FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT                                            (PART 18)

Chapter 10. The Wandandian and Conjola Districts 

This is largely an area of uncleared bushland. Features of interest lie along the highway or on the coastal strip. Other than the Wandrawandian Siltstone and the Conjola Formation, the only variation in the geology is provided by an occasional dyke or inlier of basement rocks. 
Several dolerite dykes will be seen on these excursions; they are all similar, being composed of a bluish-black olivine dolerite with phenocrysts of plagioclase up to 1 inch across. The dykes are up to 60 feet wide and are traceable for several miles. 
The deep valleys of Wandandian and Conjola Creeks have exposed highly folded strata of unknown age, usually taken to be Ordovician like the metamorphic rocks of Tolwong and Nerriga. These have been intruded by granite, with which a small amount of mineralisation is associated, probably in the Devonian period. 
Excursion 10a 
Conjola Creek January 1969
Tomerong-Conjola (17 miles) 
The main interest lies in the outcrops of basement rocks seen west of the highway. The excursion includes both the Wandrawandian Siltstone and the Conjola Formation, a dolerite dyke, a granite intrusion and copper and tin mineralisation in Ordovician slates. 
Route: Prince’s Highway south from Tomerong. 
Stop 1. The Bewong dyke crosses the highway immediately past Bewong Creek, 4 miles past Tomerong. It intrudes the Wandrawandian Siltstone, but all that is seen here is a poor outcrop of dolerite boulders. They show typical labradorite phenocrysts, each displaying multiple twinning. The phenocrysts are aligned, probably parallel to the sides of the dyke. The line of boulders, which is the usual outcrop of the dyke, may be traced for several miles in either direction. 
The spotted gum forest, indicative of the underlying Wandrawandian Siltstone, gives way to scribbly gum near Wandandian school. This marks the first outcrop of the Conjola Formation. Diversion may be made to the right at Wandandian; many timber roads lead off the main logging road, some leading down below the Conjola Formation into the underlying slates and others up through the Nowra Sandstone onto the Turpentine Road. 
Four miles past the Sussex Inlet turnoff a track leads down to the old Jerrawangala tin prospect. 
Stop 2. The easiest approach is via the overgrown track shown on the map. Mining leases were held over a length of ½ mile, extending south west from the highway. 
Tin was discovered here about 1914, both as alluvial in gullies draining into Venandree and Wandandian Creeks and to a lesser extent in quartz lodes beneath the Permian sandstone. 
It is possible to wash a little alluvial tin in the creek. Vein quartz in the slate is unlikely to reveal any cassiterite, but may show a little chalcopyrite or bornite. Some excellent examples of small scale folding may be seen in the slate and quartzite in the creek bed. 
One mile further south, a road to the right leads to Jerrawangala Lookout and eventually to Tianjara Falls (see Excursion 11a). It ascends to the cliff top through the Nowra Sandstone, which displays its usual characteristics here. The observation tower at the top affords views of the coast for many miles in both directions. Fire roads off to the right between the lookout and the highway lead close to Wandandian Creek, where granite outcrops make a contrast to the familiar scene. 
The highway descends to Conjola Creek 7 ½ miles past the Sussex Inlet turnoff. A good section of the Conjola Formation is seen on the descent and should be examined on foot if it is to be studied in detail. A weathered dyke outcrops at one point. 
Stop at a convenient point near the Conjola Creek bridge. 
Stop 3. A small granite inlier outcrops in the creek beneath the bridge. It may only be seen when the creek level is low. The granite is overlain by typical Conjola sandstones and the unconformity seems to be comparatively level. When compared with the level of the slate at the Tin Mine, this suggests that an island may have existed temporarily in the Permian sea near Jerrawangla. The prevailing dip of the Permian strata could have been expected to elevate the unconformity south of Jerrawangla; evidently the old land surface was far from level.  
Other small granite inliers occur just east of this outcrop and a small area of slate lies upstream from the bridge. 
An interesting diversion may be made to the Conjola Copper Mine by turning right just past the bridge. 
Diversion (9 miles return) 
Turn right from the highway and then immediately turn left. After 1.6 miles, turn right. The left branch may be followed for several miles into the heart of the McDonald State Forest.  
Stop 0.3 miles beyond the fork. 
Stop 1. The unconformity between the sandstone and slate is not very marked, but beyond this point slate becomes dominant. Notice how there is considerable soil creep on the slope and the slate blocks have been overturned. 
Stop at the creek crossing ½ mile further on. 
Stop 2. Conjola Creek drains a large inlier of Ordovician strata, outcrops of which are seen here in the creek bed. The gravel includes large quantities of quartzite and spotted slate, but no igneous rocks. 
The road rises very sharply beyond the creek and in wet weather would be difficult to negotiate. Stop 1.9 miles further on where an old bulldozed timber track may be seen descending on the left. The road branches just after this spot. 
Stop 3. Slate is still found here at an elevation of 600 feet. This is probably due to the irregularity of the old buried land surface, since Permian sandstone occurs 500 feet lower only ½ mile south. The road ahead continues towards Jerrawangala Lookout. 
Follow the bulldozed track down to the creek and then head upstream for ½ mile. Copper stained slate fragments may turn up in the creek now and again. Turn left at the first major tributary and locate the old workings on the right a short distance upstream. 
Stop 4. Very little was done here beyond some surface testing. The lode proved to be inferior and no actual mining was attempted. Some chalcopyrite may be found here, but the outcrop is largely oxidised. From the amount of green stained slate found downstream, it seems certain that this is not the only place shedding the mineral. 
Return to the road and then to the Prince’s Highway. Continue for ½ mile further south to Cooloo Creek (Bunnair Swamp). 
Stop 4. Granite outcrops in the creek at the bridge and on the grassy hill opposite. By walking back up the hill towards Conjola, it will be seen that the granite gradually passes vertically into arkose and finally sandstone. This is a classic example of an unconformity and the fact that the granite surface is at the same level as that in Conjola Creek (Stop 3) suggests that the granite forms the basement over a much larger area than actually appears by its outcrops. 
Excursion 10b 
The coast from Sussex Inlet to Lake Conjola 
Most places of interest are found on the coast, where outcrops of the Conjola Formation and Ulladulla Mudstone are dominant. Well preserved marine fossils are found in the Mudstone and several igneous bodies add to the interest of this attractive area. Each locality will be treated in order, from north to south. 
Sussex Inlet 
Sussex Inlet Sand Hills January 1953
The rock platform at the entrance may be reached by walking around the beach from the “Alamein” camping area, or by a new road leading out to Cudmirrah Beach. Siltstone and silty sandstone are the dominant sediments and belong to the Ulladulla Mudstone. Sandstone at the eastern tip of the rock platform and forming a small reef are though to belong to the Conjola Formation. The map show the main features of interest, from the Cudmirrah end to The Haven.  
Sussex Inlet Rock Platform October 1961
A. The siltstone here contains quite a few solitary corals, mainly in blocks recently fallen from the cliffs. B. Large numbers of brachiopods occur in the sandstone at the base of this unit. C. There appears to be a raised rock platform here, now about 15 feet above sea level. D. A thin conglomerate bed includes numerous irregular blocks of granite, slate, and quartzite. 
Erratics in Siltstone Sussex Inlet 
entrance October 1961
Some of them show striations on their surfaces, rather like those at Crookhaven Heads (see Excursion 3c).  E. Fossils are common here, particularly crinoid stems, and fronds of polyzoa. Spherical concretions like those at Culburra are also abundant. F. Fossils are less abundant here. The sandstone is thought to be the top of the Conjola Formation. G. A small fault is visible in the cliff face. H. Shells in the sand hills are probably part of an aboriginal midden. 

