FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT (PART 11)
Chapter 6
The Yalwal and Burrier Districts
Scattered throughout Australia are numerous ghost towns. Yalwal is such a place, but time has removed all but a trace of the town itself and left only the mines which once brought prosperity.
That famous cleric-cum-geologist, the Rev. WB Clarke, noted the existence of gold here as early as 1852. He reported that fine gold was to be found in Danjera and Yalwal Creeks, recommending further prospecting to the local settlers. The ‘golden era’ began in 1872 and lasted into the present century, with minor revivals ever since.
The Permian strata rise gradually to the west, exposing the basement rocks in all valleys west of Grassy Gully. The oldest strata are comparable to those known to be of Ordovician age and consist of altered clastic sediments. These are exposed only in parts of Danjera and Yarramunmun Creeks. They are overlain by sandstones and conglomerate, containing Upper Devonian marine fossils, and by nonmarine shales, rhyolite, and basalt flows. The whole has been intruded by a body of granite and a number of basic sills.
Excursion 6a
Nowra environs (25 miles return)
The Nowra Sandstone gives rise to steep cliffs, waterfalls, and pleasant little valleys. This kind of scenery is best seen within 5 miles of Nowra, where the Shoalhaven River and its tributaries have cut through to the underlying siltstones. A number of Permian fossil localities and unusual erosional features are included in this excursion.
Route: Nowra-Flat Rock Dam-Cabbage Tree Creek-Saltwater Creek-Bamarang-Wogamia-Nowra
Stop 1. Nowra Showground is the starting point of Excursion 3e, and could well be visited first on this excursion. The structural map shows that this point is on the eastern flank of a north-south trending anticline, the uplift of which has caused the high ground immediately opposite.
Proceed down Junction Street and turn right into Berry Street at the Methodist Church. Turn right at the end of Berry Street onto the Braidwood road, and shortly after right again towards Yalwal. Another 0.7 miles further on, turn right and then left to reach the wall of Flat Rock Dam.
Stop 2. This acts as a temporary storage for water pumped from the Shoalhaven River above Burrier and intended for local consumption. The dam has been thrown across Flat Rock Creek at a point where it has cut through the entire thickness of the Nowra Sandstone. A road to the right just before the dam descends to the creek and along the valley floor as far as the river, opposite the Showground. The pipeline seen on the descent supplies the Wiggins Teape paper mill at Bomaderry. Numerous Permian marine fossils have been found on the hillside where the pipeline is buried. Between the pipeline and the dam there are a number of small waterfalls marking the base of the Nowra Sandstone. Many brachiopod fossils crowd the underlying siltstone. Blocks of this rock are to be found in the creek bed.
Return to the Yalwal road. Stop just across Flat Rock Creek.
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| Dip slope of the Nowra Sandstone from near Flat Rock Creek July 1964 |
Stop 3. The Nowra Sandstone displays cross bedding which may be seen unusually well here. On the left of the road, the strike of the cross beds is very obvious and this may be seen again in the quarries on the opposite side of the road, where the face exposes the cross beds better than in a natural outcrop.
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| Nowra Sandstone quarry near Flat Rock Creek June 1965 |
(A diversion may be made to Cabbage Tree Flat by turning right at the top of the hill beyond the dam. There has been extensive stripping of the laterised surface along this road, for use as road surfacing metal. Fine views of the river and its associated sandstone cliffs may be had at Cabbage Tree Flat.)
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| Laterite Cabbage Tree Creek Road August 1963 |
Beyond this hill, the road descends the western flank of the Bhewherre Anticline and the reappearance of the Berry Formation is marked by a prominent change in vegetation. Stop just beyond Cabbage Tree Creek, where siltstone is exposed below the bridge.
Stop 4. This small exposure, due to the pipeline which will be seen often on this and subsequent excursions, has yielded many small marine fossils, especially corals. Before long, all evidence of this rock will vanish as it is weathering very rapidly.
Nowra Hill may be seen on the left as the next hill is ascended. A diversion to the river may be made at the top, otherwise take the left hand branch.
Diversion. (3 miles return) A rough track leads out to the Shoalhaven River at the western edge of Cabbage Tree Flat. Just before the descent, walk through the bush to the creek on the right. The stream drops by way of a fine waterfall directly into a deep tidal pool, accessible by boat from the river a short distance away.
