Welcome to the Organ Pipes National Park.
The Organ Pipes National Park, abbreviated as OPNP,is a national park located in the Central region of Victoria, Australia. The 121-hectare (300-acre) protected area was established with the focus on conservation of the native flora and fauna, and preservation of the geological features in the Jacksons Creek, a part of the Maribyrong valley, north-west of Melbourne. It is situated in a deep gorge in the grassy, basalt Keilor Plains.
The Keilor Plains, where the geomorphologic and geological features of heritage value are located, is part of the greater Western Volcanic Plains which cover a large part of western Victoria and extend into south-eastern South Australia and are stated to be the third largest lava plains in the world. These landforms were created by the incision of the Jacksons Creek into think lava flows that erupted about 2.5 to 2.8 million years ago during the Pleistocene Period. These lavas overlie steeply dipping Silurian sedimentary rocks.
Sedimentary rock and fossils found in the park are also dated back 400 million years, an indication that the area was once a sea. The rocks seen in the park have mostly dark grey or brown colour. Graptolites are seen as fossils of seas snails, sea worms and extinct floating animals in the rocks here. About 2.5 to 2.8 million years ago giant lava flows known as a flood basalt, spread across the land from surrounding volcanoes.
Rock exposures of Silurian mudstones and jointed structural features of basalt are spread at many locations in the form of Basalt cliffs, boulder screes and sedimentary escarpments. The geological features in the Park, all of volcanic origin, have been categorized under the broad heads namely, the Organ Pipes, Tessellated Pavement, Rosette Rock, Scoria cone and the Sandstone layer
These basalt stones were formed in the same way that the formation known as the Organ Pipes were, but have been worn down from the top by the action of the creek flowing over them.
By definition, basalt is an aphanitic igneous rock with less than 20% quartz and less than 10% feldspathoid by volume, and where at least 65% of the feldspar is in the form of plagioclase. Basalt features a glassy matrix interspersed with minerals. The average density is 3.0 gm/cm3.
Basalt is defined by its mineral content and texture, and physical descriptions without mineralogical context may be unreliable in some circumstances. Basalt is usually grey to black in colour, but rapidly weathers to brown or rust-red due to oxidation of its mafic (iron-rich) minerals into rust. Although usually characterized as "dark", basaltic rocks exhibit a wide range of shading due to regional geochemical processes. Due to weathering or high concentrations of plagioclase, some basalts are quite light coloured, superficially resembling ryholite to untrained eyes. Basalt has a fine grained mineral texture due to the molten rock cooling too quickly for large mineral crystals to grow, although it is often porphytitic, containing the larger crystals formed prior to the extrusion that brought the lava to the surface, embedded in a finer-grained matrix.
In geology and geomorphology, a tessellated pavement is a relatively flat rock surface that is subdivided into more or less regular rectangles, blocks approaching rectangles, or irregular or regular polygons by fractures, frequently systematic joints, within the rock. This type of rock pavement bears this name because it is fractured into blocks that resemble tiles of a mosaic floor, or tessellations.
Please feel free to log your find before sending answers to keep logs up to date. BUT answers and photo must be sent with in 7 days or your log may be deleted.
Q1 The name of this Earth Cache.
Q2 Look at the formation at GZ and describe it's grain size (fine, medium, large) and tell us in your own words why you think they are this size.
There are four types of tessellated pavements and they are; pavements formed by jointing; tessellated pavements formed by cooling contraction; tessellations formed by mud cracking and lithification; and tessellated sandstone pavements of uncertain origin.
Q3 What type of pavement do you think this is, and why do you think so?
Q4 Please take a picture of yourself or something with your team name written on it, even just in the sand. Please do not give anything away in your picture. (Post the picture with your log do not send with your answers.)
Most of all Enjoy.