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1. THE HAWKESBURY SANDSTONE FORMATION | |
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This magnificent sandstone, up to 200 metres thick in the Sydney region, forms the foundation of the City of Sydney. It outcrops spectacularly along the coast from the harbour to the Bouddi Peninsula. (click on image for more examples)  Sandstone Cutting on Expressway (click on image for more examples)  (mainly quartz) with a smaller proportion of feldspar, clay, and iron compounds such as siderite (FeCO3). It was formed from the sands which came from Antarctica about 200 million years ago, carried by a vast river system. Over millions of years these sands were consolidated into sandstone up to 50 metres thick on the peninsula. Hawkesbury Sandstone contains minor shale beds rich in fossils. FEATURES OF THE HAWKESBURY SANDSTONE It forms the higher plateaus of the Bouddi Peninsula with spectacular escarpments. It is a light coloured massive sandstone cream to yellow in colour, and weathers to a grey on the surface. Its internal brown markings were formed by iron-rich water which seeped through the sandstone many years after the rock had been laid down. These are known as Liesegang rings giving attractive patterning. Cross-bedding (inclined layers) were formed by water currents moving the sands across a river delta. The soils from the Hawkesbury Sandstone tend to be highly infertile and shallow, and water passes through them easily. This is shown by the low heath vegetation on the high moorlands, such as Bombi Moor and Hawke Head THE SANDS OF OUR BEACHES 300 million years ago the rocks of Antarctica were uplifted to form a massive mountain range. At this time Australia and Antarctica were adjacent and situated near the South Pole as part of Gondwana. Erosion of these mountains over millions of years produced vast quantities of sand. The sand was carried by a huge river system from Antarctica across southern Australia until it reached the east coast. It deposited the sand into the Sydney Basin and consolidated to form the Hawkesbury Sandstone. 
After the sandstone beds were laid down Australia separated from Antarctica and began drifting close to its present position. 
Later erosion broke down some of the sandstone to produce the sands of our beaches. NOTE: If you look carefully at the sand near the high tide mark on the north-eastern end of Putty Beach ('Fishermen's End') you can see a fine line of black sand which is actually zircon and rutile. These came here from Antarctica with the vast quantities of clear sands. 
PERCHED SAND DUNES Ancient windblown perched sand dunes on Bombi and Mourawaring moors occur 90 to 100 metres above present sea level. They are evidence of a colder, drier, more distant coastline in the last Ice Age. The pure white sand of these dunes was mined in the 1960s for silica used in the manufacture of high quality optical glass. The deposits still remain, but are preserved as part of Bouddi National Park.  
TESSELLATED PAVEMENTS ON THE HAWKESBURY SANDSTONE Many tessellated pavements occur on the Hornsby plateau on surfaces of Hawkesbury Sandstone. The tessellations do not penetrate deeply, perhaps 10-20 cm below the surface caused by weathering and erosion. 'Crazy Rock', adjacent to the crest of the road to Patonga, has several thousand square metres of tessellations. NOTE: Geologists nowadays conclude that heat metamorphism (volcanic activity) is NOT a cause of tessellations.  LATERITE Many people think that laterite is a volcanic rock. It is in fact produced by the weathering of sandstone. At Killcare Heights there is an extensive plateau of laterite above the Hawkesbury Sandstone. Over millions of years the surface of the sandstone has been chemically and mechanically decomposed to form soil. Under conditions of high rainfall and humidity this soil has been leached of silica and humus. What remains is a rich, soil-like layer composed of oxides of iron and aluminium.  It is soft and crumbly when formed, but it becomes extremely hard when exposed to air. It forms hard pea-shaped nodules and solid boulders. Laterite is not a mineral, nor is it a complete soil. USES OF LATERITE For gardening laterite has low fertility with insufficient nutrients, but it can be productive with the addition of fertilisers. In the past it has been quarried and used as a road base. At Killcare Heights laterite has been used for building, in particular for the walls of houses. It is dug up when it is soft, crumbly and rusty red. It can then be moulded into blocks that harden in the open air and change to a duller reddish brown colour. Note: The Hawkesbury Sandstone has many important commercial applications, especially seen in our early buildings all around Sydney. See later section on "Commercial Uses of Rocks". |