Bondi Stops 1-5

Self-Guided Geology Trip to North Bondi The North Bondi Route was undertaken starting from the north most stop near Hugh Bamford Reserve. We made our way down by foot to the rock platforms on the northern end of Bondi Beach. A total of 10 stops were observed and comments on North Bondi’s stratigraphy, coastal erosion processes, weathering of rock masses and effects of igneous intrusions are made in this section. This investigation along the coast of North Bondi reveals its landscape in a true geological manner. Bondi as seen in the geological map, is situated on Hawkesbury Sandstone which is medium to coarse grained quartz sandstone with very minor shale and laminate lenses.

Stop 1: Hugh Bamford reserve

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A 5 m high cutting of Hawkesbury Sandstone was observed. The sandstone is exposed and subject to weathering. At the stair entry/exit point of Hugh Bamford Reserve, a 5 meter high cutting reveals a cross section of Hawkesbury sandstone. There is evidence of seepage after a rainfall event (on 20/04/2013) indicating that there are defects such as along the bedding planes, allowing channels for water to flow through. Bedding planes are clearly defined and is also highlighted by the presence of vegetation in the plane. This suggests that soil such as clay and silts are present in the plane, brought there through soil

movements caused by rainwater seepage. Sand can also be inferred to be present in the plane as weathering can loosen sand particles from the sandstone. The presence of vegetation suggest that the defects in the bedding planes are created and further increased by the vegetation’s’ roots within the openings. Expansion of roots within the fissures and pores increase the level of weathering. Stop 2: Aboriginal Carvings: Aboriginal carvings which are etched into dull white Hawkesbury Sandstone which is exposed in a field of grass (North Bondi Golf Links). Parts of the sandstone are stained an orange-yellow colour due to the oxidation of iron articles in the sandstone. Black staining is also seen on the sandstone possibly from the rain runoff from the soils encompassing the sandstone. Significant weathering is seen on the sandstone, possibly from a combination of coastal processes such as wind action, wave action, thermal expansion and recrystallization.

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Aboriginal carvings which are etched into dull white Hawkesbury Sandstone which is exposed in a field of grass (North Bondi Golf Links). Parts of the sandstone are stained an orange-yellow colour due to the oxidation of iron articles in the sandstone. Black staining is also seen on the sandstone possibly from the rain runoff from the soils encompassing the sandstone. Significant weathering is seen on the sandstone, possibly from a combination of coastal processes such as wind action, wave action, thermal expansion and recrystallization. Sea salt spray can be felt on the top of the cliff and this affirms the theory that salt plays a part in the weathering of Hawkesbury Sandstone. Stop 3: Natural outcrop of Hawkesbury Sandstone

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An outcrop is a result of an uplift of rock or an erosion or surrounding rock, causing the bedrock to be exposed. The natural outcrop of Hawkesbury sandstone is stained a light yellow to orange colour, which is the result of oxidation of iron in the rock. Dark stains of grey to black colour are also seen on the top surfaces of the sandstone, again a result of water runoff from soils and vegetation growing on the outcrop. There are tilted planes indicating the presence of cross bedding at the outcrop, however metamorphism of the Hawkesbury Sandstone towards the east end of the outcrop has caused the sandstone to metamorphose into quartzite which is more resistant to erosion. There is presence of pebbles within the Hawkesbury Sandstone, revealing that during the deposition of sand particles, pebbles were deposited as well. Strength and hardness of the pebbles would alter the engineering properties of the outcrop, however the protruding pebbles suggest that the pebbles are more resistant to erosion and weathering and results in sandstone of higher strength. Figure 1

Stop 4: Top of old sandstone quarry

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The Sydney basin is composed of Hawkesbury sandstone and also shale formed by compaction of mainly clay minerals. It is often characterised by very fine soil being eroded away. The photo of the cliff face taken facing south shows the different rock layers in the cliff. Shale lenses are depicted

through the areas of the rock layers which are worn back. The different layers show unconformity in the cliff face. An old basaltic dyke was observed to have intruded into the sandstone. The dyke continues south and its presence is indicated by the narrow chasm which is left behind from significant weathering. Most of the basalt has been weathered into dark clay material seen in the photograph. The dyke is one of 4 dykes in North Bondi and its formation has affected nearby rocks through contact metamorphism. Stop 5: East side of the quarry

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Columnar joints are seen to the east of the quarry and is caused by shrinkage stresses caused by the cooling of the sandstone. Through the volcanic neck under the ourcrop, the sandstone was heated and metamorphosed into quartzite (morhs hardness scale = 7) which is a harder rock. As the rock cooled, shrinkage of the mass created stressed which resulted in long straight columns of quartzite widths of 10-20 cm. Bedding planes and cross beds which were evident both further inland and along the cliff did not affect the orientation of the forming of the columnar joints. The shrinkage joints form perpendicular to the direct of the heat source (Blackstone, 1963) which is located beneath the quary. The columnar joints formed have an average of five sides and thus have a pentagon shape. Joints along the top of the cliff were also observed as stresses from shrinkage reach to the top surface of the quarry.

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D. L. Blackstone

Columnar jointing in sandstoneRocky Mountain Geology, April 1963, v.2, p. 7-11 http://www.atnf.csiro.au/people/rnorris/SydneyRockArt/sites/BondiGolfCourse/index.htm