Return to the Yalwal road. Some 2 ½ miles further on, the road descends through the Nowra Sandstone, which may be examined on foot if desired. Outcrops of the underlying Wandrawandian Siltstone are poor. Stop near a small bridge a short distance further on.
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| Shoalhaven River and the Cambewarra Range from above Cabbage Tree Flat October 1964 |
Stop 5. From this point a rough foot track leads to The Pulpit, a prominent residual of sandstone overlooking the river. Walk through the bush on the right of the road, keeping up hill until the ground begins to drop away to the river ahead. Then keep to the ridge on the left, which leads to The Pulpit. The entire walk is less than half a mile and well worth the exertion since the view of the river is unique.
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| The Pulpit at Bamarang November 1967 |
En route notice the prominent cross bedded sandstone outcrops, covered with ferns, orchids, and moss. The highest point overlooks the little valley of Saltwater Creek and the view extends to the Cambewarra Range beyond.
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| The view from The Pulpit August 1963 |
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| The Pulpit November 1967 |
Return to the road and proceed to the bottom of the hill. Stop opposite the road leading off to the right.
Stop 6. Pebbly Wandrawandian Siltstone is exposed beside the road. The pebbles are well rounded, being composed largely of quartzite. A small mesa of sandstone, on which is situated Calymea Trig. Station, may be climbed from here. Head into the bush on the left beside the road
until the sandstone cliff is reached a short distance away. Access to the summit is possible by way of a large joint in the cliff, or by climbing over fallen rocks at the western end, where the trig. cairn is situated. There is a clear view of Nowra Hill from here.
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| Calymea mesa Bamarang July 1964 |
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| Nowra High Science Club outing to Calymea mesa July 1964 |
Turn right from the Yalwal road towards the river. The road passes around the base of The Pulpit, where the Wandrawandian Siltstone is exposed, and follows the river for two miles before ascending the Nowra Sandstone scarp again. Levee banks and swamps may be seen on the river flats. Turn right at the top and proceed for a further mile. Stop where the road descends to the river on the left.
Stop 7. This section of the Nowra Sandstone shows features not seen elsewhere. At the top are several silty members, which may mark the base of the overlying Berry Formation. Fossils similar to those seen at Cabbage Tree Creek (Stop 4) are found here and in spoil dumped on the opposite side of the road. Massive sandstone follows, with a large block detached along a joint plain lying against the cliff. In the bush at the base of the cliff may be seen a prominent conglomerate bed and some cross bedded sandstone of the type more common at the top of the formation.
The road continues to a large river flat at Wogamia, one of the earliest parts of this district to be settled.
The right hand branch at the top of the descent leads to Longreach, where a narrow flat borders the river for several miles. A weir was planned to block the salt water in this vicinity, but it could not be constructed because of the thickness of silt on the river bed.
Return to Nowra by the same route as far as the pipeline, then turning left.
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| Dendrobium orchids (rock lilies)at Longreach |
Excursion 6b
Grassy Gully and Burrier (45 miles return)
The Shoalhaven River has cut through successively older rocks upstream from Saltwater Creek and the Devonian basement is exposed near Grassy Gully.
Gold was mined here some time ago and the old mines add to the interest of this place.
Proceed as in Excursion 6a. The road continues past The Pulpit and crosses Saltwater Creek. Keep straight ahead at the Yalwal turnoff, 0.3 miles further on. One mile beyond this, the road ascends and the pipeline is seen close by the road. Comparatively fresh siltstone (Wandrawandian) is exposed here. Stop where the road reaches it highest point.
Stop 1. A flying fox maintains contact with farms across the river,
which are otherwise approached by the Budgong road. Immediately below is a cliff composed of Conjola Formation which first outcrops at river level upstream from The Pulpit on the opposite bank.
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| The ferry, Bundannon. August 1963 |
The road descends rather sharply, crosses a small stream and skirts the river with prominent Conjola sediments on the left. Stop at the far end of this section, where the road cuttings cease.
Stop 2. Charles Laseron collected fossils here sixty years ago, when access was easier by boat and the cuttings had not yet been blasted. Fossils can still be found along the water’s edge in loose rocks thrown here by the road makers.
