FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT (PART 7)
Chapter 3 (continued)
THE NOWRA DISTRICT. EXCURSIONS 3c, d AND e
Excursion 3c
Crookhaven Heads, Culburra and Calalla (41 miles)
Isolated coastal outcrops of fossiliferous siltstone provide good localities to study representative Permian marine fossils.
Route: Nowra-Pyree-Crookhaven Heads-Culburra-Calalla Point-Forest Road-South Nowra-Nowra
Take the Greenwell Point road and turn right at Pyree. The road crosses Crookhaven Creek and then the flood gates at the Saltwater Swamp, an area still of little agricultural use because of its high salt content.
Go straight to Crookhaven Heads and stop to the right of the Pilot Station. Walk through to the beach.
Stop 1. The sediments exposed here are miles from the nearest
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| Weathered dyke at Crookhaven Heads June 1963 |
rocks of known age, at Coolangatta or Currarong. They are considered by most geologists to belong to the Wandrawandian Siltstone. Although fossils are not abundant, there are a number of other points of interest.
An anticline, with its axis at right angles to the coast, buckles the strata just north of the beach. The siltstone here includes numerous erratic blocks of metamorphic rocks. Some of these have been found to be grooved on the underside in a similar fashion to pebbles found in present day glaciers where the grooving is caused by the pebbles being ground against the rock beneath the ice. This has been taken as good evidence in favour of the ‘iceberg theory’, which concludes that the Permian sea was cold and in the neighbourhood of glaciated mountains.
The beds also contain glendonites – peculiar crystalline structures up to 2 inches long. These are composed mainly of calcite, but were apparently formed from other minerals which crystallised in the icy bottom mud of the Permian sea. They have been replaced more recently by calcite, which was possibly derived from neighbouring fossils.
A weathered out dyke has produced a large channel a little further along the platform. The many pebbles found here are derived mainly from within the sandstone beds. The rock platform may be followed right around the headland and back to the road.
Return to Culburra. Park out on Wheeler’s Point and descend onto
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| Nowra High School teachers at Wheeler’s Point Culburra June 1963 |
the rock platform on the northern side.
Stop 2. The strata here strongly resemble those at Crookhaven Heads, being composed of a sandstone bed overlying siltstone, the whole being mapped as part of the Wandrawandian Siltstone. Fossils are quite common, and are frequently associated with large ‘cannonball’ concretions. These unusual structures seem to be the result of minerals in the sediment recrystallising around a nucleus, frequently a fossil. At the tip of the headland, the dip increases sharply, perhaps because of a fault just off the coast.
Lake Wollumboola, south of Culburra, is typical of many coastal lagoons, being the result of a rise in sea level followed by sand deposition across the inlet so formed. The lake shores show evidence that there has been, more recently, a slight fall in sea level.
Return on the Nowra road and turn left towards Currarong before the road descends to the flood plain. At the cross roads further on, go straight ahead to Calalla Bay. The traces of former roads in the bush are the remains of a grandiose subdivision called ‘Jervis City’ which failed many years ago.
Stop 3. At Calalla Point there are typical outcrops of Wandrawandian Siltstone. The rock is fine grained with a fair assortment of rounded pebbles and some large erratic blocks. Fossils tend to be grouped together, as at Culburra, but most are found as soft casts, as little calcite remains. At the southern end of the rock platform, where Calalla Creek enters Jervis bay, an aboriginal midden is found.
Return to the Currarong road and turn left towards the Prince’s Highway. About 1 ½ miles further on, a small quarry may be seen on the right where the road passes through a low cutting.
Stop 4. Here the siltstone is intruded by a dyke of porphyritic dolerite. There are many such intrusions throughout the Currumbene State Forest. This particular dyke is quite weathered and cannot be traced for any great distance. A few fossils may be found in the siltstone, which has been altered to hornfels near the edge of the dyke.
Continue towards Nowra. This area has proved difficult to interpret geologically, for there are only scattered outcrops. It would appear that the Nowra Sandstone has lost its identity by gradually becoming more silty. Faulting also seems to have occurred, but its extent is uncertain.
