The Hornsby Plateau is a dissected sandstone plateau lying to the north of Sydney Harbour that rises 200 metres. The plateau is a part of the larger Sydney Basin structure.[ Sydney Basin-Subregions]
Geography
The North Shore and the Berowra Valley National Park
[ Geology, topography and soils Friends of Berowra Valley] are located on the Hornsby Plateau. It is separated from the Blue Mountains Plateau by the passage of the
Colo River. The plateau rises from the
Cumberland Plain in the south along a warp line increasing from the proximity of
Cattai to
Botany Bay. The plateau's elevation ranges between 200-220 metres, with gullies that fall 80-100 metres deep along drainage lines. Steep slopes with drastic ascension of over 100 metres are present.
[ Geology of the Sydney Basin Sydney Vignerons Association Inc.] The soil type is generally
, which has low fertility and is normally less than 500 mm deep.
[Chapman, G.A. & Murphy, C. L. 1989, Soil Landscapes of the Sydney 1:100 000 Sheet, Soil Conservation Service of NSW, Sydney.]
Geology
Rock deposits in the
Sydney area were
Tertiary period freshwater sediments, which were a rough
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.
[ Sydney Basin - landform Office of Environment and Heritage (New South Wales)]
The plateau was scoured by freshwater streams, which perforated a labyrinth of V-shaped 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.[Herbert, C. & Helby, R. (eds) 1980, A Guide to the Sydney Basin, Dept. of Mineral Resources, Geological Survey of NSW, bulletin no. 26.]
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 igneous rock throughout the plateau.
See also