The Springs, Swan Lake 
The water level in this lake, like most lagoons on the south coast, seems to have fallen in the recent past. There is a drop of about 15 feet down a steep bank to the present beach, which may be 100 yards away in some places. 
The grey siltstone contains numerous marine fossils, particularly where the road drops down to the lake shore. 
Cudmirrah and Berrara 
There are a lot of interesting things to be seen on a walk along this section of coast. It is best to start at the southern end of the rocks, where Canal Lagoon (Berrara Creek) runs into the sea. Sandstone and silty sandstone, dipping gently to the north, are seen for the first few hundred yards. A few fossils may be found in them. Then follows a large dolerite dyke, striking approximately at right angles to the coast. It is about 60 feet wide and composed of the same porphyritic dolerite seen at Bewong Creek. The sediments are arched up by the intrusion. This dyke may be traced inland and for some distance out to sea on aerial photographs. 
North of the dyke, fossils become more and more abundant until one thin bed of calcareous siltstone is reached, packed with a variety of marine fossils. Some parts of the rock are almost solid limestone, and contain large numbers of brachiopod spines. 
Cudmirrah Faunal Reserve 
This is another in the important chain of reserves designed to protect a large variety of habitats. Off the southern end of the beach, the ‘Walter Hood’ was wrecked in a storm on April 26th, 1870. A monument on the hill reminds us of the 11 men who lost their lives in this disaster. Fragments of tiles, glass, and other scraps may be found on this beach, washed ashore from the wreck. 
Red Head (Bendalong) 
Extensive outcrops of volcanic rocks occur on the headland. There appear to be several flows, intersected by numerous dykes. The whole is probably of Tertiary age. Most of the rock is very weathered; the appearance of the cliffs from the sea gave the locality its name. 
The remains of an old jetty remind us of the silica industry, established here about 1920. The volcanic rocks, in the form of flows and dykes, have caused extensive metamorphism of the underlying Conjola sandstone. This has largely taken the form of silicification. Quartz has been deposited from solutions accompanying the intrusion which converted the sandstone into a hard, compact quartzite. 
Today, instead of being shipped to Port Kembla, the silica is taken by road to be made into refractory furnace bricks. Extensive quarries are still being operated. 
Questions 
1. Discuss the nature of the basement in the Conjola district. 
2. Describe the field occurrence of dykes in this area. 
3. Discuss the sedimentary structures observed in the siltstones at Sussex Inlet and Berrara. 
4. Present evidence for the postulated existence of granite beneath the Permian sediments at Wandandian. 
5. Why has there been little agricultural development along this part of the coast? What other industries have developed? 
6. What effect is the present development of coastal holiday resorts having on the natural assets of this area? 

Wreck Bay from the entrance to Sussex Inlet January 1953 

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