Download The story of granite and Bega Cheese

A journey through the earth history of
Australia’s Coastal Wilderness
Part 2 –
Granite and Bega Cheese
Twin Sisters Tors, Bodalla State Forest
2. The story of granite and Bega Cheese
Our journey starts at Moruya about 300 km south of Sydney, on the northern extent
of Australia’s Coastal Wilderness. The story begins with the Moruya Granite that has
historic ties with Sydney Harbour Bridge, built between 1924 and 1932. The massive
supporting bridge pylons are built from dressed granite rock obtained from a quarry
at Moruya.
North Pylon – Sydney Harbour Bridge
Moruya is a pleasant town next to a tidal river on a coastal flood plain set in a fertile
grassy landscape against the mountainous backdrop of the rugged eastern
escarpment.
The historic Moruya Granite quarry is on the north side of the Moruya River 3 km
downstream from the Moruya Bridge. The grey granite, specked with black mica, is
massive, with very few fractures. Not a single block was rejected by the masons on
the bridge construction site.
Historic Moruya Quarry
Adjacent wharf
The dairy farm grasslands, cleared of eucalypt forests, are typical of the rich soils
formed on granite rocks. The grasslands here are the northern margins of the
granites of the Bega Batholith. This huge suite of granite is composed of over 130
separate plutons (including the Moruya Granite) covering some 9000 square km of
south eastern Australia. Granites of the Bega Batholith, extend from Moruya in the
north east to the south at Cape Conran, and Mallacoota in the Victorian part of
Australia’s Coastal Wilderness. A tiny granite outcrop in Canberra may be the north
westerly extent of this batholith.
A batholith (from Greek bathos, depth + lithos, rock) is a large emplacement of
igneous intrusive (also called plutonic) rock that forms from cooled magma deep in
the earth's crust. Batholiths are usually made up of granite or chemically similar
rock.
A pluton in geology is a single body of intrusive igneous rock that crystallized from
magma slowly cooling below the surface of the Earth. (Wikepedia).
The Bega Batholith granites crystallised from melted rock deep in the earth’s crust
some 420 to 400 million years ago (Mya), being the early-mid Devonian period.
These granites formed in response to the interaction of tectonic plates at the eastern
margin of the old supercontinent of Gondwana, shown in the figure. Crustal melting
took place landwards of the plate boundary, the so-called Palaeo-Pacific oceanic
trench, with the oceanic plate sliding under (subducting beneath) the thick
Gondwanan continental crust. As the oceanic plate sank, under increasing
temperature and pressure and reactions with water saturated sediment, the rock
materials melted and then rose like huge hot air balloons up through the crust (the
plutons).
If such molten rock, or magma, reaches the surface it can form explosive andesite
volcanoes that are common today around the Pacific Ocean ‘ring of fire’ (actually a
series of tectonic plate boundaries). If the magma cools below the surface, it forms
granite – a generic term that covers a wide range of silica rich igneous rocks.
Variations in chemical composition give granites their diverse colours and textures
that distinguish one pluton from another. These variations make a diversity of
building stones. Examples of this diversity are:
The Bega granites are similar in appearance to number 3, with dominance of the
white form of feldspar rather than the pink form.
Igneous rock (from the Latin igneus meaning of fire) is one of the three main rock
types, the others being sedimentary and metamorphic. Igneous rock is formed
through the cooling and solidification of magma or lava. Igneous rock may form
either below the surface as intrusive (plutonic) rocks or on the surface as extrusive
(volcanic) rocks. (Wikipedia)
Driving south from Moruya, you can see fine exposures of the granite in the road
cuttings along the highway.
Granite breaks down easily to produce the rich soil beloved by farmers, and a
landscape of gently rounded hills. Right through the region almost all of the dairy
farms producing the famous Bega Cheese are on granite based soils, identified by
the granite boulders dotted across the paddocks. These boulders are easily seen
because the native forests were cleared by early European settlers for farming and
grazing. Almost all of the cleared land between the coast and the escarpment was
formerly forests growing on granite based soils.
Google Earth
The cleared land in this Google Earth image shows the area of the granite based
soils used for dairy farming.
The granite breaks down as water penetrates joints and fractures and rots the
feldspars and smooths out sharp edges to produce the characteristic clusters of
rounded boulders, known as tors.
Dairy farm on granite landscape, Coolagolite
In the high country of the escarpment, granite tors are scattered through trees and
grassland along the Snowy Mountains Highway between the top of Brown Mountain
and the Maclaughlin River east of Nimmitabel. Further south in South East Forest
National Park, a particularly beautiful site to enjoy forested granite landforms is about
25 km east of Bombala at Myanba Creek, with its falls and lookouts.
Myamba Lookout South East Forest National Park
Granite tors, Myamba Creek
Myamba Creek
Spectacular views of granite escarpment can be found at Tuross Falls north east of
Nimmitabel in Wadbilliga National Park. The route to this site also takes the visitor
through outliers on the Monaro Volcanic Province (described elsewhere on the
journey) on the escarpment.
