The Hornsby Plateau is a dissected sandstone

plateau In geology and physical geography, a plateau (; ; : plateaus or plateaux), also called a high plain or a tableland, is an area of a highland consisting of flat terrain that is raised sharply above the surrounding area on at least one side. ...

lying to the north of

Sydney Harbour Port Jackson, commonly known as Sydney Harbour, is a ria, natural harbour on the east coast of Australia, around which Sydney was built. It consists of the waters of Sydney Harbour, Middle Harbour, North Harbour and the Lane Cove River, Lane ...

that rises 200 metres. The plateau is a part of the larger

Sydney Basin The Sydney Basin is an Interim Biogeographic Regionalisation for Australia, interim Australian bioregion and is both a structural entity and a depositional area, now preserved on the east coast of New South Wales, Australia and with some of its ...

structure.

Geography

The North Shore and the Berowra Valley National Park 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 Botany Bay (Dharawal language, Dharawal: ''Kamay'') is an open oceanic embayment, located in Sydney, New South Wales, Australia, south of the Sydney central business district. Its source is the confluence of the Georges River at Taren Point a ...

. 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. The soil type is generally lithosols, which has low fertility and is normally less than 500 mm deep.

Geology

Rock deposits in the

Sydney Sydney is the capital city of the States and territories of Australia, state of New South Wales and the List of cities in Australia by population, most populous city in Australia. Located on Australia's east coast, the metropolis surrounds Syd ...

area were

tertiary Tertiary (from Latin, meaning 'third' or 'of the third degree/order..') may refer to: * Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago * Tertiary (chemistry), a term describing bonding patterns in organic ch ...

freshwater sediments, which were a rough

gravel Gravel () is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentation, sedimentary and erosion, erosive geological processes; it is also produced in large quantities commercially as crushed stone. Gr ...

ly layer up to 8 metres thick located beneath

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...

and sand at a mean depth of 6 metres. As time went by,

weathering Weathering is the deterioration of rocks, soils and minerals (as well as wood and artificial materials) through contact with water, atmospheric gases, sunlight, and biological organisms. It occurs '' in situ'' (on-site, with little or no move ...

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 The Tasman Sea is a marginal sea of the South Pacific Ocean, situated between Australia and New Zealand. It measures about across and about from north to south. The sea was named after the Dutch explorer Abel Janszoon Tasman, who in 1642 wa ...

. Thus, these movements created the Blue Mountains and Hornsby Plateaux, with the lagging behind of the peneplain becoming the Cumberland Plain. The plateau was scoured by

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

streams, which perforated a labyrinth of V-shaped

valley A valley is an elongated low area often running between hills or mountains and typically containing a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over ...

s 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 Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

layers:

Hawkesbury sandstone Sydney sandstone, also known as the Hawkesbury sandstone, yellowblock, and yellow gold, is a sedimentary rock named after Sydney, and the Hawkesbury River north of Sydney, where this sandstone is particularly common. It forms the bedrock f ...

, consisting mostly of sandstone, but with some

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

, and the overlying Ashfield Shale, which supports richer vegetation. There are also a few volcanic intrusions throughout the plateau.

References

{{Coord, -33.70490, 151.09901, format=dms, type:city_region:AU-NSW, display=title Landforms of New South Wales Plateaus of Australia Geography of Australia Hornsby, New South Wales Hornsby Shire Geography of Sydney Landforms of Sydney Dissected plateaus

Geography

Geology

See also

References