This little known (other than to locals) scenic location is close to the little town of Metcalfe, not far from Castlemaine and about 40 minutes from both Bendigo and Daylesford. A layer of bedrock provides an interesting canvas for the Coliban River and contains rock pools, crevices, and other interesting features.
It's a perfect location for a picnic, where you can walk along the river, over the rocks, and just enjoy the beauty of the stunning features.
The Geology
The waters of the Coliban emerge at elevations of over 600 metres in the southern part of the Great Dividing Range. The high catchment area can have rainfall of over 1 metre annually, and so the flow down the Coliban, and the nearby but larger Campaspie, can be quite substantial.
The river flows down across the volcanic plains of north-western Victoria, and here at the Cascades, it flows over a substantial area of exposed granite. This gives us a perfect opportunity to see how time and water can make quite remarkable features in the hard rock.
North of here, there are rolling foothills of granitic soils, where granite and sandstone have been broken down into small particles over millions of years, and this abrasive material is picked up by the rivers as sediment.
Water Erosion
In a rock river bed, water can make erosive inroads in a number of ways. Here the water has come down through significant sedimentary layers as discussed above and can be rich in minute abrasive particulate matter. This adds to the erosive power of the water, and the effect is suffered by this exposed granite in this section of the river bed. The hills around the feature give the water some velocity as it enters this section, adding to the effect. Sediment loaded water, with a rapid rate of flow, can be quite a potent force in the landscape.
Rill erosion occurs when runoff water forms small channels as it concentrates down a slope. These rills can be up to 0.3m deep. If they become any deeper than 0.3m then the result is termed Gully erosion.
Potholes are vertical, more or less cylindrical holes drilled into the river bed by swirling sediment-filled water and often enlarged by the circular action of trapped pebbles.
If the potholes are deep, further erosion can turn them into all sorts of weird and wonderful three-dimensional shapes and structures.
A braided channel is a type of channel that is divided into smaller sub-channels by small, temporary islands called eyots. Braided channels develop in rivers with a lot of sedimentary loads, a steep gradient and where the discharge of the river changes regularly. When the volume of the load exceeds the river’s capacity or the discharge of the river drops, the river is forced to deposit its load in the channel and islands of sediment (eyots) form.
Parallel cuts.
When the river level fluctuates a lot, separate streams develop and cut their own channels, side by side, through the bedrock. If the flow contains sufficient sediment, sometimes the current cannot carry it all and this contributes to the braided channel-like structure.
An unrelated feature is a Dyke, which occurs when a or sheet-like body of magma cuts vertically or almost vertically through and across strata, though some dykes are steeply inclined.
They are formed when molten rock of one type or other is injected at high pressure in between earlier flows of magma which have cooled or are in the process of cooling. They form a layer of mineralised rock sandwiched between the existing layers of rock forming a band of stripe-like formation, often of a greatly contrasting colour, which has a striking appearance. If you'd like to know more about dykes, I have an earthcache at Stanthorpe in Queensland featuring a fantastic dyke.
You may log your find here at the Cascades Earthcache immediately, but for it to stand, you must send me your answers to these questions within10 to 14 days of your visit, and post the required photo. You can contact me via your profile, or at earthcaches@jamieson.id.au which might be seen quicker.
Q1. At the listed location, can you see the dyke at or near your feet, or closeby? Describe it's nature, and whether you can see it emerge on the other side of the channel?
Q2 A few metres upstream of the listed coordinates, you will find some potholes and at least one of them is shaped like a figure eight. How do you think that happened?
Q3. Please upload a photo of yourself, or some personal object, at some other area of the cascades, and upload it with your on-line log.