The Moogerah (thunder) peaks are located in a region that once slept beneath the belly of a volcano in the ancient Main Range, which erupted some 24 million years ago, and spreads across the Fassifern Valley. Reynolds Creek cuts through the valley to form two distinctive peaks – Mount Edwards (originally named Mount Banister) and Little Mt Edwards.
Prolonged erosion has removed most of the eastern side of the volcano leaving steep slopes and a retreating escarpment which is now the edge of the Main Range. As this occurred and some of the underlying rocks were eroded, any plugs, dykes or sills composed of resistant rock types – particularly those of rhyolite and trachyte – remained as steep peaks.
Mount Edwards, rising 634 metres above sea level, is a large trachyte plug which was formed when magma filled vertical pipe-like fissures. Trachyte is an igneous volcanic rock with an aphanitic to porphyritic texture and it contains around 65% silica.
Trachyte consists of various minerals: Essential alkali feldspar, and relatively minor plagioclase and quartz or a feldspathoid such as nepheline may also be present. Biotite, clinopyroxene and olivine are common accessory minerals. Trachyte is hard-wearing and does not absorb water. As you look around the summit and walk the trails you will see an extraordinary range of rock types in a myriad of colours – this is caused by magmatic differentiation.
Trachytes often have minute irregular steam cavities which make the broken surfaces of specimens of these rocks rough and irregular. Trachys means ‘rough’ in the Greek language.
DO NOT STAND ON THE EDGE OF THESE CLIFFS.
Wander around the flat summit and look at the exposed cliff faces to your left and right.
Please send your responses to the CO on message centre. Then you can log the Earthcache online. I will contact you if any problems with your answers. False logs will be deleted – you need to visit GZ.
Questions:
Q1 - What is the predominant shape of the rocks on the cliff faces?
Q2 - How were the vertical columns formed?
Q3 - Suggest why the cliff faces have not crumbled away?
Q4 - Describe the rock the permanent survey marker is attached to?
Upload a selfie at the summit if you would like to. This is optional.