Located along the Gorge Trail within Magpie Creek is a tillite specimen. The rock face has been subject to large amounts of erosion, which has carved a steep slope from the top of the specimen down towards the Sturt River/Warriparri. It is known as "The First Waterfall".
Amongst the rock are several clasts that have been embedded. A clast is a fragment of mineral, which come in various sizes and colours.
Most clasts that are smaller than a grain of sand (<1/16 mm) are derived from minerals made of clay. Most clasts that are larger than a grain of sand (>2 mm) are actual fragments of rock. These fragments are commonly made up of fine-grained rock like basalt or andesite.
If the clasts are larger still, they are most likely made of coarse-grained rock such as granite, quartzite or gneiss.
There are two types of clasts at GZ and are made of quartzite and granite.
(Source: https://opentextbc.ca/physicalgeologyh5p/chapter/what-is-a-rock/)
The clasts in the tillite specimen are round in shape, indicating that they were embedded in the stone during the flow from a glacial river. This type of river has a source of water which melts from a glacier.
514 million years ago during the Delamerian Orogeny (a time in which Australia experienced folding and uplift), the clasts were subject to change due to the geological movements. The most common change during this time was the clasts splitting into several parts.
Quartzite and granite are very different rocks. This means that the clasts originated from very different places. Also, the rocks which the clasts originated from are much older then its current host.
The rock surface has multiple layers, when answering the EarthCache questions you will only need to focus on the two lower layers.
Logging Requirements:
Walk down to GZ and answer the following questions:
Step up onto the first level and observe the clasts at your feet.
Q1. What are the rough size of the clasts?
Q2. What type of rock specimen is located here? (Quartzite or Granite). Explain why gave this answer.
Q3. Do these clasts show evidence of splitting during the uplift and folding of the Delamerian Orogeny time period? If yes, describe the evidence to back up the answer you gave.
Now step up onto the second layer. Observe the singular large semi-circle shaped clast at S35 02.254 E138 35.220, it is on the right hand side of the rock when facing AWAY from the river behind you. It is beneath the overhanging rocks above you.
Q4. What is the rough size of the clast?
Q5. What type of rock specimen is located here? (Quartzite or Granite). Explain why gave this answer.
Q6. Does this clast show evidence of splitting during the uplift and folding of the Delamerian Orogeny time period? If yes, describe the evidence to back up the answer you gave.
Based on all information you have so far, answer this last question:
Q7. Which rock (Quartzite or Granite) do you think was the more resistant to the uplift? Why?
Q8. Take a photo of you, your stamp or personal item with the area around you (Please don't include any spoilers).
Please send answers via the Geocaching message centre or to my email, answers posted in the logs will be deleted.
You can log the find immediately, if answers are supplied to CO within 14 days.
Getting to GZ:
I have included several waypoints with the best spots for parking, entry to the trail and the descent to GZ.
Please be careful as loose rocks and steep descents are present on the way down.
Please also be wary of snakes! I encountered a brown snake on the way down the trail when preparing this EarthCache.
I do hope you enjoy the EarthCache lesson I have prepared and appreciate the area!
I have reached the GSA's highest level:
