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, New South Wales, 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.
Geology Description
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.Flannery, Tim, "Introduction: The sandstone city" in Flannery 1999 pp. 8–9 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.Flannery, Tim; 'The Stone', in Deirmendjian, 2002. 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 in length. Based on these profiles, characteristic channel-scale architectural elements are estimated to be at least wide, with individual macroforms 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 . Although these dimensions are large, they are measurably smaller than those of channels and bars in the modern Brahmaputra River of Bangladesh. 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.
Formation
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. The Hawkesbury Sandstone is cut by several major fault and fracture systems, including the north-northeast-trending Homebush Bay Fault Zone, which extends for at least across the Sydney region.
Evidence such as unidirectional paleoflow, freshwater biota, and abundant mudrock intraclasts indicates deposition in a fluvial environment. Its sheet-like geometry, low variability in paleocurrent directions, numerous erosion surfaces, and scarcity of in situ mudrocks further suggest formation within a braided river system, comparable to the modern Brahmaputra River in South Asia. An alternative interpretation proposes a more subdued sediment source and a comparatively arid river system, comparable to the modern Cooper Creek, Warburton, or Diamantina River in the states of Queensland and South Australia. A different view favours a tidally influenced delta, where herringbone cross-bedding records bi-directional flow. The issue remains contentious.
The Narrabeen Group preserves evidence for deposition in a range of environments, including alluvial fans, braided and meandering rivers, and delta plains. A braided river system, characterised by massive floodplain deposits up to thick, has been proposed for the massive sandstones. Fluvial modelling by Miall and Jones (2003) further suggests that the Hawkesbury Sandstone comprised channel belts exceeding in width. Currents washed through it, leaching out most of the minerals and leaving a very poor rock that made an insipid soil. They washed out channels in some places, while in others, the currents formed sand banks that show a characteristic current bedding or cross-bedding that can often be seen in cuttings. At a time in the past, monocline formed to the west of Sydney. The monocline is a sloping bend that raises the sandstone well above where it is expected to be seen, and this is why the whole of the visible top of the Blue Mountains is made of sandstone. The Hawkesbury Sandstone is also cut by numerous igneous intrusions, including the Great Sydney Dyke, a major Jurassic dolerite dyke extending for at least across inner Sydney. The dyke intruded the sandstone approximately 163 million years ago and is generally several metres wide, although its width and geometry vary considerably along its course. Sandstone immediately adjoining the intrusion is locally fractured, jointed and thermally altered by contact metamorphism.
History Hewing and working the stone
The quality of the sandstone known to Sydneysiders as yellow block became well known early. Called on by the Colonial Architect, for example, to be used in the main buildings of the University of Sydney, the stone was supplied from the Pyrmont quarries where there were at least 22 quarrymen working by 1858. Among them was Charles Saunders, licensee of the hotel 'The Quarryman's Arms' who became Pyrmont's biggest quarrymaster. Pyrmont yellowblock not only had good hardness, texture, and colour, it was also suitable for carving and so it could be incorporated into buildings in the form of sculptures and finely carved details. The sculptor William Priestly MacIntosh, for example, carved ten of the explorers statues for the niches in the Lands Department building in "Pyrmont Freestone".
From the beginnings of the colony in 1788, settlers and convicts had to work with the stone, using it for building and trying to grow crops on the soil over it. The sandstone had a negative effect on farming because it underlay most of the available flat land at a very shallow depth. In the late 19th century, it was thought that the sandstone might contain gold. Some efforts were made at the University of Sydney to test this idea. Reporting on them in 1892, Professor Liversidge said "The Hawkesbury sandstone and Waianamatta shale was, of course, derived from older and probably gold-bearing rocks hence it was not unreasonable to expect to find gold in them." The sandstone is the basis of the nutrient-poor soils found in Sydney that developed over millennia and 'came to nurture a brilliant and immensely diverse array of plants'. It is, for example, the "heartland of those most characteristic of Australian trees, the eucalypts". As plants cannot afford to lose leaves to herbivores when nutrients are scarce so they defend their foliage with toxins. In eucalypts, these toxins give the bush its distinctive smell.Flannery, Tim, "Introduction: The sandstone city" in Flannery 1999 p. 15
Saunders's quarries, known locally as Paradise, Purgatory. and Hellhole, were so named by the Scottish quarrymen who worked there in the 1850s. The names related to the degree of difficulty in working the stone and its quality. The best stone was 'Paradise', a soft rock that is easy to carve and weathers to a warm, golden straw colour. The Paradise quarry was near present-day Quarry Master and Saunders Streets, Purgatory quarry was near present-day Pyrmont Bridge Road, and Hellhole was where Jones Street now is, near Fig Street. Before World War I, quarries opened up in other Sydney suburbs, such as Botany, Randwick, Paddington and Waverley.
The men who worked the stone were highly skilled and organised. Their trade union was the first in the world to win the eight-hour working day in 1855. The daily wages for quarrymen and masons in 1868 has been cited as ten shillings, while labourers earned seven to eight shillings per day at that time. Stonecutters were subject to a range of lung diseases such as bronchitis, pneumonia, and a disease known as "stonemasons' phthisis", now known as a form of silicosis or industrial dust disease. In 1908 questions were asked in the Legislative Assembly in the parliament of New South Wales about how likely the men cutting sandstone in Sydney were to contracting the disease and whether the Government should grant medical aid to them.
Early building