Twin Sisters Tors, Bodalla State Forest

2. The story of granite and Bega Cheese

A journey through the earth history of Australia’s Coastal Wilderness

Part 2 – 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.

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

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).

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.

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

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)

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 mud-laden 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.