This cache is SirJ-Oz's eighteenth cache. An Earthcache with 4 questions to answer.
Anniversary Rocks
The area indicated by the coordinates will put you on the Anniversary Outer Circle Trail, but that is not why you are here. You are here to view part of the geological feature known as the Anderson Creek Formation.
Please take care while you are taking your observations, depending on the time of day there will be passing cyclists, joggers and pedestrian traffic on the path.
The bedrock of the Melbourne district is of Middle Palaeozoic age belonging to the Silurian and Devonian periods (354-441 million years ago). However, these rocks are overlain in wide areas by much younger rocks, mostly of Tertiary and Quaternary age (2-65 million years ago). The oldest rocks of the bedrock are Early Silurian in age (441 million years ago). They outcrop on the west side of Melbourne in the Maribyrnong Valley around Keilor where they are called the Springfield Formation, and in the north-east around Warrandyte, where they are known as the Anderson Creek Formation. Fossils are uncommon in these formations. Younger rocks are found east of Keilor and south of Warrandyte, ranging in age from Late Silurian (418 million years ago) in the city area (the Melbourne Formation) to Early Devonian (384 million years ago) at Lilydale (the Humevale Formation).
The Anderson Creek Formation consists of massive Upper Silurian siltstones interbedded with thin sandstones and occasional groupings of massive, laminated and current bedded greywackes, conglomerates and clast beds. The Formation forms a semi confined fractured rock aquifer system. Observations have shown it to be massive and to have a low primary porosity and permeability, however jointing and fracturing have imparted secondary porosity and permeability. The storage capacity of the Anderson Creek Formation and its ability to act as a conduit for the passage of groundwater is dependent on the frequency and degree of interconnection of joints and fractures, which generally are unpredictable.
If you look towards the South-East of the posted coordinates, you will see an exposed rock face. These rocks were formed during the Lower Silurian age about 430million years old. Belonging to the Anderson Creek Formation. Soil that develops from weathering of these rocks is mostly light grey clay that is nutrient poor and generally rubbish for growing stuff in. The exposure of this rockface is due to the cutting made for the Outer Circle Railway.
Post your answers to SirJ-Oz
Questions
Q1. Include your caching name and the name of the cache.
Class of Rock
There are three main classes of rocks:
Igneous
Rocks that are formed when molten magma or lava from a volcano cools and solidifies.
Sedimentary
Rocks that are formed when layers of sediment (sand, pebbles) are compacted.
Metamorphic
Rocks that are formed when rock layers are subjected to heat and pressure that causes a change in the structure of the rock (formation of crystals).
Q2. What class of rock do you think this is (and why do you think that)?
Layers and Tilt
Layers are generally originally laid down horizontally, and often became tilted due to tectonic activity.
Q3. Looking to the southern end of the exposed rockface, what is the approximate angle (from horizontal) of the tilt?
Colour
Colour is one of the first noticed and most obvious characteristics of a rock, but it is also one of the most difficult to interpret. With the exception of grey and black, which mostly results from partially decayed organic matter, most rock colours are the result of iron staining. Ferric iron (Fe+3) produces red, purple, and yellow colours (from minerals like hematite and limonite). Ferrous iron (Fe+2) produces greenish colours.
Some broad interpretations may be made from a rock's colour. Red colours mean well oxygenated environments, such as river channels, some flood plains, and very shallow marine. Green colours mean an environment low in, or lacking, oxygen, often associated with marine environments. Dark grey to black colours mean anoxic conditions, which may mean deep water, but could also be a swamp environment. The conclusion is, environmental interpretations can only be made in relation to the other evidence present with the rock.
When and how these colours originate in sedimentary rocks has been in long debate but many of the colours are digenetic. Thus, environmental interpretations from colour must always be viewed suspiciously.
Q4. Based on the colour of the rock, what inferences can you make about its composition?
Once you complete the EarthCache requirements you can post your find without delay, as per the EarthCache guidelines. You will also need to verify your find by sending me a message and provide your answers to the questions. Logs from teams that have not provided answers in 1 week, will be removed.
Please don't post any of the answers (or hints to the answers) in your on-line log
Sources:
http://www.earthlearningidea.com
https://apps.epa.vic.gov.au/EnvAuditFiles/53X/45567-1/45567-1_a.pdf
http://www.emelbourne.net.au/biogs/EM00636b.htm
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