Geology and history of the Dog Rocks area:
Dog Rocks outcrop is an intrusion of Devonian Granite, estimated at 350 million years of age. GZ is pretty close to the highest point of the outcrop, with my GPS saying I was at 101 metres ASL
In geology, "intrusion" means that over millions of years, magma, which is molten rock in the core of the earth, has been pushed upwards from below the surface, forming an igneous rock body and as it heats the rock through which it travels, these can change in their nature, becoming metamorphic rock. Intrusions can be quite small, like the Dog Rocks outcrop, or they can be mountain ranges like the nearby You Yangs. Intrusive bodies are formed as molten rock pushes its way upwards through the crust, filling cracks and fissures in the overlying "country rock", but never reaching the surface. In the case of both the You Yangs and Dog Rocks, these overlying deposits were soft sedimentary rocks, which have since weathered away, leaving the harder igneous rock exposed.
This outcrop is granite, identified by Victorian Resources Online as Dog Rocks Granite. It has a characteristic colour and is coarsely porphyritic. This means that the larger mineral grains, known as phenocrysts, are much coarser than the surrounding finer mineral grain, which is known as groundmass. The larger phenocrysts are formed deep underground as the rising molten rock is insulated by the mass of overlying crust and cools slowly, but as it forces its way closer to the surface, the rate of cooling increases and the finer crystals of the groundmass form. The colour of granite is primarily dictated by the groundmass and depending on the minerals in the area, this can vary widely.
This material is similar in lithology (physical makeup and characteristics) to the nearby You Yangs and is of a similar age.
The surface of the Dog Rocks outcrop also includes small boulders of Cambrian greenstone (lepidolite), which has been intruded by the granite. This outcrop is in one of the 6 Cambrian Greenstone belts identified in Victoria.
Greenstone was frequently used by indigenous people as axe heads. Thought to be among the oldest rocks in Victoria, this dense mineral is hard and holds an edge well, so it was regarded as a valuable commodity and it was widely traded throughout the South East of Australia. It was quarried here, as well as several other sites throughout Victoria, including Mt William near Lancefield, where a very large quarry has been found. As described below, few traces of these quarries now remain at Dog Rocks.
The height of this outcrop meant that during the Miocene era (23 million - 5 million years ago), when the surrounding area was covered by the sea which laid down the surrounding limestone plains, Dog Rocks stood out above sea level as an island.
This surrounding Miocene limestone forms the basis of the historic limestone and cement industries in the nearby Batesford and Fyansford areas. Some activity continues today, although the largest nearby is now down near the Anglesea Road at Warun Ponds. The remains of more recently abandoned operations can be clearly seen driving around the area. Depending on how you approached GZ, you may well have passed a limestone quarry, active or disused cement works, or the remains of the overland conveyor.
During the early pastoral years of this district, well-meaning farmers covered much of the outcrop with overburden removed from the Batesford limestone quarry. Overburden is the dirt which sitting above the mineral of interest in an open-cut mine. As you can imagine, this significantly reduced the amount of visible granite and it has also obliterated many of the remains of the greenstone quarries. It is thought that had the outcrop not been covered, it would more closely resemble the You Yangs, with sparse soil among the granite boulders supporting a dry bushland, rather than the rolling green pasture you see today. The sanctuary across the road gives a bit of a hint as to how it may have been, as this was less cultivated during those years, having been set aside in the 1850's as a reserve.
The view to the north, with the You-Yangs in the background, shows the Miocene limestone country and the escarpments carved by the nearby Moorabool river. Across the road from GZ, the area is a wildlife and flora sanctuary, established in 1856, when the current owner's grandfather purchased the property.
There is plenty of room at GZ, so please ensure that you pull completely off the road as the outcrop has apparently lost its novelty or beauty for some of the locals and they pass by at a reasonable pace !
To claim this cache:
To claim this cache, you must send me an email, with two pieces of information, both of which you can find by examining the boulder at GZ and consulting the cache notes.
Firstly, what is the background colour of the Dog Rocks granite and what do you think causes it to be that colour, or colours ? Don't be fooled by the lichens and mosses, you need to check the exposed areas of rock.
Secondly, on the northern side of this boulder, you will see a particular feature in the rock itself. Please describe this feature and how it differs from the surrounding rock. Why do you think it differs from the surrounding rock ? Note that depending on how long the surrounding grass is, you may have to push it aside to see the feature.
Feel free to log your find without waiting for my answer, but note, I will also have to delete the log if no email contact is made within 7 days of your log. If you would like a reply, please ensure you send your email address along with the message.
This area is very popular with the local photographers and although it's not a requirement, photos of the area are encouraged, so we can enjoy these rocks in a variety of seasons and weathers. Please don't post photos which show the feature in the rock at GZ though, or I will reluctantly have to delete the log.
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