TUGGERAH LAKES DISTRICT

A change in vegetation provides clues to underlying geological structures. This igneous dyke has weathered to mineral rich clay. Some time after the Sydney Basin formed small but explosive volcanoes blasted through the overlying fractures. The volcanic necks or diatremes are filled with breccia, a mixture of ash, basalt and country rock that has collapsed into the vent. The enormous forces that drive continental shift have caused north-south and east-west fracturing in the Sydney Basin. These fracture zones influence the course of the Wyong River: - Alluvial sandstones and conglomerates were laid down on floodplains. Shales formed in the abandoned channels and billabongs. - Bayhead Delta like those where shales and cross-bedded sandstones are formed. - Mudstones result from fine sediments like those being deposited in Tuggerah Lakes. They often contain fossils. - Some Sandstones were formed from Barrier Beaches like this one at Wyrrabalong. - Sandstones laid down in Tidal Deltas like The Entrance may contain ripple marks and animal burrows.