The Kaniyang Wiilman bridge was built for trail users to cross the Collie River and walk the Wiilman Bilya trail. This Earthcache can be easily accessed by taking a walk along the Wiilman Bilya trail to a small waterfall.
Earthcache Lesson
Geology is amazing. Changes in rocktypes have the ability to affect modern day landscapes. Think of mountain ranges, cliff faces, coastal headlands and even waterfalls. The underlying geology has an influence in all of those features including this waterfall on the Wiilman Bilya trail.
The rocks along the trail belong to the Yilgarn Craton, a very old, relatively flat and stable land mass consisting of granites and greenstones, felsic and mafic rocks, respectively. As you walk the trail most of the rocks are granites or granite gneiss. These rock are formed ~5km in the Earth’s crust, enough to melt fully or partially melt the crust. Can you imagine the pressure these rocks would feel being buried so deeply.
In the image below is a dark coloured dolerite (mafic rock) cross cutting an older pink granite (felsic rock).
The key to this Earthcache lesson is to look at the constituent minerals of the rocks at GZ and determine what they are. To do this you need to understand the difference between the rock types here.
Mafic versus Felsic
Mafic is a term to describe rocks that are lower in silica and higher in iron and magnesium and the minerals are darker in colour. The dolerite dyke in the image above is mafic. They can be grey, green and black and many shades of these colours, though they do have lesser amounts of light coloured minerals.
Felsic is used for light coloured rocks, high in silica and low in iron. Examples of a felsic rocks are granite, a very common rock type. Pink, beige, white and pale grey are typical felsic rock colours. The granite in the image above is orange pink in colour and is a felsic rock.
Most of the rocks along the walk are granitic, a felsic rock rich in quartz and feldspar.
Earthcache questions
Look at the photo within the photo gallery. The rock circled holds the answers to the following questions.
Q1. Look at the photo attached and find the rock circled. Describe the two dominant colours of the minerals present. Based on these colours, determine if the rock is mafic or felsic.
Q2. Do you think this rock is more resistant than the surrounding rocks? If so, does it affect the waterfall’s position?
Q3. Well a task really, please take a photo of you (not just a member of your group) or an object belonging to you with the water in the background.
Please log the cache straight away. You can submit the answers to the questions within a few days but logs without answers will be discreetly deleted within 14 days.
The cache was placed for WA goes BIG in 2022 hosted by WestOz Geocaching over the June long weekend.
