Bridge over Snowy Waters EarthCache

This cache is north of Buchan. The waypoint for Buchan Caves is 37^o 29.822S 148^o 10.362E. Once you get to Buchan, continue north up the Buchan–Gelantipy Road until you pass Seldom Seen. Take the right fork in the road, after Seldom Seen, to Little River Falls and McKillops Bridge. To find the cache, continue past Little River Falls until you reach a hair-pin bend in the road at the aptly named Turnback Track. Stop here and take in the view of the Snowy River Valley. Also examine the descending road. If you and your passengers feel up to the next part of the drive continue down this road until you reach McKillops Bridge. An alternative route to McKillops Bridge via Orbost, Bonang and Tubbut involves slightly less white knuckle driving.

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.

McKillops Bridge is only one of three bridges across the iconic Snowy River, not including the spillway wall road on the Jindabyne Dam as a bridge. This amazing bridge was built before WWII and is a tribute to the engineering skills and tenacity of those involved for more reasons than one. Prior to the damming of the mighty Snowy River as part of the Snowy Mountains Hydroelectricity Scheme the river ran deep and wild. A stock route crossed the river at this point and a barge helped travellers make the perilous crossing for a number of years. Take a moment to consider how much water must have been in this valley during flood events that saw the waters lapping the decking timbers of this bridge.

The rocks here are granodiorite. They formed from a massive intrusion of magma into the other colder rocks. If you look carefully you will see lots of fragments of other dark coloured rocks, unmelted, trapped in the light coloured rock. These are bits and pieces of the rocks that the magma pushed its way into. They are called xenoliths. This rock pre-dates the Snowy River Volcanics which are described in the 'Gorgeous' Earthcache.

This Earthcache has one waypoint but two spatial locations. The first location gives a birds eye view. From this vantage point, looking south, the rocks are well exposed in the drier parts of the river bed immediately below. The exposed rocks are mostly a pale grey coloured granodiorite. However, another light coloured rock seems to form a stripe across these grey rocks. If you look carefully you will see that together with the large obvious stripe there are numerous smaller ones. This is special type of igneous intrusive rock called a dyke. [Dike is the American spelling].

A dyke is a thin sheet-like intrusion that cuts across the layers and/or other structures in the intruded rocks. Dykes usually result from magma being pushed into thin fractures in the rock. Low viscosity magma that flows easily may do this more often but dykes of all compositions are known. Dykes often happen when the magma from the main intrusion has begun to cool and the remaining magma is enriched in materials not yet crystallised. The result is a dyke of a related but different composition to the intruded rocks. Often this also means the rock will be a distinctly different colour to the host rocks.

The second spatial location for this waypoint is on the rocks themselves. It is suggested you park your vehicle at the visitor hut on the east side of the river [37^o 04.940S 148^o 24.843E] and walk back across the bridge to the west side before descending to the valley floor. At the visitors hut you will also find some neat displays that may help you log your visit.

Down at river level you can see how the granitic rocks are intruded by the dyke. The dyke has many small off shoots that seem to snake out into the surrounding rock. This indicates that the fracture wasn't a simple plain break in the rocks but a set of complex fractures rupturing the rock more chaotically, albeit trending in a general direction.

Most dyke rocks cool relatively quickly and therefore tend to have very small crystals despite being igneous intrusive rocks. Thicker dykes may have enough heat to keep their interior magma warm for longer, producing a chilled margin of smaller crystals with larger crystals towards the middle. Some dykes also develop flow structures as the magma in the interior keeps moving while the cooler margin material solidifies. Very large dykes may also contain enough heat to metamorphically alter the adjacent host rocks.

There are some other cool features in the granodiorite, evident in the outcrops just below the bridge. The rocks exposed here are beautifully sculpted by the abrasive action of the river. Swirl pools contain pebbles that continue their erosive action every time the river rises and its turbulent waters make the pebbles spin. Look carefully at the nice clean rock surfaces and you will also see rock fragments entombed in the magma before it cooled. The dyke is also broken and off-set in several places indicating that it too has been broken by small fault movements after it had cooled. If you look very carefully you may also notice the dyke rocks and those just next to it stick up a little from the surrounding rocks. This is because this rock is a tiny bit harder than the others and resists erosion a little longer. Differential erosion like this is common with dyke rocks and often forms spectacular ridges in the landscape.

Note on the road: The road from Little River Falls to McKillops Bridge and from McKillops Bridge to Bonang is an all weather gravel road. It is suitable for 2-wheel drive vehicles during dry weather although vehicles with very low clearance and/or low profile tyres may bottom out on rougher sections. In several sections the road is only a single vehicle wide with vertical rock walls on one side and long steep to vertical drops on the other side. It is unsuitable for articulated trucks or caravans. This road traverses one of the most remote areas in Victoria and breakdowns on it may result in the necessity to camp out. Mobile phones do NOT work in this region and there are no public or private phones within easy walk. Traffic volumes on this road can be very low, especially in the cooler months during weekdays. Ensure your vehicle is roadworthy and you are prepared for breakdowns or other delays, such as tree or rock falls blocking the road, before attempting this journey.

To log your visit to this site you need to send photos of your visit and answer the following questions to GeoGeckoEd (through profile above):

a) What was to be the official opening date of the first McKillops Bridge and how many days before the opening was it washed away?
b) What is the general trend of the main dyke exposed at this point [take a compass bearing if you can]?
c) Is the dyke exposed on the east side of the Snowy River?
d) What colour were your knuckles when you arrived at McKillops Bridge? Were they anything like the colour of the dyke?

Once verified, you can log your visit, but you must include the number of people in your group who visited this Earthcache.