Geology
400 to 500 million years ago during the Ordovician and Silurian periods of geological time, the area that is now Victoria was a part of the ocean and had various types of sand and mud being deposited on it. Over the intervening millions of year these sand and mud stones have been compressed, metamorphosed (heated), deformed into folds on all scales and finally uplifted and eroded to form a rolling terrestrial landscape. This type of rock forms most of Melbourne's south and eastern suburbs and can be seen in many road cuttings throughout the Melbourne suburbs (some particularly prominent ones are along the Eastern Freeway).
About 5 million years ago, the whole area which is now covered with the northern suburbs of Melbourne, would have been gently rolling hills and valleys, similar to the eastern suburbs. Around 4.5 million years ago an extensive period of volcanic eruptions began over the whole of the western half of Victoria. The basaltic lava that erupted for the next roughly 4 million years was all very low viscosity (very runny) and slowly filled all the valleys and eventually formed the plains we see today. They extend from here, through Ballarat and as far west as Port Fairy and Portland.
The Victorian Newer Volcanic’s (as this lava deposit is known by geologists) is extremely hard. When the Eastern Freeway was being built in mid 70’s and later the Domain and Burnley tunnels were being constructed for the Monash Freeway, engineers were surprised at just how hard the Newer Volcanic deposits are. Drilling with a Alpine Miner AM 105 is usually done at an average speed of 1.5m/sec, this was slowed down to around 3mm/sec!! We also used the entire worlds supply of spare drilling teeth for these machines and new ones had to be manufactured!
The outcrop
This outcrop is an example of columnar basalt, which forms as lava cools from liquid to a solid. Columnar basalt outcrops are fairly common sites on basaltic lava planes and this one is significant in Melbourne and Victorian geology as it is near the very eastern edge of the Newer Volcanics deposit. Darebin Creek (~70 m to the east) forms the edge of this massive lava plane.
To log this cache:
ai) What is the height of the tallest column you can find? (bring a tape measure!)
aii) Look at the surrounding landscape, read the cache description and make a guess at how far into the ground the columns might go, including why you think this. (the number isn't the important part of this question, the thought process you went through to get it is.)
b) Examine the rock carefully and describe what you see, including any bubbles or crystals you can find.
c) Can you think of a reason for the bubbles being distributed in the rock the way they are?
d) OPTIONAL: Give a local and overseas example of this type of geological formation
e) OPTIONAL: Take a photo your GPS on top of the distinctively shaped rock. Include yourself if you want to.
If you insist on logging this cache before receiving permission to log, can you please make the log a NOTE. You can change it to a 'find' when permission has been received and your logs will still be in the right order. Thanks.
Other earthcaches on western lava plain include:
A Deep Volcanic Outlook
Niddrie Quarry
Coast and Reef
Organ Pipes
Rosette Rocks
Sediment Rocks
Logging Etiquette: Geocache hiders sometimes go through a great deal of planning to place their caches. As a result, they'd like to hear your feedback on whether you liked or disliked any aspect of the hide, or if you feel that some cache maintenance is required. Single word, acronym, or emoticon logs may be easier when you have a lot of caches to log, but it doesn't tell the hider or other finders anything about your adventure (or lack thereof) in finding the cache. Please keep this in mind when entering your log.
Don't forgot to log your attendance at:
