FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT (PART 22)

CHAPTER 14. The Nerriga District

The Permian sediments reach their highest level east of Nerriga and it is in this area that spectacular examples of erosion are most common. 

Extensive denudation has exposed the underlying basement rocks over a wide area. These are a continuation of the rocks exposed at Yalwal and Ettrema in the north. 

The Budawang Range reaches a height of 3,711 feet at Currockbilly Mountain and a similar height at Mount Budawang a little further south. This prominent ridge is the backbone of the basement, consisting essentially of a narrow belt of resistant Devonian strata flanked by Ordovician slates. Marine fossils occur in both these groups as well as sporadic mineralisation. Alluvial gold has been extensively mined, but reefs have rarely proved profitable. 

The core of the area described in these pages is a wild confusion of deep valleys and remarkable sandstone peaks. It is a place to be seen only by the bushwalker and not for the ordinary tourist. It is beyond the scope of this book to describe any but the most accessible places, but the geological features of these are representative of the whole. 

Excursion 14a 

Quilty’s Mountain (18 miles return) 

From Sassafras, a road runs south towards the Clyde River. It provides access to many fine views of the Clyde and its tributaries. Quilty’s Mountain is remarkable for its aboriginal Bora Ground, one of the few reminders of the former inhabitants of this district. 

Route: Sassafras-Round Hill-The Vines-Quilty’s Mountain 

Turn left from the Braidwood road 1 mile past Stony Hill and 34 miles from Nowra (see Excursion 9a). 

For the first mile, the road crosses basalt country, outcrops of which may be examined in many places. Thereafter, the sandstone-basalt boundary is crossed several times and great contrasts in their respective vegetations can easily be seen. 

Some 3.7 miles from Sassafras, a view to the east opens up, including the Tianjara mesas. The uppermost part of the Clyde gorge lies in between, a short distance to the east. Stop at Round Hill, a prominent residual beside the road a little further on. 

Stop 1. The sandstone outcropping here is typical of the Berry Formation in this area. It resembles that seen at Stony Hill and Tianjara. From the summit (2,412 feet) there is a panorama of the Clyde gorge, the Endrick River, and the mountain ranges in the south. Mt Talaterang and the top of the Pigeon House are also visible. 

A little further south the road skirts Newhaven Gap which provides access to the Clyde canyon, emerging upstream from the Coal Measures exposures (see Excursion 11a, Stop 4). The road reaches its closest point to the gorge 2.2 miles south of Round Hill. 

Stop 2. The cliff edge is only a few minutes walk from the road. There are numerous open joints in the sandstone, some of which yawn like great caves well back from the cliff and provided a convenient disposal unit for the sawmill which formerly operated here.  

It is possible to descend into the valley by way of some of the more accessible joints. Far below, the base of the Permian sediments is marked by a prominent bed of sandstone, below which outcrop the slates of the basement.  

The road deteriorates past this point and the next 2 miles should be covered on foot. Basalt is reached after the track has descended a wide gap in the sandstone plateau. Shortly afterwards, the track swings to the north. 

Stop 3. This locality, known as The Vines, is covered by thick vegetation, in contrast to the scrubby sandstone country nearby. A narrow belt of basalt may be traced northwards into the valley of the Endrick River as far as Nerriga. Apparently the lava flowed down the valley in earlier times from a point south of The Vines and since then the Endrick River has removed most of it by erosion. 

Quilty’s Mountain (or Mount Endrick) lies just west of this point. A track ascends to the bare rock outcrops near the summit, where aboriginal motifs in the form of native animals are depicted on the ground by heaps of small stones. This locality should be treated with all the respect it deserves as a historic monument and none of the stones should be moved. 

From The Vines, the track may be followed to Nerriga (12 miles). Otherwise, the return journey must be made over the same route. 

Excursion 14b 

Nerriga-Braidwood (68 miles) 

The Permian sediments reach their western limit at Nerriga and their present boundary with the basement rocks trends towards the coast, reaching it at Bateman’s Bay. On this excursion, outcrops of Upper Ordovician and Upper Devonian rocks are prominent and occupy most of the surface. 

Route: Nerriga-Oallen-Corang River-Charley’s Forest-Mongarlowe-Braidwood 

This excursion begins where Excursion 9a finishes – at the western edge of the sandstone plateau, east of Nerriga. The base of the Nowra Sandstone is easily found. The unconformity with the Ordovician basement is 0.7 miles further down the hill at a point where the road bends to the right. 

Stop 1. There is no marked unconformity visible as such, but there is a distinct change of slope and in vegetation at this point. The basement rocks consist of quartzite and phyllite intersected by numerous quartz veins. 

From this point on, the road descends more steeply and soon the Endrick River is reached. Cars may be parked on the left just before the bridge. 

Stop 2. The river gravel is representative of the rocks upstream and of no particular interest. The road cutting, however, reveals an interesting section of steeply dipping slates, with many quartz veins, overlain unconformably by semi-consolidated river gravel. This strongly resembles that in the nearby stream and is plainly of recent origin. Notice its irregular base, representing part of the old river bed. 

The Clyde River gorge from near Newhaven Gap April 1969 

From the Endrick River, the road ascends to a comparatively level, fertile area underlain by the same Tertiary basalt seen at The Vines (see Excursion 14a, Stop 3). Outcrops may be seen on the right 0.6 miles from the river. Silicified logs are known to occur beneath the basalt west of the road and enquiries at nearby farmhouses should yield further information. The Endrick River drops over a large waterfall just beyond these paddocks and copper minerals have been found in the gorge below. 

