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The bolder beach at Mimosa Rocks. The boulders strewn on the foreshore are weathered columnar rhyolite

Rock formations at Bunga Head
Columnar jointing in rhyolite at Mimosa Rocks
The black rock is mudstone, also known as coffee rock, formed on the floor of an ancient lake
Maffic intrusions
Turbidite broken up and mobilised in the liquid of a mafic dyke
Upturned turbidite bands intruded by a mafic dyke
Quartz bands at Nelsons Beach

Mimosa Rocks National Park

Latitude -36.582834, Longitude 150.052664 Mimosa Rocks

Located in the Sapphire Coast Region of NSW, nearest town Tathra

Source: A journey through the earth history of Australia's Coastal Wilderness




Link to Detailed Map


The underlying geology of Mimosa Rocks National Park is dominated by highly folded and faulted sedimentary rocks such as slate, siltstone, shale and greywacke which were deposited during the Ordovician period (430-490 million years ago). Additional sediments accumulated during the later Devonian period (355-410 million years ago). During this time, coarse-grained granite and gabbro were intruded into the sedimentary rocks at Doctor George Mountain to the east of Bega and Goalen Head in the northern part of the park, while rhyolite was extruded over the top of the older rocks at Bunga Head to a depth of more than 140 metres. This volcanic activity has produced striking coastal formations such as the rock pedestals at Goalen Head, columnar jointing at Bunga Head, and the "castle-like" sea stacks of the Mimosa Rocks at Aragunnu (Packham 1969; TEC 1991).

Poorly-consolidated sediments such as gravels, sands and clays were also deposited at various times during the Tertiary and Quaternary periods (the last 65 million years). Occasionally exceeding 50m in thickness, these deposits cover substantial parts of the coastal zone northwards from Gillards Beach where they are being actively undercut by wave action. In places, these sediments grade into gravel beds of water-worn pebbles of quartz and quartzite which may be covered by recently-deposited aeolian sands. They include the so-called "coffee rock" found at Aragunnu Bay and elsewhere, which is probably an eroded podzol soil that has been hardened by humic groundwater.

In Mimosa Rocks National Park, volcanic eruptions occurred on the floor of a deep lake. The results can be seen in the cliffs at the northern end of Aragunnu Beach in Mimosa Rocks National Park. Here there is an extraordinary array of various kinds of volcanic materials mixed with black mudstone. The story revealed in this complexity is that of a deep freshwater lake floored by black muds. Fishes swam in its waters and an occasional lycopod (a fossil plant like a modern tree fern) floated in and sank to the bottom. Flattened remains of lycopod (club mosses) trunks can be seen in the black mudrocks at Picnic Point, south of Aragunnu indicating one of the earliest Gondwanan forest environments. Club mosses are common around the world today but are only present in a ground hugging moss-type form, a far cry from these, the world's first forests.

A volcanic dome rose in the ancient lake, shattering rocks and incorporating the pieces into the breccia in this photo. This site is located at the headland between Mimosa Rocks and Aragunnu Beach

Into the Aragunnu lake environment rose a bulging volcanic dome which eventually burst through the lake floor and began to erupt underwater. The lake water boiled and fumed, shattering bedrock into a mosaic of interlocking pieces. The erupting lava also broke up, forming a mass of fragments. As the dome grew, large chunks of lava were shed down its sides. The result, geological chaos.

The coastal cliffs and rock platforms of the Mimosa Rocks National Park are formed from a mixture of rock types including rhyolite, gabbro, sandstone, siltstone, conglomerate and metasediments. Many of these rock exposures exhibit spectacular large and small scale features, such as folds, faults and intrusions.

Getting There

Mimosa Rocks National Park is located on the far south coast of New South Wales between Bermagui and Tathra. Entry to the park is gained by turning east off the Tathra Bermagui Road to several locations including Nelsons Beach, Middle beach, Middle Lagoon, Bithry Inlet, Picnic Point and Aragunnu which gives access to Bunga Head. There are picnic and camping sites and toilets at Aragunnu Beach and Picnic Point. There is a wheelchair-accessible boardwalk and lookout at Aragunnu.

Allow around two and a half hours from the car park to walk to the columnar jointing at Mimosa Rocks headland and back. You will need additional time to investigate the volcanic dome. To find the Volcanic dome remains, go east from the carpark and you will come to the headland at the northern end of Aragunnu Beach. Walk north across the headland and look for the brecciated rocks.

Weathered upturned bedding at Nelsons Beach

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