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Darebin Parklands
The unique nature of rock outcrops within Darebin Parklands makes it an excellent study site for amateur or professional geologists. The associations within the park are regionally common, but globally rare. Silurian rocks are not internationally prolific and Victoria hosts a significantly large unit beneath Melbourne’s eastern suburbs. Combined with this Silurian unit is an extensive unit of Tertiary basalt. The basalt’s eastern limit is the Darebin Creek and the Yarra River to the south. Briefly to the north, and moreover to the west, the Victorian flood plain basalt creates the gentle undulating topography of the plains of southwestern Victoria.
General Geology
The three main rock groups, Igneous, Sedimentary and Metamorphic are discussed below. You can think of these as families of rocks. Within each family there are many individuals.
Igneous
Igneous comes from the Latin ignis, or “fire”. Igneous rocks are new, born from within the earth. They comprise either fresh material from the mantle or reignited continental material that has been subducted and melted. Those igneous rocks produced at or above the Earth’s surface are extrusive (volcanic), those formed beneath the surface are intrusive (plutonic). They can also be a combination of the two. Creation of igneous rocks can be spectacular (e.g. by way of volcanic eruption or rapid continental rifting); or it can be rather passive (e.g. slow intrusions or oozing gas-rich eruptions) and is commonly the result of tectonic activity at or adjacent to plate boundaries. More than 95% of the Earth’s outer 50 kilometres consists of igneous rocks. Igneous rocks are defined by their mineral composition and their texture. Examples of igneous rocks are granite, rhyolite, andesite, basalt, diorite, gabbro, peridotite and komatiite
Sedimentary
When rocks break down due to the processes of weathering and erosion, sediment is created. Sedimentary rocks are formed from particles accumulated and consolidated on the earth’s surface after the rocks breakdown. Accumulation zones may be a lake, an ocean, floodplains, and quiet parts of a river or stream, desert dunes or even glacial fallout. Sedimentary rocks are named based on the size and relationship of the particles within them. Features of the rocks give clues as to the environment of deposition, such as different organisms present at the time, waves, seasonal aridity, wind and current directions, tidal flows, major climate change, Examples of Sedimentary rocks include sandstone, limestone, mudstone, siltstone, greywacke, conglomerate and breccia.
Metamorphic
Created when pre-existing sedimentary or igneous rocks are subjected to heat and/or pressure, metamorphic rocks will often retain many of characteristics of the original rock. They are generally more complicated than the other two rock types and can be quite spectacular. Metamorphic features can reveal things about rocks such as whether they have been buried, been in contact with volcanic products, or even been flushed with hot fluids.
Darebin Parkland - Silurian Outcrop's 1 & 2
There are several significant outcrops of sedimentary rock throughout the park. This rock is of the Silurian period. In ancient seas, 430 million years ago, fine mud and sand sediments were laid down horizontally and slowly developed into rock. Eventually pressures under the earth pushed these rocks to the surface resulting in the dramatically different landform on the Ivanhoe side, as compared to the volcanic Alphington side. The darker layers are made up of iron oxide, with the layers being like tree rings in allowing geologists to age the rock. Silurian rock produces excellent caves, with several small examples in the large escarpment further down stream. The Wurundjeri people occasionally used these caves for shelter when passing through this area.
Silurian Sandstone
The Silurian sandstone is a Sedimentary Rock. These rocks consist of particles of variable size bonded together and hardened to form a solid rock. This rock unit was produced after a deep ocean accumulated fine sediments over a very long period. The layers alternate between sand and silt sized particle. Each layer represents a period or event of deposition.
There are no fossils known to exist within the Silurian rocks of Darebin Parklands. Within the same unit further to the north, huge populations of Brachipoda appear in intermittent layers. They have all died together most likely buried by a sudden large influx of sediment. Their size is small and the shells articulated.
Logging requirement:
To Log this EarthCache, please send an email with answers to the following questions to our Geocaching.com account.
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Your caching name and the EarthCache name
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You will be standing on one of the outcrop's, what is the approximate width of each layer of rock.
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Looking approximately 40 metres upstream (North), you will see the second outcrop, unlike the area you are standing on, it has what appears to be one large flat area, wht is the approximate Height and width of the flat component of the outcrop?
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If you like include a photo of yourself at GZ, but please don’t include a picture of any of the questions as we will need to delete the photo.
Sources:-
http://www.dcmc.org.au/parklands/geology.php