Activity 4:
Geomorphic Systems
Geology of the Sydney Harbour
Sydney Harbour is made up of mostly sandstone and shale formed during the Triassic period (about 220 million years ago). The eastern part of the harbour (which includes Balls Head) is predominantly Hawkesbury sandstone. Sandstone is up to 200 metres thick in the Sydney region, it forms the foundation of the City of Sydney. It outcrops spectacularly along the coast from the harbour to the Bouddi Peninsula.
Sandstone is a very hard rock that tends to break away in large blocks leaving the boulders and vertical cliffs that characterises the Sydney coastline. To the west the harbour is predominantly made up of Ashfield Shale (Source: Australian Museum).
The Sydney Basin was formed when the earth’s crust expanded, subsided and filled with sediment between the late Carboniferous and Triassic, approximately 300 to 50 million years ago (mya). Early stages of development occurred while the Earths continents were joined to form Pangaea and Gondwanaland as tectonic forces folded the Australian and Antarctic continents to form the Lachlan Fold Belt. These same forces formed a large mountain range, which slowly weathered and eroded away, filling the fold belt with layers of sediment, deposited into a shallow sea. Over time, tectonic forces has led to the uplift of these layers to form the Great Dividing Range, which is now weathering and eroding to form the landforms we see today, such as headlands, cliffs and valleys.
Read more here: https://australian.museum/learn/minerals/shaping-earth/the-sydney-basin/ and https://www.environment.nsw.gov.au/resources/nature/sydneybasin.pdf
Coastal and river processes (weathering, erosion, transportation and deposition)
Weathering describes the breaking down or dissolving of rocks and minerals on the surface of the Earth. Water, ice, acids, salts, plants, animals, and changes in temperature are all agents of weathering (National Geographic Society). There are three main types of weathering:
-Biological weathering: weathering caused by the movements of plants and animals. This could be the movements as a result of growth of roots or moss/lichen. It could also include the movements of animals moving around on the rock which breaks down the particles of rock.
-Chemical weathering: the process of chemicals in rainwater making changes to the minerals in a rock. Carbon dioxide from the air is dissolved in rainwater, making it slightly acidic. A reaction can occur when the rainwater comes into contact with minerals in the rock, causing weathering.
-Physical weathering: occurs when physical processes affect the rock, such as changes in temperature or when the rock is exposed to the effects of wind, rain and waves.Water can get into cracks in a rock and, if it freezes, the ice will expand and push the cracks apart. When the ice melts, more water can get into the larger crack; if it freezes again it expands and can make the crack even bigger. Wind can cause weathering by blowing grains of sand against a rock, while rain and waves cause weathering by slowly wearing rock away over long periods of time.
Source: British Geological Society https://www.bgs.ac.uk/discovering-geology/geological-processes/weathering/
Biological Weathering
Chemical Weathering
Physical Weathering
Erosion is the geological process in which earthen materials are worn away and transported by natural forces such as wind or water.
Erosion can take place in a variety of ways:
Hydraulic Action is the sheer force of water crashing against the coastline causing material to be dislodged and carried away by the sea.
Compression occurs in rocky areas when air enters into crack in rock. This air is trapped in cracks by the rising tide, as waves crash against the rock the air inside the crack is rapidly compressed and decompressed causing cracks to spread and pieces of rock to break off. Compression is one of the main processes that result in the creation of caves.
Abrasion is when rocks and other materials carried by the sea are picked up by strong waves and thrown against the coastline causing more material to be broken off and carried away by the sea.
Attrition is when material such as rocks and stones carried by waves hit and knock against each other wearing them down. As these materials are worn down sand and rounded beach pebbles are formed.
(Excerpt taken from: https://www.gsi.ie/en-ie/geoscience-topics/natural-hazards/Pages/Coastal-Erosion.)
Transportation refers to the movement of eroded material. Both wind and water can be the agents that play out this movement. At Balls Head, it is the wind, wave and tidal actions that transport sediments.
Deposition is the laying down of sediment carried by wind, flowing water, the sea or ice. At balls head, material has been deposited to create small beaches on the North and West sides of the headland.
Read more about weathering and Erosion here:
https://education.nationalgeographic.org/resource/weathering/
Balls Head and Berry Bay
Balls Head is a headland that has been formed through erosion and Berry Bay is a bay has been formed by erosion and deposition.
Headlands are formed when the sea attacks a section of coast with alternating horizontal bands of hard and soft rock. The bands of soft rock, such as sand and clay, erode more quickly than those of more resistant rock, such as chalk. This leaves a section of land jutting out into the sea called a headland. The areas where the soft rock has eroded away, next to the headland, are called bays. (Source: BBC Bitesize)
Activity:
Use Google Earth and use the street view function.
Explore various sections of Balls Head Reserve, looking for evidence of weathering, erosion and deposition.
Annotate a map of balls head such as the one here: https://www.hellosydneykids.com.au/wp-content/uploads/2017/11/Balls-Head-Reserve-small.jpg
Topography of the Sydney Harbour and Balls Head
Topography is the study and description of the physical features of an area, for example its hills, valleys, or rivers.
Bathymetry is the study of depths and shapes of underwater terrain. In other words, it is the underwater equivalent to topography. The underwater topography (bathymetry) of the harbour has an average depth of 13 metres, with deep channels as deep as 40m and shallow areas from 3 to 5 metres deep.
Click on the clip to the right to see a flythrough of the Sydney Harbour Sea floor. See below to view the topography of the Sydney Harbour and Balls Head.