FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT (PART 19)

Chapter 11  Tianjara and the Little Forest Plateau   

Among the landmarks of the Shoalhaven district is the Tianjara group of mesas. They lie about 5 miles south of the Braidwood road, near Tianjara Falls. Their flat surfaces, bounded by cliffs and steep slopes make them obvious from all directions, but particularly from the north. They are easily visible from Bundanoon, Saddleback, and Durras Mountain. 
Extensive outliers of the Berry Formation cap the coastal hills south from Tianjara Falls to the Little Forest Plateau. This is uninhabited country, used as a military practice area and out of bounds from time to time. The sediments of the Berry Formation are rather coarser than those further north. The presence of the mesas may be due to the peculiar resistance of the strata in this area. Some geographers have suggested that they may be relics of a former land surface, uplifted and largely removed before the more recent uplift which has been responsible for such features as Tianjara Falls and the Ettrema Gorge. 
The views of the Clyde River gorge, the surrounding sandstone cliffs and the panoramas of beaches and coastal lakes make this good bushwalking country. There is a minimum of undergrowth on the higher land, but the valleys tend to be thickly vegetated. 
Excursion 11a 
Tomerong-Tianjara- Rotary Lookout (34 miles) 
The Tianjara mesas and spectacular examples of cliff recession are the main features of this excursion. The road may not be passable south of the mesas; however the features to be seen at the other end can be approached via the Pointer Gap just as easily (see Excursion 11b). 
Route: The Braidwood road from Tomerong to a point 0.8 miles before Tianjara Falls, then south to the Porter’s Creek Dam road, near the Rotary Lookout. 
Take the Braidwood road from Tomerong. The road soon rises from the well timbered Wandrawandian Siltstone country through the Nowra Sandstone onto the Berry Formation. Some 2 ½ miles further west, the Parma Monocline is ascended. Sandstone is seen for a short distance, but the road continues on the Berry Formation at an elevation of 600 feet for several more miles until it reaches Flat Rock Creek. 
Stop 1. The Nowra Sandstone outcrops here in the creek. It shows well developed cross bedding. One mile downstream, the creek leaves the swamp country by way of a waterfall and joins Parma Creek in the upper part of Hell Hole (see Excursion 8a, Stop 3). 
The Wandrawandian Bore, sunk in 1890 in search of coal, is located near Condy’s Creek, about 1 mile south of the bridge. 
Continue towards Braidwood. At the Turpentine Corner, the road joins the Braidwood road from Nowra. The Berry Formation is widespread along the ridge and is responsible for some very fine forest land along the Turpentine Range. Shortly after the corner, the Dean’s Gap fire road leads off on the right towards Yalwal (see Excursion 6d). 
A distant view of Jervis Bay may be seen from the road 4.6 miles past Turpentine. Gravel pits along the road are extracting iron enriched sandstone and weathered siltstone. Several fire roads on the left in this vicinity lead, by way of steep descents, to the Prince’s Highway near Wandandian.  
A diversion may be made along a fire road leading off to the right 5.6 miles past Turpentine. It follows the sandstone ridge separating Yarramunmun and Boolijah Creeks and eventually divides, giving access to both valleys. In both cases, after a descent of 800 feet, slate is found unconformably underlying the Permian sediments. 
The Old Wool Road branches to the left 6.9 miles from the Corner. It leads down to Wandandian by way of a complex series of logging roads (see Excursion 10a, Stop 1). 
Views of the main Tianjara mesa are seen ahead soon after this, and the road branches off the Braidwood road 9.1 miles from Turpentine and O.8 miles before Tianjara Falls. 
The left hand branch 4 miles from the Braidwood road leads to Jerrawangala Lookout (The Twelve Mile Road: see excursion 10a, stop 2). Keep to the right and stop after the road has ascended the steep side of Kangaroo Hill. 
Stop 2. This is the smallest of the mesas, reaching just over 2,300 feet above sea level. The silty sandstone outcropping here contains a few marine fossils. A good view of the whole countryside may be had from the top of the hill.  