Continue through Nerriga and turn right along the Goulburn road 2 miles past the town. This road passes through an area where alluvial gold was once extensively mined. Jerricknora Creek was one of the chief centres, especially towards its confluence with the Shoalhaven River. Stop where some old mine heaps may be seen on the left, 5 miles from the Braidwood Road. 

Stop 3. A series of shafts on the hill side mark the outcrop of a quartz reef; quartz and slate are plentiful on the heaps. Alluvial gold found in the nearby creek was probably traced to this outcrop. Similar small mines abound in the area, but most of them are of little interest. 

Continue to the Shoalhaven River at Oallen Crossing. 

Stop 4. This is a favourite place for fossicking. Gold may be panned from gravel in the river itself or from the extensive gravel deposits alongside the road just before the bridge. There is nearly always someone here to give advice about the best places to try! 

From Oallen, the road may be followed on to Bungonia (26 miles) and thence to Bungonia Caves (see Excursion 9c). Otherwise, the road should be retraced to Nerriga (7 miles) and then right towards Braidwood again. Stop at the Corang River 5 miles further on. 

Stop 5. Some alluvial gold may be washed here. The river gravel consists mainly of the local low grade metamorphic rocks with some pieces of Permian conglomerate. A ½ mile past the bridge the road crosses the remains of an old water race. Built in the 1890’s, this channel carried water 24 miles from the upper Corang River through tunnels and across bridges to supply a head of pressure for the hydraulic sluicing of gold bearing gravels. The race is visible for miles upstream from the bridge. 

New road cuttings south of the Corang River reveal good exposures of steeply dipping slate. Blocks of red jasper occur 2 miles south and in this vicinity there are many outcrops of old river gravel. 

Turn left for Mongarlowe 2.7 miles beyond the river. The road crosses Wog Wog Creek and begins to climb slowly. Outliers of Permian sandstone may be seen a mile or so to the east. Stop at a gate on the left 2.4 miles from the creek. It is possible to drive a short distance along this track. 

Stop 6. Access may be had from this point to the Corang group of outliers and the underlying Upper Devonian strata. The track leads down to a creek crossing and shortly after a branch leads up the hill to the right. Outcrops here are mainly of slate. About ½ mile from the creek, the first Devonian rocks are met. These are rhyolites and they form a narrow belt a few hundred yards across. Much of the rhyolite is silicified and has become masses of chalcedony containing cavities filled with small quartz crystals. Common opal also occurs. 

Beyond the rhyolite, beds of sandstone strike north-south across the track. These dip steeply to the east and are intersected by numerous quartz veins. They belong to the Upper Devonian Merimbula Formation and marine fossils have been found in the sandstone in this vicinity. The location is to the right of the track, 400 yards beyond the rhyolite boundary.  

Should it be desired to climb Corang Peak, the track must be followed for another ½ mile before a left branch is taken towards the mountain. Corang consists largely of the Nowra Sandstone. The Yadboro Conglomerate forms the lowest outcrops of the Permian here. The view south from Corang reveals Wog Wog Mountain in the near distance with Currockbilly beyond. The axis of the Budawang Range is essentially that of a syncline in which the Devonian rocks have been preserved. Quartzite ridges are prominent along the range and form the highest points. 

Return to the Mongarlowe road. Some 2.2 miles further south, First Curradux Creek is crossed. Stop just before the second crossing where there is a small roadside quarry on the left. 

Stop 7. Steeply dipping beds of black siltstone contain Upper Ordovician graptolite fossils. These are confined to one narrow zone in the middle of the quarry, where the rock is fairly soft and quite fissile. It is interesting to speculate on the sparseness of fossil evidence in these ancient rocks. Without these rare discoveries, the age of these beds would be only vaguely known. Do not remove, from this or any other place, any fossil material for which you have no use. 

Occasional pieces of agate may be found in the creek, brought down from rhyolite outcrops a short distance upstream. Continue towards Mongarlowe. Good views of the Budawang Range and Corang are seen a little further south. Notice the thin vegetation on this side of the range. The coastal side is much more thickly clad, presumably because of its higher rainfall. 

Stop at a roadside quarry a little over 3 miles further south, near Charley’s Forest. 

Stop 8. Greenish altered rhyolite and shale outcrop here. It is possible that the rhyolite has been faulted against the Ordovician slates here, as a considerable area of quartz occurs near the boundary just north of the road. 

Eight miles south of Sapling Yard Creek, the road passes through Mongarlowe. This was once a prosperous gold mining centre and the Mongarlowe River was extensively dredged.  

The left branch of the road leads to Monga and Clyde Mountain, while the right goes to Braidwood (8 miles). On the Monga road there is much evidence of former mining activity. The Braidwood road passes onto granite 5 miles from the town, from which point on there is a marked change in topography and vegetation. 

Questions 

1. Discuss the changes observed in the topography when passing from (a) basalt to sandstone at Sassafras, (b) sandstone to slate at the Endrick River and (c) slate to granite near Mongarlowe. 

2. Put forward theories to explain the joints seen in the sandstone near the edge of the Clyde gorge. 

3. What evidence is there to suggest that there has been volcanic activity in the Endrick River area since the uplift of the country began? 

4. Compare the shapes of Corang Peak and Currockbilly. What has caused this essential difference? 

5. What observations could be used to distinguish an outcrop of Upper Devonian sandstone from a Permian one?