There are several distinct vegetation changes along the road, from scrubby sandstone types to spotted gum forest. While this is due to a change in the underlying rock type, it is difficult to interpret precisely.
Turn right at the highway and return to Nowra (see Excursion 3e).
Excursion 3d
Bomaderry Creek and the Good Dog Intrusion (15 miles)
This excursion includes the Good Dog Lamprophyre intrusion and a number of interesting features within the catchment of Bomaderry Creek.
Route: Nowra-Moss Vale Road-Cambewarra-North Nowra-Nowra.
Proceed through Bomaderry and along the Moss Vale road. Stop at the bottom of the first hill, where Bomaderry Creek is seen on the left.
Stop 1. The creek gravel contains a lot of red jasper in addition to the normal rock types derived from the Cambewarra Range. Downstream, the creek enters a picturesque gorge cut into the Nowra Sandstone, emerging as a tidal stream ½ mile upstream of the bridge on the highway.
Examine the geological map of Bomaderry. It would appear that folding in the Pleistocene epoch deflected the stream southwards and the gradual uplift of the land entrenched the meandering stream into its gorge.
Continue towards Cambewarra and 400 yards further on turn right into Bell’s Lane. Follow this to the most distant farm on the lower slopes of the mountains. Seek the landowner’s permission before continuing on foot.
Stop 2. The Good Dog intrusion is the largest of at least 30 lamprophyre bodies intruding the Permian sediments on the Cambewarra Range. It has a roughly circular form and may be a volcanic neck of Triassic or Jurassic age. The rocks are all characterised by a high percentage of well crystallised ferromagnesian minerals – hornblende, biotite and augite, in that order – set in a greyish groundmass of intermediate composition.
The lower slopes of the mountain are strewn with boulders of these peculiar rock types, while the actual outcrop is obscured by the dense vegetation above. The best place to examine the rock is in the creek behind the farm, the bed of which is full of lamprophyre boulders.
Return to the Moss Vale road and continue towards the Cambewarra Range. Turn left (Barfield Road) just beyond the first road cutting on the mountain. Turn right after ½ mile into Tannery Road and follow it until the road descends to Good Dog Creek near the reservoir.
Stop 3. By following Good Dog Creek downstream for 200 yards, two small dykes and a sill can be seen intruding dark Berry Formation siltstone. Upstream, there are extensive gravel banks, in which blocks of shale and tuff containing marine fossils are quite common.
Return to Cambewarra by following Tannery Road back to the bitumen. Follow Hockey’s Lane, directly opposite, until the ford on Tapitallee Creek is reached.
Stop 4. This creek, the main tributary of Bomaderry Creek, carries the same type of gravel as the latter. The bed of the stream has frequently changed over the years and there are terraces, swamps, and stranded gravel deposits to illustrate this.
Continue to Illaroo Road (bitumen), turn left and proceed towards Nowra for 1 mile. Turn right near the water tower and continue along this road until a cross road (McMahon’s Road) is reached (¾ mile). Turn right, then left after ½ mile. This road leads down to the river at Humbug Reach, where permission to enter must be sought from the landowner.
Stop 5. The river takes a sharp bend, turning back on itself and leaving only a narrow neck of land across the meander. In flood, the river uses this as an alternative route, but because of the tidal nature of the stream at this point, it is not likely to adopt this course permanently.
Return along McMahon’s Road to Nowra, turning right at Illaroo Road. A fine view of the river may be seen from near the power line. Turn right to the golf links, a short distance from the Prince’s Highway.
Stop 6. A good section of the Nowra Sandstone is seen here and
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| The Nowra Sandstone at the Golf Links April 1969 |
may be studied in detail. It is practically complete; the Wandrawandian Siltstone, although not visible, would outcrop behind the golf links. Note the cross bedding and the steeply dipping joints.