Upper Tuross River Wadbilliga National Park
Tuross Falls
A stop off near Tilba Cemetery
A side trip well worth the effort. Heading south, turn left at the turnoff to Central Tilba
from the Princes Highway, instead of turning right to the village. Follow the sign to
the cemetery towards the coast. The forestry map shows the beach as Wallaga
Beach. There is a car park and access to the beach just below the second grave
enclosure. The geoheritage treasure is the headland south of the cemetery. The site
is best visited at low tide.
Approaching the headland, the distinctive features of almost-vertical beds of
brownish Ordovician sedimentary rocks stand out. We will meet these very old rocks
at other places along the coast. Here, we’ll look at the igneous dykes and quartz
veins intruding the Ordovician beds. The first clue is the white slash in the centre of
the photo below.
The white dykes are chemically similar to the nearby Dromedary intrusion, and were
intruded at the same time as the Dromedary mass [Mount Dromedary, now called
Gulgaga]
An intrusive dyke is a slab-like igneous body whose thickness is smaller than the
other two dimensions. Thickness can vary from sub-centimetre scale to many
metres, and the lateral dimensions can extend over many kilometres. When
intruded, a dyke shoulders aside pre-existing layers or bodies of rock; this means
that a dyke is always younger than the rocks that it cuts through.
So far so good but the going now gets weird. On the same headland there is another
network of dykes of granite composition, quite unlike the chemistry of Dromedary,
and a granite outcrop in the sand.
Granite dyke
Granite outcrop
We can see the granite intruding into, breaking up and assimilating the Ordovician
beds.
Granite intruding into the turbidite
The white rock cutting across the centre of the turbidite is not another dyke but a vein
composed of quartz, common in these highly deformed old rocks. The beds cut by
the vein are Ordovician sedimentary rocks called turbidites.
Turbidites were deposited on the deep ocean floor from very fluid sand and mudladen avalanches cascading down continental slopes. Sand and mud gradually
settled out of suspension. Successive avalanches are often separated in time, but
can build up thick piles of sediment. The same processes occur today down the
continental slope off eastern Australia.
Geological study of this complex geological site has identified the granites as part of
the Bega Batholith. The nearest granite outcrop, the Cobargo Pluton, is 14km away.
The two sets of dykes on this headland are separated by some 323 million years.
Instead of returning to the cemetery, walk to the other headland at the northern end
of the beach. This headland is entirely made up of the Cretaceous lavas of the
Dromedary province encountered at 1080 Beach (the next headland north).
In the front boulder the mixing of chunks in the lava soup can be easily seen.
Granite shaping the landscapes
The term ‘granite’ embraces a wide range of rocks differing in their composition,
chemistry, depth and temperature at the time of formation. The Bega Batholith is
made up of over 130 plutons. Many of them are quite different in their crystalline
texture and composition and this has helped to map the extent of each pluton.
Some granites are more susceptible to weathering than others, so that they break
down more easily into sand and soil. Contrasts in weathering and susceptibility to
erosion in different rock types, including granites, is a major factor in determining the
shape and structure of the landscape we see today. One example is in the Cobargo
area shown below.
Google Earth
The grassland on the Cobargo Granite stands out as it has been cleared of native
forest to expose fertile soils well suited to pasture and dairy farming. The area
producing prime cheese coincides with the geology map.
The Ordovician sedimentary rocks form elevated, erosion-resistant ridges where they
abut the granite. Hot, molten granites intruded into the Ordovician rocks deep in the
crust, cooking them in the process, resulting in a band of hard heat-altered rock
against the granite. Erosion and weathering over millions of years has left the
contact zone standing out as rugged ridges, while the granite has washed away or
formed deep soils.
Just to the south of the Cobargo Granites is Mumbulla Mountain, composed of a
quite different granite type. This is far more resistant to erosion and stands out as a
rugged range above the adjacent Cobargo granite landscape cleared for agriculture.
Mumbulla Mountain from Princes Highway
Google Earth
Similar examples of the landscape that reflect different erosion rates occur across the
region. The view from Piper’s Lookout on the Snowy Mountains Highway at the top of
the steep climb from Bemboka on Brown Mountain shows rolling grassland stretching
almost to the coast 50 km away. As at Cobargo, the grassland represents areas of
fertile soil weathered from Bega Batholith granites, now cleared for pasture.
Forested hills to the left mark the cooked rocks intruded by the granite.
View from Pipers (Brown Mountain) Lookout
The weathered granite is well exposed in road cuttings seen during the ascent to
Piper’s Lookout. The yellow material is a mixture of soft clay and sand formed from
breakdown of minerals in the granite. Occasional large rounded boulders embedded
in this material are all that is left from the once massive rock. When wet, the clay and
sand has no strength and readily slumps downhill, making this a road builder’s
nightmare.
The Bega Granite story does not end here, it continues well inland from Piper’s
Lookout. Boulder outcrops (tors) appear among the forest and in grassy areas along
the Snowy Mountains Highway. Formation of the steeply sloping escarpment, its
face cut by deep gorges, is another fascinating story. This is described in Part 3 of
our journey.