FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT (PART 15)
Ettrema and Beyond – the Great Gorges. Excursion 9a
One must never underestimate the erosive power of running water. Almost as fast as the uplifting process begins, many streams large and small begin to level the land again. Nowhere is this better seen than in the gorge country west of Nowra, where scrubby sandstone plateaux give way to great canyons, and tinkling waterfalls demonstrate the ceaseless battle between the two.
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| Ettrema Gorge January 1969 |
From sea level at Nowra, the sandstone surface rises, almost imperceptibly, to a height of 2,500 feet in the west. The Shoalhaven River and its tributaries have removed this resistant layer along their courses and exposed the older rocks below. Unlike Kangaroo Valley, these gorges have little to attract the settler since their few level areas are comparatively infertile and access is difficult indeed. It has taken the discovery of minerals – gold, silver, and lead – to cause man to descend into them.
Much of the higher land remains undissected, still bearing its shield of Nowra Sandstone or even part of the overlying Berry Formation. These in turn have been further protected from erosion by a covering of basalt, poured out in a time before the great gorges had formed and now about to be carried away to the sea by streams which may not have then existed.
Settlement is sparse in the area bounded by the Shoalhaven River, Danjera Creek, and the Braidwood road. In all of this country, an area of hundreds of square miles, there are only four or five isolated farms. The rest of the landscape is a confusion of bare rock, sandstone cliffs, and wild valleys – a scenic area well worthy of preservation in the form of a National Park.
Beneath the Permian sediments lies an interesting series of older Palaeozoic rocks. These have already been met in Chapter 6, where the rocks at Yalwal took the form of granite and an Upper Devonian sequence. Similar rocks, as well as older Silurian and Ordovician sediments, are found in all the valleys west of Yalwal. This sequence of older rocks was eroded to a near sea level surface in early Permian times and later buried by the marine sediments we call the Shoalhaven Group. Much of the land surface west of the Shoalhaven River, for example near Bungonia, appears to be this old surface exhumed from beneath the Permian sediments.
Because of the rough nature of the country, roads are necessarily few, and this restricts the places of interest which may be reached by car. The grandeur of the scenery may only be appreciated by those willing to walk. It is likely that no-one has ever set foot on some of these remote hill tops, nor ever examined the rocks in some of the deep valleys, so there is much to attract the curious visitor. However, some words of caution: this is dangerous country for inexperienced hikers, and the possibility of rockslides and flash floods is always present. Only those who are physically fit and sufficiently experienced should leave the beaten track.
Excursion 9a
The Sassafras Tableland (27 miles return)
The Braidwood road, which may be traversed from either Tomerong or Nowra, follows the divide between the Shoalhaven tributaries on the north and those of Wandandian Creek and the Clyde and Endrick Rivers on the south. Its surface, essentially that of the Nowra Sandstone, rises gradually from near Nowra Hill (200 feet) to Sassafras (2,500 feet). Basalt outliers are dominant in the west; otherwise the surface is formed from the Nowra Sandstone or the Berry Formation.
Route: Tianjara Falls via the Braidwood road to the descent at the western edge of the plateau.
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| December 1963 |
Stop 1. Tianjara Falls is the only place along this road where the encroaching valleys may be examined with ease. The water drops about 100 feet over a cliff of Nowra Sandstone, at which point Tianjara Creek changes its name to Yarramunmun Creek. Some aspects of the geology of the older rocks exposed further down the valley were studied in Chapter 6.
Typical potholes are exposed just above the falls; some of the water (most of it in dry seasons) vanishes into one of these and emerges some way down the cliff face. The creek above the falls is flowing in an old valley which apparently predates the present cycle of erosion.
The cliff top on the right may be followed for 1 mile before it veers to the east. A descent of 800 feet into the valley at this point will reveal typical Ordovician slates beneath the Permian strata. On the western side it is possible to descend with care at several points not far from the falls. It is a hike of 15 miles along the top from here to Yalwal. Care should be taken to keep to the ridge between Yarramunmun and Boolijah Creeks to avoid unnecessary detours. A fire road extends along part of this route, and this provides magnificent views of the valleys on both sides.
Proceed towards Braidwood. After 1 ½ miles, the road crosses Boolijah Creek, here not much more than a swampy watercourse. It is, in fact, the main headwater of Danjera Creek. There seems to be good evidence for the theory that Danjera Creek has captured part of the upper Clyde River. The whole of this plateau is besieged on all sides by encroaching valleys. Those on the coastal side appear to be the most effective, since their streams flow down the dip of the sandstone.
Stop at Stony Hill, the only prominent hill seen along this road, 4 miles from Tianjara Falls.
Stop 2. Silty sandstone of the Berry Formation outcrops here. The rock differs noticeably from the Nowra Sandstone and also from the typical Berry siltstone seen near Nowra. Evidently the larger particles brought into the Permian sea at this stage were not carried more than 10 miles from the shore, with the result that there is a gradation within the Formation from sandstone in the west to siltstone in the east.
Just past Stony Hill, the road ascends the Sassafras basalt outlier. The change in soil type is obvious enough, and with it goes a change in vegetation. Stop 0.6 miles past Stony Hill, where basalt outcrops are seen by the roadside.
Stop 3. This is a typical olivine basalt. It was probably poured out as a flow fairly late in the Tertiary period. This event may have occurred before the uplift and certainly before the Shoalhaven Gorge was carved. Some geologists have suggested that a fault or monoclinal fold has elevated the country around Sassafras. Although this has not been substantiated, there does seem to be a change in the dip of the strata in this vicinity.
Beyond Sassafras, where rich farmlands provide a welcome change from the monotonous sandstone country, the road passes over similar terrain for 5 miles until the Touga road is reached. Excursion 9b leaves here. Some 1.3 miles further on, the road begins to descend to the Endrick River. Stop where the road drops beneath cliffs of sandstone on the right.
Stop 4. This marks the western edge of the Shoalhaven Group sediments. The Nowra Sandstone is similar in most respects to its outcrops further east and cross bedding is quite common. Marine fossils appear to be absent, however.
A short climb and walk to the west will reveal a view of the Endrick River valley, the Shoalhaven River, and the distant hills towards Goulburn. South of the road is the valley of Bainbrigge’s Creek, a tributary of the Endrick River.
Excursion 10a commences here, but otherwise the same route must be followed to return to the coast.
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| Ettrema Gorge November 1967 |
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