Excursion 3e
Nowra Hill and Falls Creek (17 miles return)
This short excursion covers a wide range of stratigraphic and erosional features, and is probably the most profitable of all the excursions based on Nowra.
Route: Nowra Showground-South Nowra-Falls Creek-Nowra Hill-Nowra
Stop 1. The Showground is situated at the top end of Junction Street, beyond the shops and the Council Chambers. At the Hanging Rock (now fenced to avoid accidents) a fine view of the drowned river valley may be had. Other points of interest include the Nowra Sandstone (similar to the golf links outcrop), the levee on the left bank of the river, and the dip slope of the sandstone on the opposite bank towards the bridge.
Nowra Creek, which enters the Shoalhaven on the left below the showground, is tidal for 1 mile upstream. Within this tidal part is an isolated group of mangrove trees, miles from their nearest neighbours. They may be relics of the days when the river was more saline than at present.
Proceed to the Prince’s Highway and go south as far as the brickworks, on the right through South Nowra. Obtain permission before entering.
Stop 2. The brick pit is in black siltstone of the Berry Formation, and the deep soil formed from this rock type is clearly visible. Solid unweathered siltstone is found only in the deepest part of the pit, at the water table. Above this level, the rock is considerably altered, at most to reddish clay.
Numerous crinoid stems and other rare fossils may be found here, but are mostly very weathered. The best place to look is in the quarry floor, where they are least altered.
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| Dyke cutting through the Berry Formation at South Nowra brick pit August 1967 |
There are several reverse faults in the face behind the pool and these may be traced to the opposite side. A small, very weathered dyke of vesicular basalt runs parallel to these faults, its course no doubt being influenced by their’s.
Continue south to Falls Creek. At the Currumbene Creek bridge,
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| Parma and Currumbene Creeks February 1965 |
Berry siltstone outcrops in road cuttings and in the creek bed, where there are numerous marine fossils. Stop on the left side of the highway between the school and the Parma Creek bridge. It is a short walk through the bush to the falls.
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| The Falls at Falls Creek June 1963 |
Stop 3. The falls owe their origin to a bed of siltstone within the Nowra Sandstone, which allows the rock above to be undercut permitting gradual retreat of the lip. The lower part of the Sandstone outcrops downstream, and 500 yards below the falls tidal water is reached.
The easiest descent is on the left side of the smaller fall, from the bottom of which the undercutting siltstone may be followed beneath both falls, and the rock ascended on the opposite side of the pool.
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Behind the Falls at Falls Creek June 1963 |
There are numerous pot holes in the sandstone, both in the present bed and above it. If the water level is low, most of it will be seen to vanish into a prominent open joint, emerging a short distance away.
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| Pot holes in the bed of Parma Creek February 1965 |
The sandstone here is typical of most outcrops. Few, if any, fossils occur and the most prominent feature is the number of small erratics of metamorphic rocks.
Return towards Nowra. About 2 miles from Falls Creek take the road on the left leading to Nowra Hill and HMAS ‘Albatross’. After 1 mile across level country the road ascends Nowra Hill. Outcrops of siltstone are seen from this point on. Park just beyond the highest point on the road, from where a track leads to the summit.
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| View from Nowra Hill June 1963 |
Stop 4. The view, especially if seen from the summit, is very extensive. Nowra Hill seems to be merely a residual of the Berry Formation, with no particular structural origin.
Blocks of grey siltstone along the track past the gate are richly fossiliferous. Do not destroy these, since this only a small outcrop and of considerable interest. More fossils may be found in the nearby road cutting.
Continue through the Naval Air Station, and then turn right towards Nowra. The road descends after a short distance and few outcrops are seen from this point on.
Questions
1. Discuss the probable origin of the main features of the Shoalhaven River flood plain. 2. Describe the most common sedimentary structures observed in the Nowra Sandstone. 3. List geological reasons for the growth of Nowra at the expense of towns on the flood plain. 4. Give an account of the various structures observed in the brick pit at South Nowra.
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Fifth Form Nowra High School Geology Excursion July 1966 |
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