Sydney 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. Sydney sandstone was deposited in the Triassic Period probably in a freshwater delta and is the caprock which controls the erosion and scarp retreat of the Illawarra escarpment.

The Hawkesbury sandstone forms the bedrock for much of the region of Sydney, Australia. Well known for its durable quality, it is the reason many Aboriginal rock carvings and drawings in the area still exist. As a highly favoured building material, especially preferred during the city's early years—from the late 1790s to the 1890s—its use, particularly in public buildings, gives the city its distinctive appearance. The sandstone has traditionally been interpreted as the product of a large braided river system, comparable in scale and depositional style to the modern Brahmaputra River in Bangladesh.

The sandstone is notable for its geological characteristics; its relationship to Sydney's vegetation and topography; the history of the quarries that worked it; and the quality of the buildings and sculptures constructed from it. This bedrock gives the city some of its "personality" by dint of its meteorological, horticultural, aesthetic and historical impact. One author describes Sydney's sandstone as "a kind of base note, an ever-present reminder of its Georgian beginnings and more ancient past." Sandstone escarpments box in the Sydney area on three sides: to the west the Blue Mountains, and to the north and south, the Hornsby Plateau and Woronora Plateau. These escarpments kept Sydney in its bounds and some people still regard the spatial boundaries of the city in these terms.

The Hawkesbury Sandstone is a Triassic sheet sandstone widely exposed across the Sydney Basin, especially along the coastal areas near Sydney. The ripple marks from the ancient river that brought the grains of sand are distinctive and easily seen, telling geologists that the sand comes from rocks formed between 500 and 700 million years ago far to the south. This means that the highest part of the visible lines almost always faces approximately south. It is a very porous stone and acts as a giant filter. It is composed of very pure silica grains and a small amount of the iron mineral siderite in varying proportions, bound with a clay matrix. It oxidises to the warm yellow-brown colour that is notable in the buildings which are constructed of it.

Photomosaics were constructed for two cliff sections south of Sydney, one extending 5.6 kilometres (3.5 mi) in length. Based on these profiles, characteristic channel-scale architectural elements are estimated to be at least 2.7 kilometres (1.7 mi) wide, with individual macroforms 5–10 metres (16–33 ft) high, indicating the constructional depth of typical channels. Hollow elements (scoop-shaped units interpreted to have formed at channel confluences) reach depths of up to 20 metres (66 ft). Although these dimensions are large, they are measurably smaller than those of channels and bars in the modern Brahmaputra River of Bangladesh. 6 kilometres (3.7 mi) of sandstone and shale lie under Sydney. Sedimentation is interpreted to have been influenced by strong currents operating in shallow marine, littoral, estuarine, fluvial, lacustrine, and aeolian settings.

Crushing strengths and fire resistance tests carried out on Sydney sandstone showed that the compressive strength was 2.57 tons per square inch, or 39.9 megapascals (MPa). The crushing strength for ashlar masonry and lintels averaged 4,600 pounds per square inch (31.7 MPa). Recent tests have recorded compressive strengths of up to 70 MPa. In fire resistance tests, designed to assess the resistance to collapse of a building in a fire, the sandstone came through better than some of the very hard stones, especially the granites. (The stone was subjected to temperatures approaching 800 degrees Celsius, for 15–30 minutes and plunged into cold water.)

Other types of sandstone found in Sydney include sandstones in the Mittagong formation, Newport Formation Sandstone, Bulgo Sandstone, Minchinbury Sandstone, and other sandstones which occur within other layers of sedimentary rocks; such as sandstones within Ashfield Shale, Bringelly Shale and Garie Formation. Bald Hill Claystone is considered by geologists to be a variety of sandstone. Iron and aluminium oxides are found within laterite, which was formed by the weathering of Hawkesbury sandstone.

The formation was deposited during the Anisian stage of the Middle Triassic, which spanned approximately 247.2 to 242 million years ago. At that time, Australia and Antarctica were joined as components of the supercontinent Gondwana. From around 280 million years ago, eastern Antarctica began to rise through a major mountain-building episode comparable to the ongoing uplift of the Himalayas. This uplift generated a vast river system that drained northward into the Sydney Basin, transporting and depositing sediments and minerals eroded from Antarctic rocks. Comparable sedimentary patterns can be observed today in river systems fed by the Himalayas. The sand was washed from Broken Hill, and laid down in a bed that is about 200 metres thick. Following burial and lithification, the sandstone underwent gentle folding, fracturing, and faulting, was intruded by basaltic dikes and sills, and was later uplifted—most prominently in the west—forming the Blue Mountains.