Hornsby Plateau

Rock deposits in the Sydney area were tertiary freshwater sediments, which were a rough gravelly layer up to 8 metres thick located beneath silt and sand at a mean depth of 6 metres. As time went by, weathering transformed much of the sandy silt into siliceous clay. At the end of the Tertiary period, or about 80 million years ago, earth movements created an upheaval of 600 metres in most of coastal eastern Australia, due to the rifting and opening of the Tasman Sea. Thus, these movements created the Blue Mountains and Hornsby Plateaux, with the lagging behind of the peneplain becoming the Cumberland Plain.^[5]

The plateau was scoured by freshwater streams, which perforated a labyrinth of V-shaped valleys with intruding rocky ridges into the Hawkesbury Sandstone. The drainage activity is ascertained by the Hawkesbury Sandstone's impotence, which were employed by early watercourses. As the creeks carved deeper into the plateau, they began undercutting the sandstone walls of the valley, and as such, blocks of sandstone fell down, thus creating cliff lines.^[6]

Due to its hilly topography, the plateau was slow to develop. It has two main sedimentary rock layers: Hawkesbury sandstone, consisting mostly of sandstone, but with some shale, and the overlying Ashfield Shale, which supports richer vegetation. There are also a few volcanic intrusions throughout the plateau.

• Woronora Plateau

33°42′18″S 151°05′56″E / 33.70490°S 151.09901°E / -33.70490; 151.09901