Continue towards Burrier. One mile further on, instead of turning left, go straight ahead to the gravel dumps.
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| Shoalhaven River nearBurrier February 1966 |
Stop 3. These pebbles are representative of the geology
upstream – porphyry, rhyolite, quartzite, and hornfels are all found here. Tidal water penetrates as far as this point, where the first rapids are found.
Return to the main road, and turn right towards Grassy Gully. The road to the right just beyond a group of farm buildings leads to the pumping station. The Grassy Gully road leads up a steep hill, crosses the pipeline and winds around the hillside for several miles. Stop 2.6 miles past the pumping station road, where the road crosses a small creek.
Stop 4. The base of the Conjola Formation is exposed in a small creek bed downstream from the road. The lowermost sandstone has created a waterfall immediately below which are outcrops of basalt of Devonian age. A boulder horizon marks the actual unconformity.
Thin papery shale is wrapped around some of the boulders and may contain nonmarine fossils. Brachiopods have been found in the sandstone immediately above the unconformity.
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| Unconformity September 1963 |
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| Pebbles in the old basalt surface September 1963 |
The entire outcrop is duplicated in a tributary coming in on the left a short distance downstream. Fissures in the old basalt surface are still filled with pebbles washed in as the marine subsidence took place. This exposure is 400 feet above sea level and considerably higher than the basement rocks along Yalwal Creek, Grassy Gully, and the Shoalhaven River. High ground evidently persisted here as an island while much of the surrounding country was being submerged in early Permian times.
Continue towards Grassy Gully. 2 miles further on, the road passes between mullock heaps right on the level of the unconformity.
Stop 5. The basement rock exposed here is a silicified, flow banded rhyolite, in which the gold mineralisation occurs. The mines of Yalwal and Grassy Gully extracted oxidised, near-surface deposits only, since at deeper levels the gold yield was too small. For this reason, most workings are shallow and above the water table. The primary, unoxidised ore usually contains pyrite and arsenopyrite but rarely visible gold.
Further shafts and tunnels occur towards the creek, where alluvial gold was mined. Most of the Devonian outcrops seen are of rhyolite, but a sheet of basaltic rock, possibly a contemporaneous flow, occupies much of the lower part of the valley. Tuffaceous sediments are associated with it.
Beyond Grassy Gully, the road climbs steeply again and heads towards Yalwal Creek. There are many other tracks here, some leading down to the Shoalhaven River while others are merely timber tracks. The most obvious route leads to private property, but a fire road to the left a short distance before the gate leads down steeply to Yalwal Creek.
Stop 6. The creek crossing, being entirely of gravel, may be hazardous, but beyond the creek the road ascends steeply, leading towards Tolwong. Many interesting features may be seen on foot, however.
At this point, the base of the Permian is only a short distance above creek level. The gravel includes granite, rhyolite, basalt and quartzite.
The Ettrema Creek junction is on the right about 1 mile upstream and should be visited. Ettrema Creek evidently supplies a tremendous quantity of gravel, most of it composed of quartzite with an occasional piece of granite and basalt. Cavities in the basalt may contain sulphide minerals. An outcrop of sandstone on the southern bank right at the junction with Yalwal Creek contains numerous poorly preserved brachiopods, probably of Upper Devonian age. Ettrema Creek may be followed upstream for several miles. The vast gravel banks continue, and folded Devonian strata outcrop prominently beneath the Permian sandstones.
Yalwal Creek also exhibits folded strata, especially at a point 1 mile further upstream. A little beyond this, rhyolite outcrops on the eastern bank and is intruded by granite on the west.
Downstream from the creek crossing, nonmarine Permian strata outcrop between the unconformity and the Conjola Formation. Glossopteris has been found here. The Shoalhaven junction is just around the corner. The proposed Shoalhaven Dam, to be built towards the end of the century, could well be built less than a mile upstream from its junction with Yalwal Creek. Devonian quartzite outcrops in the river bed and has been extensively drilled in the search for a suitable site. At this spot, a low point in the cliffs on the southern side would permit a spillway to be built, diverting excess water into Yalwal Creek.
The return journey to Nowra must be made by the same route.
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