Cracked not Sawn EarthCache
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Difficulty:
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Terrain:
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Size: 
(large)
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The above co-ordinates will take you to the car park of the easy walk to Sawn Rocks and is also a good area to start collect some information for this earth cache along with a walk down the track to the rocks.
Sawn Racks are a fantastic example of columnar jointing from volcanic activity 21 million years ago. The columns are 40m high and some sources suggest they go a further 60m into the ground. They consist of polygon columns which are also known as organ pipes. The shapes are typically hexagons which is the main shape that is formed with this type of feature although some sources list them as octagon (8 sided) while others refer to them as mainly pentagons (5 sided). One of your tasks will be to see which is the main type of polygon at this feature.
The columns are actually cracked and not sawn. The columns are formed by a basalt lava flow from the extinct Nandewar Volcano. The cracking phenomenon is the result of the slow and even cooling of molten rock (the basalt cools from outside towards the centre) which enable individual crystals within the rock to align perfectly with each other. Typically the tensile stresses caused by the cooling tend to produce 6 sided polygons (Hexagons). The slower the cooling the large the columns produced. Over millions of years the pipes have been exposed by the erosion from the creek of the softer material.
The Nandewar Volcanics were active for a period of 4 million years (21 to 17 Million years ago) and is attributed to the tectonic plate moving over a hot spot (fixed melting zone) deep under the earth’s crust and coming up through various weaknesses to form volcanoes. (The Warrumbungles and Mt Canobolas also relate to this hot spot). The activity started further north than Sawn Rocks and finished to the south. The volcano grew by initial burst of melting under the lower crust and then later by the eruptions of lava from a large molten chamber centred under the volcano at a depth of about seven km below the surface. More earth science information can be seen on the “Hot Spot” information board.
The volcanics through this period pushed out 400 cubic kilometers of lava.
To get to Sawn Rocks from the car park follow the easy walking path at the end of the car park for approximately 750m until you reach the viewing platform. It is said the best photographic times are at noon. If you wish to explore further you can take the steps down to the creek and see many of the fallen pieces of pipes.
Getting There
Sawn Rocks is in the northern section of Mount Kaputar National Park. To get there from Narrabri travel north along the Newell Highway towards Moree for 3km. Turn right into Killarney Gap Road (Bingara Road). After about 35km, you'll see the sign to Sawn Rocks on the right.
To get to Sawn Rocks from Bingara travel approximately 65km east along Killarney Gap Road.
Tasks
1. On the A “Hot Spot” to visit information board name the two volcano’s and there age when they were active. Which one was the active when Sawn Rocks was formed?
2. On the way to the viewing platform observe the surrounding ground material matrix and compare its hardness to Sawn Rocks. Access to the creek may also help answer this question.
3. At the viewing platform there is a plaque which gives some reasons for the columns breaking off over time. What is the main factor?
4. How many sides do most of the polygons have?
Optional
• Post up your picture showing off Sawn Rocks with your GPSr
• In the creek measure the diameter of the polygons.
Additional Hints
(No hints available.)