Wednesday, July 20, 2011

Rocked OUT!

Our faithful and ever reliable gang of interested and enthusiastic helpers arrived at the meeting place nice and early and then we waited to see whether anyone else would turn up to be shown some of the interesting aspects of Bendigo's geology that Eric was prepared to demonstrate and explain to us all - it was cold and wet - a thoroughly miserable morning, after all!

We need not have worried about this as, despite the very unpromising weather, several families did attend and we were able to start our activity on time.

Eric explained some of the geological features of the White Hills landform by referring to a geological map of Bendigo on which he had mapped many of the interesting details surrounding us.

We crossed the road without incident and examined the roadside geology, which Eric explained to us all.  The damp, dull morning weather proved to be absolutely ideal for taking photographs and also for collecting samples of the coloured clay.  Most of us simply accumulated this on our boots and clothing although some interesting clay balls were carefully moulded and taken home afterwards by a few children.  Of course, sharp eyes soon discovered a little lizard which Anne Bridley identified for the finder as Bougainville's Skink (Lerista bougainvillii) - a description and photograph is on Page 45 of the Bendigo Field Naturalists Club's book, Frogs and Reptiles of the Bendigo District.

Eric explained how these soft rocks had been formed millions of years ago deep in the ocean, as layers of fine sediment, and how they were compressed into thin layers which were folded and buckled by massive earth movements to eventually form the layers of Sedimentary rock that we were looking at.  We were all told the names of these upward and downward folds - Anticlines, and Synclines - and the intriguing names that some examples had been given by the earlier residents of this area, for example, the Slaughterhouse Anticline.

Bougainville's Skink (Lerista bougainvillii)

Next, we looked for examples of crystal deposits in the sedimentary rocks and found many good examples of Pyrites showing the cubical shape of its crystals.

Most of these crystals had weathered away because of their exposure to air and water (we know a little bit what this felt like because of the weather!) and now just appeared as rusty looking lumps in the rocks or sometimes as square-edged holes showing where the Pyrites crystals had been.

The striking red, brown and yellow colours, that were already being appreciated by some children for their potential as coloured ochre, result from minerals, mainly iron compounds, which are present in the rocks.  Some of these coloured chemical compounds are able to  dissolve in rainwater and form coloured bands in the rocks.  Rob Moors explained how the children could investigate this process by using one end of a strip of blotting paper to soak up colour from some coloured water made with a water-soluble Texta Pen.  Colours which have been mixed to produce the colour in the pen are able to separate into different coloured bands as they soak up into the strip of blotting paper. Many naturally occurring vegetable extracts also behave in this way and are able to form separate bands of different colours on Blotting paper.

We walked up onto the remnants of the White Hills Gold-mining area and carefully avoided the many holes which had been left by the miners. 
At the top of the rise, Eric explained the inverted nature of the landscape.  We were standing on (in) the ancient bed of the Bendigo River which had formed the rounded quartz pebbles that made the soil so rough and difficult to walk over.  The tall hills which had surrounded this ancient river valley had all eroded away over millions of years of weathering - we were asked to imagine what it would have looked like back then.   Eric had asked the group about the special feature of the mine holes left by Chinese gold-miners and we had a brief but interesting discussion about which explanation - 'miners were avoiding dark corners where Evil Spirits could lurk', or  'rounded holes were safer because they were less prone to collapse', was more likely to be true.

There was also time for us to notice some of the native plants which grow surprisingly well in this difficult clay and gravel soil and also, unfortunately, several introduced plants which grow there as weeds - Boneseed is an example of an introduced plant which grows very well in these soil conditions.

Several of the native birds which live amongst these shrubby plants flew around us and many others could be heard calling while we were concentrating on the geology and plant life of the area.

Jan thanked Eric for showing us around and explaining what we were walking over.  She also thanked the resilient and weatherproof members of the Family Nature Club for coming despite the weather.  We all experienced various aspects of the environment - Geology, Native Vegetation, Introduced Weeds, Native Birdlife, and Wintery Weather - all at the same time in this very interesting area of Bendigo, the White Hills.

Eric also offered to take us Fossil Hunting to find Graptolites, but in another area of Bendigo and during warmer weather because there is a lot of sitting down and careful searching involved.  We will certainly accept his kind offer and will work out when this will happen.

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