FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT (PART 6)
Chapter 3: The Nowra District
Nowra is strategically built on the first flood-free site upstream on the south bank of the Shoalhaven River. It was not an important settlement until about 1880, when Terara was extensively flooded. Soon after, the coming of the railway and the construction of the present road bridge assured its development as a rural centre. Today, it is a rapidly growing town, with developing industry, tourism, and defence establishments.
Several factors dominate the geology of the area. The Nowra Sandstone has provided a resistant surface, broken only by the Shoalhaven River and a few of its tributaries. Where this sandstone has dipped below sea level at Nowra, an extensive flood plain has developed, the origin of which is of considerable interest and will be explained later.
Coolangatta Mountain is the only unusual feature in the area. It is really an outlying part of the Cambewarra Range, which it resembles geologically. It is a local tradition to consider it to be an extinct volcano, perhaps because of its shape and the volcanic rocks found near its summit.
Excursion 3a
Nowra-Shoalhaven Heads (30 miles return)
Much of this excursion is through alluvial country; otherwise siltstones of the Berry Formation are dominant.
Route: Nowra-Bomaderry-Bolong-Coolangatta-Shoalhaven Heads-Berry-Nowra
Take the Prince’s Highway north from Nowra. Turn right along Bolong Road towards Shoalhaven Heads. The Nowra Sandstone, seen at the river, gives way to recent sediments along Bolong Road. Several modern factories are passed and the road approaches Coolangatta Mountain over miles of river flats. Broughton Creek was formerly used by small ships moving upstream to Berry. Just past the bridge (right hand branch) extensive quarries are seen on the left.
Stop 1. The Berry Formation siltstone seen here is very fresh and unweathered. Fossils however, are rare. The siltstone has been used as road metal and as a filling to prevent river bank erosion.
Proceed to Coolangatta. Much of this land is easily flooded. Old buildings, part of the original Berry estate, are seen after the road leaves the river. ‘Coolangatta’ was first settled by Alexander Berry in 1820. For many years the Berry family managed the estate, draining waterlogged meadows and clearing the slopes. The original homestead was destroyed some years ago but many interesting relics remain and may be inspected by arrangement with the owner.
Stop opposite the turnoff to Shoalhaven Heads.
Stop 2. Coolangatta Mountain (992 feet) may be climbed quite easily from this point, but permission must first be sought from the landowner. Siltstone and tuff of the Berry Formation compose the bulk of the mountain, with a small remnant of the Bumbo Flow at the summit. The view is extensive, but partly obscured by thick vegetation.
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| The view from Coolangatta Mountain December 1961 |
Proceed to Shoalhaven Heads. Turn left at the river bank.
Stop 3. In recent years, the river mouth has silted so much that the bar has become a beach closing the Heads completely. The water now takes the longer route via The Canal, entering the sea at Crookhaven Heads (see Excursion 3b). Floodwaters now take longer to drain and this is a problem which must be resolved before very serious flooding occurs.
Return to Coolangatta and turn right towards Berry. The few outcrops along this road are all of Berry Formation siltstone. The extensive swamps on the coastal side are due to the sand hills behind Seven Mile Beach preventing normal drainage. The hill behind Farmeadow is capped by an outlier of the Kiama Sandstone.
Turn left at the Prince’s Highway. From here to Bomaderry, the underlying rocks are all Berry Formation siltstone. Six miles from Berry and 500 yards past Wiley’s Creek, some river gravel is seen in a cutting on the right.
Stop 4. This represents the former course of a stream flowing from the Cambewarra Range. The gravel is typical of the present stream beds and may be the remnants of a former bed of Wiley’s Creek. Boulders in nearby paddocks indicate more of its old course.
Return to Nowra. Stop at the park to the right of the bridge on the Bomaderry side.
Stop 5. These outcrops are typical of the Nowra Sandstone, which is most often a gritty, quartz sandstone with some cross bedding and layers of marine fossils. These may be found in the cliff face below the park and in the road cuttings towards Bomaderry Creek.
