A journey through the earth history of
Australia’s Coastal Wilderness
Part 4
- Gulaga – The first of the Three Mountains
Gulaga across granite landscape from the south, near Cobargo
Gulaga, behind Tilba Tilba
From Bermagui
Yuin traditional performing arts
4. Gulaga - The first of the three mountains
No journey through Australia’s Coastal Wilderness would be complete without
noticing three massive mountains towering over rolling landscapes. Gulaga (Mount
Dromedary) is the backdrop of Narooma and Bermagui, Mumbulla (Biamanga)
dominates the skyline north of Bega, and Balawan (Mount Imlay) forms a dramatic
backdrop to the sweep of Twofold Bay at Eden.
These three mountains are of deep significance to the Yuin people and their
descendants. They are used today for ceremony and teaching. When visiting,
please pay your respect to the rocks, soil and plants, for these natural features are
an integral part of the culture of the indigenous communities of the region. Today,
Gulaga is a national park co-managed by the New South Wales National Parks and
Wildlife Service and the Indigenous traditional owners.
Heading 10 km south from Moruya at the Tuross Head turnoff there is the first close
view of one of the three mountains, the 806 metres high Gulaga. British naval
explorer Captain James Cook named Mount Dromedary for its camel-like humped
appearance on his voyage north along the east coast of Australia in 1770.
Gulaga to the south over Tuross Lake from Hector Williams Drive
Australia had of course already been discovered and settled by the ancestors of the
nation’s Indigenous people over 50,000 years ago.
Gulaga is an integral part of the mid-Cretaceous story. The Gondwana
supercontinent started to break up in the earlier Jurassic Period, as India floated off
to its destiny of crashing into Asia. The crumpled front end of this ongoing collision is
called the Himalayas. In early Cretaceous time the remainder of Gondwana started
to rupture on its eastern margins, the line of ultimate separation being marked by a
string of volcanoes with the future east coast of Australia on one side and what was
to become New Zealand and the submerged continent of Zealandia on the other
side.
Zealandia is a nearly submerged continental fragment that sank after breaking
away from Australia 60–85 million years ago (Mya), having separated from
Antarctica between 85 and 130 Mya. It may have been completely submerged by
about 23 Mya and most of it (93%) remains submerged beneath the Pacific Ocean.
Zealandia is 3,500,000 square km in area, larger than Greenland or India and
almost half the size of Australia. (Wikipedia)
Much of the volcanic evidence of the rupture is underwater on Zealandia and in New
Zealand. The Australian side of the volcanic activity is seen in the Whitsunday
Igneous Province and Shoalwater Bay in Queensland, Swansea, the continental
slope off Sydney and the Dromedary Igneous Province in NSW, the Gippsland Basin
in Victoria and the Cape Barren - Mount William area of north east Tasmania.
This volcanic activity is inexorably bound up with the formation of the Tasman Sea
and the existing coastline. This is covered in Part 7 of this earth history journey.
The Dromedary Province is dominated by Gulaga. The province however includes
Montague Island 9 km off Narooma and surrounding outcrops of volcanic rocks up to
5 km away from the mountain. There is also evidence of related igneous outcrops
offshore up to 50 km from the mountain.
Montague Island is a nature reserve renowned for its wildlife particularly seals and
penguins, as well as other sea birds. It is an ideal place to view migrating whales.
Accommodation is available in the historic lighthouse keepers’ cottages. Access is
by commercial boat operators from Narooma and Bermagui.
The rocks of Gulaga Mountain itself are mid-Cretaceous, dated at about 98 Mya.
The mountain dominates the regional landscape as seen from the two photos below.
Adapted from Google Earth
Gulaga from Bermagui
Gulaga is part of a volcanic province. There is evidence of volcanic activity in the
area, particularly around Tilba Tilba Lake. The volcanics are also dramatically seen
as lava outcrops in the surf at 1080 Beach in Eurobodalla National Park and as
dykes injected into Ordovician turbidites on the adjacent headland.
Tilba Tilba Lake
basalt soils, Tilba Tilba
Lava outcrop 1080 Beach
nearby Dromedary related dyke
The southern end of 1080 beach shows a good example of juxtaposition of the dark
(mafic) lavas and lighter coloured granite-like monzonite similar to the material
making up the Gulaga structure.
Tilba Tilba Lake outlet to the sea 1080 Beach
The mountain today is the remains of a large glob of igneous rock intruded into the
older landscape rocks to within 350 metres of the then surface (since eroded away).
