Earthcache located east of Carnarvon in the Shire of Upper Gascoyne
How to get here: Approximately 40 kilometres east of Carnarvon along the Carnarvon-Mullewa Road to Gascoyne Junction you will encounter a sign to turn left to Rocky Pool. The road from Carnarvon to the turn off is sealed except for a couple of kilometres at the junction of North West Coastal Highway.
Background Information
As a permanent water-hole on the Gascoyne river, Rocky Pool has become a popular picnic spot in the last 50 years. Over the centuries, countless birds and animals have come here to drink or to shelter in the cool shade close to the water.
Pools such as this provide food and habitat for native plants and animals. They also help recharge the groundwater basins that are essential to the horticulture industry around Carnarvon.
Local Aborigines too, would have known of this place long before the coming of Europeans. Then, as drovers, shepherds and squatters began to move out into the rangelands with their herds this would have been an important watering point for thirsty stock and men alike. Kingsford Smith may have boiled the billy here on many occasions, on his mail run to Bangemall.
Geology
Rocky Pool lies within the Gascoyne Sub-Basin of the Carnarvon Sedimentary Basin. The Sub-Basin contains sedimentary rocks of Palaeozoic, Mesozoic and Cainozoic age separated by regional unconformities.
Rocky Pool lies between outcrops of lateritized Toolonga Calcilutite (lime mud-stone) of Mesozoic age. The river channel, which averages 600m in width, is reduced to 75m between these outcrops. These rocks extend underneath the channel forming an impermeable base to the river-bed sand.
Vegetation
The banks of the river are lined by River Gums in a zone about 60m wide. Occasionally they may form broader zones especially where benches incised in the clayey silt are developed by the river. The trees tend to protect and stabilize the banks during floods. Occasionally River Gums grow in the river bed and on the silt-capped islands, but the most common species growing here is the Cadjeput. This species forms dense stands immediately upstream from Rocky Pool and some semi-permanent pools.
Interesting Information
Formed in the Gascoyne river and named for its distinctive reddish rocky surrounds, the Pool is very much a port of the river. Like most watercourses in this region the Gascoyne only flows after significant rainfall, and the river "coming down" is a major event in the lives of those who live on or near to it.
The Gascoyne catchment (with its major tributary, the Lyons) stretches some 600 kilometres Inland, to beyond the Great Northern Highway (North of Meekatharra). With such a huge area to drain there is little wonder that significant floods occur after rare heavy rainfalls. The highest flow attained by the river was in February 1960 when the water level reached 7,62 metres. A year later it reached 7,61 metres, while during Cyclone Steve in March 2000 it climbed to 7,50 metres.
Some of these floods had a devastating impact on Carnarvon and its surrounds - particularly those occurring before the levee banks were built near town in the 1960s. More commonly, these big flows have been welcomed for the replenishment of much needed water supplies. In the long intervening dry spells billabongs like Rocky Pool are an important oasis, nurturing both body and soul.
Why is here a pool?
In order to understand why here is this pool and why it is at the said location it is necessary to speak about the mechanisms of bringing water down a valley in a river here the Gascoyne River. These mechanisms are disturbed at Rocky Pool in the underlying geology by a special effect along the river direction which is called an anticline (as can be seen in the right image below) in the earth layers which leads to a complete different model regarding the water transportation along the river at this location.
Before coming to the final conclusion on the existence of the pool here it is necessary to think about the coexistence of all responsible mechanisms.
- Water transportation
3 main effects transport water in the Gascoyne river - here we understand this as a generic term - into the western direction to Carnarvon.
The main amount of water is transported directly in the river bed, but as heard above this only happens after significant rainfalls ("the Gascoyne only flows after significant rainfall"). The speed of this water flowing down the river is by far the highest, but as mentioned earlier it does only be effective during or short after rainfalls.
Second amount of water being transported to Carnarvon is in the sedimentary layer of sand, clay and rocks (called Older alluvium formation in the image below). The horizontal size of this water leading layer is not only direct under the river bed but also side-wards up to 50m on each border. The speed of this water flowing in this layer is slower than that in the river bed, and it is also dependent from the amount of water in the river itself and the local relations between sand, clay and rocks.
Third amount of water is the ground water, which flows on the water unpermeable layer (called Toolunga formation in the below image) under the sedimentary layer.
The effects of these mechanisms, whose influences in total can only be estimated, and therefore build a rather inaccurate model of this situation, change over the year, mainly by the amount of available water, and affect each other, this too influenced heavily by the amount of currently flowing water.
Regarding this does not yet explain any special behaviour along the river, though any change in the depth and broadness of the water leading layers 2 and 3 leads to a different amount of water being transported down the river.
- Special geology "Anticline" at Rocky Pool
As outlined above in the Geology paragraph the Gascoyne river flows in a more or less continuous geologic landscape. This continuity is heavily disturbed here by the Rocky Pool anticline which crosses the river nearly rectangular as can be seen in the below image right.
This special effect in the earth layers along the river direction (as can be seen in the right image above) leads to a complete different model regarding the water transportation along the river at this location. When regarding the cross-section in the left image above along the line A-A' it is obvious, that the depth of the river bed (Alluvium) is minimized, and the layers below (Sandstone and Siltstone) are much higher and almost reach the surface of the earth.
What to do here
Take a walk around the Rocky Pool admiring the beautiful ghost gum trees, float in the tranquil waters and explore the shallow pools for fossils that have been washed down from the Kennedy Ranges. Bring a picnic, or cook up a meal on the barbecues provided. Rocky Pool offers an opportunity to relax and appreciate the beautiful tranquil environment.
Logging conditions
To log this EarthCache, you must do the following and email us the results. You can go ahead and log your find, (ie. you don't need to wait for permission) but if we don’t think you have honestly completed the required tasks we will delete your log (after emailing you first of course):
1) Tell how many there were in your group.
2) How many layers of the underlying earth are affected by the described anticline?
3) Which of the described amounts of water responsible for a remarkable water transport is affected most by this anticline?
3b) Only for specialists: do the other layers probably transport any water along the river outside of rain fall times? Can you justify your answer by any reason?
4) In your opinion: are these effects minimizing any water flow also valid on the left and right side of the river? Can you eventually justify this answer by any observation at the pool site?
5) Optional task: take a photo of Rocky Pool with your GPS unit in the photo
Please email your answers to questions 1) - 4), please DO NOT post them with your log. Just optionally post your photo
For communications the GC Message Center is preferred - because its message delivers automatically this GC number and your GC nickname - but email works as well if you do not forget to provide these two items.