself back together, forming a very solid rock. Sometimes the mix of hot rock and super heated gases collapses in a dense column onto the landscape and rushes down slope as a pyroclastic flow that sweeps all before it. The resultant rock is called an ignimbrite.
Rhyodacite is a name given to volcanic igneous rocks that contains a high proportion of silica. Rocks of this composition always contain minerals such as quartz and feldspar in very high proportions. Unlike granodiorite, the coarse grained, plutonic igneous rock that forms from slow cooling of magma of the same chemical composition, pyroclastic rhyodacite cools relatively quickly as it flys through the air and very few large crystals can be seen in the resultant rock. Microscopic inspection reveals that much of the rock is composed of fragments of frothed up magma that cooled so quickly that they remain glassy rather than crystalline. Those crystals that are present and visible to the eye grew in the first stages of cooling before the magma erupted. This results in a fine grained rock studded with varying amounts of much larger crystal grains. In rocks like this the large crystals are called phenocrysts. Rocks with this texture are called porphyritic. Many of the rocks in the Snowy River Volcanics are therefore quartz – feldspar porphyries. Many of the rocks also have a high percentage of fragments of other rocks that were blown to pieces in the eruption. The relative amounts of these lithic fragments to crystal grains is often used as a way of distinguishing rocks of one eruptive event from those of another.
If you look closely at any rock fragment in this region you should be able to see crystal grains and rock fragments suspended in a mass of the fine grained material. Quartz crystals are usually clear and glassy, feldspar crystals are usually white and often have rectangular outlines. Lithic fragments look just like bits of rock (which may also contain crystals and other bits of rock) and are often a reddish or greenish colour. The fine grained material does not look glassy to the eye. The fine grained fragments of volcanic glass have started to chemically alter and turn into clay minerals. With the exception of the quartz, all the minerals in these rocks will eventually weather into clay minerals.
Log your find as soon as you can but please also answer the following questions, sending your answers to GeoGeckoEd (through profile above):
a) Find some local rocks at the viewing site and examine them carefully. Using the visual estimation guide provided, would you call this rock a crystal rich volcanic rock or a lithic rich volcanic rock?
b) Using the visual estimation guide provided, is this rock a quartz porphyry or a feldsapr porphyry?
b) Looking at the landscape of the gorge in front of you, what is it about the geology and geography of this gorge that makes it such a refuge for the rare and endangered Brushtailed Rock Wallaby? There are at least two or three possible answers. Maybe you can think of more but any answer must relate to this gorge. These rare wallabies don't live anywhere else in this part of Victoria, only this gorge.
Answering the questions ensures your log is preserved.