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| Nowra Sandstone at the Golf Links April 1969 |
Excursion 3b
The Shoalhaven flood plain (30 miles)
Features of the flood plain and the development of the present system of distributaries are the main points to be studied on this excursion.
Route: Moss Street Nowra-Terara-Numbaa-Comerong Island-Numbaa-Brundee-Pyree-Greenwell Point-Nowra
Leave the Prince’s Highway on the northern side of the shopping centre. Turn left and proceed towards Terara. Stop at the High School on the right. The school has a permanent display of local rock specimens and geological maps. Permission should be sought from the Principal if you wish to view these.
Stop 1. The school grounds, or the hill behind, allow good views of the flood plain and these will assist in explaining its origin.
On the northern or river side, a small levee bank, on which building development is taking place, flanks the river. It appears to be above the present flood level. From the bridge at Nowra, the river leaves its meandering course and flows almost straight to the sea at Shoalhaven Heads. It would appear that, during the Quaternary Ice Age, the sea level fell on a world-wide scale by up to 400 feet. This allowed many streams to incise themselves into valleys that are now below sea level. The Shoalhaven was apparently confined in a sandstone gorge, the partly submerged remains of which are visible above the bridge. When the sea level rose towards its present level, the valley was drowned. Evidently the sea level, only 10,000 years ago, exceeded its present level by 10 feet or more and all of the plain east of Nowra was submerged. The former valley was gradually filled with sediment and when the sea dropped to its
present level, the river took a more direct route to the sea. The filled valley became the site of a series of lagoons, now mainly swamps.
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| Nowra High School during a flood June 1964 |
Evidence for these hypotheses has come from wells and borings which have brought up shells and estuarine sediments from beneath the plain. At least 200 feet of sediment lies on the bed of the river at Nowra. Similar histories could be written for Port Jackson and Botany Bay.
Proceed towards Comerong Island. Note the flood drain from Worrigee Swamp. Terara and Numbaa are only small communities
today, but were prosperous in the early days before severe flooding compelled their partial abandonment.
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| Shoalhaven River at Numbaa August 1963 |
Stop 2. The Canal began as a trench dug in 1821 by Berry’s convicts. Its purpose was to provide a safe entry into the Shoalhaven River by way of the Crookhaven River. Today, the river uses this route in preference to its former course and has greatly enlarged the Canal. This is causing serious problems in flood prevention and bank erosion.
Comerong Island, across the Canal, has a long ocean frontage which is really an extension of Seven Mile Beach. Much of the island is unsuitable for dairying because of the high salt content of the soil.
Return towards Nowra and turn left at Jindy Andy Lane (2 ½ miles from the Canal). Turn left again when the Greenwell Point road is reached. Flood drains in this vicinity have turned up thousands of estuarine shells; the whole area is barely above sea level.
Continue to Greenwell Point.
Stop 3. The Greenwell Point ‘island’ is one of a number of hills rising above the flood plain. A bore put down during an oil search penetrated 390 feet of silty sediments, perhaps the lower part of the Berry Formation as seen at Coolangatta. Occasional marine fossils are found here in surface exposures. From Greenwell Point, several other ‘islands’ of sedimentary rock may be seen nearby. Their presence here seems to be related to the fact that the Permian sediments have been uplifted by a broad anticline extending from Point Perpendicular.
Return towards Nowra, but turn left at Jindy Andy Lane. Stop at Crookhaven Creek, 400 yards further on.
Stop 4. Salt water extends 8 miles from Greenwell Point to these
flood gates. Upstream an occasional mangrove struggling for survival shows that, in historic time, tidal water penetrated inland at least another 3 miles – possibly into Worrigee Swamp.
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| Worrigee Swamp during a flood June 1964 |
Continue towards Nowra. Three miles further on, at Worrigee, a group of houses and an old quarry are seen on the right.
Stop 5. A dolerite dyke is exposed here and was formerly quarried for use as road metal. It has a north-south strike in contrast to the east-west strike of most known South Coast dykes.
The road on the left just past the quarry leads to a council shale quarry, where occasional marine fossils may be found.
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