The 12 Days of Cachemas (TDOC23) is a series of 78 caches, events and adventure labs placed across Victoria to celebrate Cachemas in July in 2023. The series is a joint initiative of Geocaching Victoria and the local community to release caches in a different category each day across the state. We hope you have as much fun finding this series, as we had placing them. Now on to the cache.
On the second day of Cachemas, Signal gave to me,
two earthcaches,
and a wherigo up in a tree.
Logging Task:
Please send your answers to me via the Geocaching Message Centre (preferred) or via here. Unless your answers are received within 10-14 days from logging, your Find log may be deleted. Make sure that you read the information contained here to assist in your understanding of the geologic lesson.
Standing at GZ, you will see a granite boulder. We know it is granite because of its Phaneritic texture and the minerals seen within it.
1. Describe this boulder, include details like size, colour and three minerals you can identify.
2. Is the boulder similar to other bedrocks in this area? If it is where are they? If it is not, how did it get here and what is it called?
3. Please include a photo with some personal item or yourself if you wish in your log looking toward the lake at the grid you passed as you entered the O'Keefe Rail Trail from the trail you walked down.
The story so far:
Lake Eppalock in Central Victoria has an interesting geological history.
Millions of years ago an extensive glacier once carved its way in a northerly direction across Victoria, with only a few outcrops surviving millions of years of erosional processes.
A glacier is essentially an extremely powerful frozen river of ice. As it moves over a landscape it erodes channels and valleys, literally tearing rocks from the bedrock and transporting the rocks long distances. When the ice melts, the boulders are left in places where you least expect them. The stranded rocks are called glacial erratics. Glacial erratics are recognised because they're large chunks of bedrock that are different from the bedrock they're deposited on. Here is a tip for you, if you encounter a large boulder that looks like it has fallen out of the sky (for example, there are no rocky cliffs nearby), you've probably found a glacial erratic.
Lake Eppalock is home to it’s very own glacially excavated valley with a well-preserved glacial sequence in a N-S belt about 38 km long and 8 km wide. The north shore of Lake Eppalock at Moorabee has a continuous outcrop over 1 km long including glacial pavements, erratics transported from distances hundreds of kilometres away and tillites.
During the Permian (299-251Ma) Victoria was located at latitudes further South, nearer to the South Pole, attached to a giant landmass called Gondwana. Gondwana itself was one of two land masses that formed the super continent Pangaea. The early Permian was a time of glaciation, with giant ice-sheets covering the southern-most parts of Gondwana, including Africa, America and Australia.
There are two main types of glaciers, small valley glaciers like that found in mountainous regions like New Zealand and continent-scale ice sheets like that in Antarctica. It is not yet understood which type moved across Victoria.
There are many glacial deposit types which can be seen along the shores of Lake Eppalock, however they are only part of the story.
As glaciers advance, they scour and pick-up (erode) rocks and transport them either along the base of the glacier, creating smooth erosional surfaces on the rock beneath by abrasion, or along the top surface of the glacier. When the glaciers retreat a number of deposit types form, including tillites and diamictites. A diamictite is a poorly sorted sedimentary rock. An example of a diamictite can be seen at GZ.
There is a long period of erosion after the Permian during which most Permian rocks disappear. Newer Volcanic Lava Flows millions of years younger uncomfortably overlie both the Ordovician Castlemaine Supergroup and Permian Derrinal Formation. The flows are isolated in a north-south trend cutting straight through the guts of the lake, following the important metallogenic Fosterville Fault. A great exposure of this rock unit is located at the Spillway.
Glossary:
Phaneritic - This texture describes a rock with large, easily visible, interlocking crystals of several minerals. The crystals are randomly distributed and not aligned in any consistent direction. A phaneritic texture is developed by the slow cooling and crystallisation of magma trapped within the Earth's crust and is characteristic of plutonic rocks.
Glacier - is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries.
Tillite - sedimentary rock composed of compacted glacial till.
Diamictite - is a type of lithified sedimentary rock that consists of nonsorted to poorly sorted terrigenous sediment containing particles that range in size from clay to boulders, suspended in a matrix.
Glacial Erratic - is glacially deposited rock differing from the type of rock native to the area in which it rests. Erratics, which take their name from the Latin word errare ("to wander"), are carried by glacial ice, often over distances of hundreds of kilometres.
Granite is primarily composed of three main minerals: feldspar, quartz, and mica. These minerals, along with some smaller amounts of other minerals, combine to give granite its characteristic appearance and properties.
Feldspar is the most abundant mineral in granite and can be found in different forms, such as orthoclase, plagioclase, or microcline. It contributes to the overall color of granite and can range from pink, white, or gray.
Quartz is another important mineral in granite. It is usually clear or translucent and gives granite its hardness and resistance to weathering and erosion.
Mica, specifically biotite and muscovite, is the third major mineral in granite. Mica adds a shimmering effect to the rock and can be seen as dark-colored flakes or layers within the granite.
In addition to these primary minerals, other minor minerals like amphibole, pyroxene, and sometimes even trace amounts of minerals like garnet or zircon can also be present in granite, although their presence is less common and typically less noticeable compared to the primary minerals.
Bibliography
Geology for Dummies 1st Ed, Alecia M. Spooner.
The Earth Science Teacher Vol 2.4 Dec 17 2022, Gary Lewis
Lake Eppalock (weekendgeology.com)