Latitude -34.650809, Longitude 150.862541
Located in the Illawarra Region of NSW, nearest large town Kiama
The Bombo Quarry site is an impressive and fascinating place where the remains of ancient columnar jointing provides us with our own Aussie version of the Irish Giant's Causeway. The site also enables visitors to see examples of weathered out dykes, volcanic breccias, the contact between the Kiama Sandstone Member and the overlying porphyritic basalt (known as the Bombo Latite Member). These features adjacent to the Tasman Sea present some amazing photographic possibilities.
In 1979 a heritage site nomination was proposed by the Geological Society of Australia (NSW Division) for the Bombo Quarry. The importance of the geological features of this area was brought to the attention of the NSW Heritage Council by Dr Suzanne Wass of Macquarie University's School of Earth Sciences (now of professor Sue O'Reilly). The quarry was owned by the Metropolitan Water, Sewerage and Drainage Board and, at that time, it was proposed that a pollution control plant be constructed on the floor of the disused quarry. Following site inspections and lengthy consultations a Permanent Conservation Order was placed over the site in 1983. It was transferred onto the State Heritage Register in 1999.
Two units of the Late Permian Gerringong Volcanics are exposed on Bombo Headland. The Kiama Sandstone Member forms a narrow wave-cut platform and adjacent vertical cliff face around the south-eastern extremity of the quarry. To the north the sandstone dips below sea level and is overlain by approximately 20m of porphyritic basalt: this formation is termed the Bombo Latite Member. The contact between the two units is well-exposed in the cliff section at the eastern end of the headland.
The red-brown colour of the sandstone (due to oxidisation of haematite) contrasts markedly with the grey-black latite, which displays spectacular columnar jointing elsewhere in the quarry. Isolated columns up to 5 meters in height stand adjacent to the coast between the northern and southern parts of the quarry; just slightly to the north, the sea wall exposes cross sections of the columns 1 to 2.5 metres in diameter, resulting in a 'Giant's Causeway' appearance.
Agglomerates or volcanic breccias have developed in some areas of the quarry, as in the southwestern portion near the access road. This lithology is readily distinguished from the latite by its chaotic appearance and light-coloured matrix.
A capping of cream-coloured weathered latite, still retaining the characteristic porphyritic texture, may be studied at the top of the northern and western quarry faces. This sharply-defined zone of surface weathering and soil formation overlies relatively fresh rock exhibiting columnar jointing.
In later geological eras, the Bombo Latite Member was intruded by at least 5 basaltic dykes of probable Tertiary age which flowed around and between the columns of latite, often taking 90 degree changes in direction. Early investigators (Jaquet 1905 & Harper 1915) mapped and described these dykes, but subsequent development of the quarry provided further exposures of some and obliterated others. Today dykes are mainly exposed in the northern half of the quarry but at least one extends across the floor of the excavation in the vicinity of the isolated columnar stacks. They are of interest due to their inclusions of xenoliths and xenocrysts, which are believed to represent fragments of the earth's mantle incorporated in magmas originating from within that zone. Sussmilch (1905) described xenoliths of hypersthene gabbro, augite peridotite, enstatite peridotite and pyroxenite occurring as rounded fragments and boulders embedded in the basalt. From a deeper level of what was probably the same dyke, Wilshire & Binns (1961) recorded hornblendite and glimmerite as the dominant xenolithes. Present exposures of most of the other dykes appear to lack macroscopically visible xenolithes. (Percival)
The Headland can be accessed via the Kiama Walking Trail: turn off the highway at the Bombo Beach exit and then turn right at the first roundabout and follow the signs to Bombo Beach. You will pass under a small railway bridge and then proceed to the car park. The walk to the quarry begins here: when the road forks follow the road which is closest to the coast, as the other road goes to the water treatment plant. The road leads through a swampy area into the quarry. From the car park it is about a 2 hour return walk going right to the end of the quarry, and stopping here and there to have a good look. Take water to drink, a hat and sunscreen as the area is very exposed.