Geological studies have suggested that the bulk of the lavas presently near Tilba
Tilba Lake (near current sea level) erupted prior to formation of the oldest of the
Gulaga intrusive mass (which altered the lavas by heat). The 1:100,000 Narooma
Geological Map shows an outcrop of these lavas between 400 and 500 metres on
the northwest side of the mountain (above Rixons Hill Road). This fixes the location
of the older sedimentary rock land surface at about 400 metres above current sea
level, generally about 275 metres above the ridges of similar rock nearby today. The
lava must have covered this part of the region to a depth of least 1 km. It has since
eroded away leaving the lava outcrop remnants and revealing the old pre-eruption
landscape.
The process can be seen on Montague Island, part of the igneous province. The
lighter coloured intrusive rocks forming the southern two thirds of the island have
large crystals of feldspar that clearly cooled slowly at depth. The northern third of the
island is made up of darker volcanic rock. The lava again must have covered the
region to a far greater depth than the northwest Gulaga lava outcrop. The lighter
coloured rocks (granite like banatite), like Gulaga, have intruded into the lava. The
most logical explanation is that the site was closer to the volcanic eruption centre and
thus deeper under the exposed slopes of the volcano.
Montague Is lava in foreground lighter intrusive rocks behind
Lava cliff
Tuff (from the Italian tufo) is a type of rock consisting of consolidated volcanic ash
ejected from vents during a volcanic eruption.
The presence of lava and tuff indicates the proximity of a stratovolcano.
A stratovolcano, also known as a composite volcano, it is a tall conic volcano built
up of many layers (strata) of lava and volcanic ash. They are characterised by a
steep profile and periodic eruptions. They are the most common type of volcano.
Wikepedia
It is exciting to think about what this complex could have looked like looking south to
north on its mid-Cretaceous sub-polar inland plain some 400 km from the then coast.
The fingerprint volcanic features at or near Gulaga have been superimposed on a
modern example (Mt Hecka in Iceland astride the Mid-Atlantic Rift):
By the late Cretaceous, the volcanic cover had been largely eroded away leaving
‘Gulaga’ as a 200 metre hill on a gently rolling, lava covered landscape. The
mountain we see today is prominent because the surrounding volcanic rocks have
further eroded away, leaving the more erosion-resistant intrusive igneous mass
behind.
A full interpretation of the area will have to wait until there is detailed geological
investigation of the adjacent seabed and, possibly more importantly, the matching
Gondwanan crust that rifted-off when the Tasman Sea opened. This sits about
900km away under water, south east of Lord Howe Island. The mother of all
Cretaceous volcanoes could be one of the submerged mountains evident there on
bathymetry scans of the sea floor. The Dromedary Province could be a former
sideshow on one of its flanks. There is an increasing body of work identifying
widespread Cretaceous volcanic material (both 101 and 97 Mya groups) across the
entire length and half the width of Zealandia. The main volcano action was to the
east. There are some enticing off-shore targets worth sampling:
Google Earth
These possibilities may include the volcanic centre or be just other pimples on a
bigger volcano which was rafted away on the other side of the opening Tasman Sea.
The potential size is not an issue - stratovolcanoes can be huge:
Mt Kilimanjaro, Kenya
Gulaga today is well worth the visit, or even a climb. The track starts at Pam’s Store
in Tilba Tilba. Park information is available on:
http://www.environment.nsw.gov.au/nationalparks/parkHome.aspx?id=N0165
Pam’s Store, Tilba Tilba
A note of warning to the trip navigator, there are two separate villages, Tilba Tilba
and Central Tilba, the latter and larger village, somewhat confusingly, often called
Tilba.
The national park boundary is 1 km along the track and then rises through a distance
of 5.5 km along a steady graded climb to an altitude of 750 metres through wet
temperate coastal and mountain forests with patches of rainforest.
The site has a wealth of indigenous and historical heritage which adds to the
richness of the travel experience. The nearby National Trust listed historic Central
Tilba Village is worth a relaxing visit.
Further along the coast road, between Bermagui and Tathra, the traveller will drive
through Tanja. This is a fertile pocket of dairy country surrounded by dry forests.
Applying the ‘dairy farm rule’ of geology, the site is a pocket of granite or volcanics.
Igneous rock based landscape, Tanja 15 km north of Tathra
The rocks are the same age as the Dromedary Igneous Province known as the Tanja
Syenite, which is a rock of similar composition to granite (but less quartz) formed in
the same Cretaceous igneous event.