Plenty Gorge Park lies broadly along the dividing line between two distinct landscape types. The western side is characterised by a flat basalt plain formed initially over 400 million years ago, and more recently (a million years ago) experiencing a further lava flow.
The newer volcanic period filled the lower part of the ancient Plenty River valley with basalt lava, disrupting, diverting and reorganising the entire drainage of the area. The western side of the park is completely underlain by basalt. The eastern side of the park consists of the undulating hills and ridges formed during the Silurian period, occurring between 400 and 430 million years ago.
Sediments of sandstone and shale form the soil types visible on the eastern side of the Plenty River and in the cliff faces of the Plenty Gorge. Tectonic plate movements have folded the landscape, which is demonstrated in the gorge and the undulating nature of the park.
Basalt, which is colloquially called bluestone in Victoria, is a hard, dark extrusive igneous rock. Extrusive means it comes mainly from volcanic eruptions. The nearby Blue Lake was originally a quarry for extracting basalt (bluestone).
Sandstone and Shale are fine grained sedimentary rocks composed of layers of clay or mud and tiny fragments of other minerals. From GZ, if you walk down the path for about 100 meters you will see numerous examples of these types of rock. You may also observe that the layers which were originally horizontal are now vertical strata indicating that the landscape has been folded in the past.
This area was once the site of major quarrying activity. Basalt was extracted from the hole which is now Blue Lake. Most of this Basalt was crushed into gravel in the vicinity of what is now the car parks and toilet facilities.
WARNING – Please do not attempt to approach GZ by going cross country from the car park. There are dangerous cliffs in the area. There is a proper path around to GZ.
From the path at GZ if you look up about half way up the steep cliff you will see a prominent rock beside a tree. You do not need to climb up to it. Observations made from the path can satisfy the conditions for claiming this geocache.
You may log your find immediately but to complete the process you must answer the following questions and email them to me, Ron.haha.
Q1. What is the approximate shape, size and colour of this rock, and therefore, what type of rock do you think it is?
Q2. What is your opinion of how the rock got to be where it is?
I will not be overly pedantic about the accuracy of your answers but your answers should demonstrate that you have read the description, visited the site and given some thought to your answers.
I will delete your log if I have not received an email within 7 days of you logging the find or if you post the answers or a photo of the rock in your log.
Congratulations to Long John Silva for the FTF.