One mile further south, the road approaches the largest mesa. 
Stop 3. The climb to the summit is only about 300 feet, but the view is greater than might have been expected. In the north, the gradual slope towards Nowra is most remarkable while the Clyde River gorge is just a short distance away to the west. Outcrops of silty Berry Formation sandstone are common near the top of the hill. 
The highest mesa, on which Tianjara trig stands (2,520 feet), is a short distance further south. Stop where the road reaches its highest point just west of the hill. 
Stop 4. The land surface is very swampy and gives the impression of being a ‘fossilised landscape’ much older than the nearby canyon of the Clyde. A view of the Clyde may be had by walking 1 ½ miles west from the road. It was in this part of the valley that coal was discovered about 1886. This locality may be reached by way of a gully 1 mile west of Tianjara Trig. The Coal Measures occupy the river bed for a short distance and several tunnels have been driven into coal seams by prospectors. 
The Clyde Coal Measures are first encountered in the river bed at the foot of a small waterfall. The Shoalhaven Group sediments overlie the Coal Measures with a noticeable angular unconformity. Recent investigations indicate that the Clyde coal seams are older than the Greta Coal Measures in the Hunter Valley, with which they were formerly correlated. 
Permian plant fossils (Glossopteris, Gangamopteris and Noeggerathiopsis) occur in the shales below the unconformity. A thickness of 135 feet of sediment lies below this point before the basement rocks are met at an elevation of 1,260 feet. Numerous plies of coal, including a seam 3 feet thick, may be seen by following the river downstream. An adit has been driven on this seam for about 40 feet. Elsewhere, other tunnels have been driven to prospect the coal, which is generally of good quality. The variable thickness of the seams and the unpredictability of their extent made mining an impossibility. 
Return to the Tianjara road and continue south from the mesas. Three miles further on, the road reaches a narrow defile with steep slopes on either side.  
Stop 5. This is the Second Devil’s Pinch. Claydon’s Creek is encroaching from the west and the head of Conjola Creek is eating away the plateau from the east. Large open joints show the process by which the cliffs retreat. 
The Clyde River valley and The Castle June 1966
Views of the Clyde River and The Castle open up as the road approaches a second defile, where a further stop may be made. 
First Devil's Pinch April 1968
Stop 6. At the First Devil’s Pinch, prominent open joints are very obvious on both sides of the road. On the eastern side, the slope drops 1,600 feet to Conjola Creek in a distance of 2 miles. The Conjola Copper Mine is in this vicinity. 
Several miles further south, the road meets the Porter’s Creek Dam road (Excursion 11b). Turn left and then right to the Rotary Lookout. 
Stop 7. From the Lookout there is an extensive coastal panorama. Of particular interest is the large area of cleared ground where the Milton Monzonite intrusion occurs. Its better soils are in marked contrast to those typically formed from the Permian sediments. The Pointer Mountain, part of the intrusion, rises prominently one mile to the east.  
The sandstone scarp may be descended on foot by way of Egan’s Gap, ¾ mile along the cliff top. From here a track leads down to Little Forest Road and so to the Prince’s Highway 
(7 miles from Rotary Lookout). 
Milton intrusion from Little Forest June 1966
Excursion 11b 
The Little Forest Plateau (18 miles return) 
The whole of the Shoalhaven Group from the base to the Berry Formation may be examined on this excursion. In addition, a small part of the Milton intrusion and outcrops of the Clyde Coal Measures and basement rocks are seen. 
The views from the Plateau are very worthwhile and there is an extensive area of good bushwalking country available which does not involve strenuous climbing. 
Route: Porter’s Creek Dam road from the Prince’s Highway. Turn west from the highway, 5 miles north of Milton. 
Stop 1. Outcrops of a basic rock occur here. They may be part of a dyke or of the Milton intrusion. The level surface of the Little Forest Plateau is very marked from here. Further north, the Tianjara mesas are prominent. 
Proceed towards the mountains. Within a mile, the road turns sharply to the left and then to the right. An intrusion of monzonite outcrops alongside the road and on the hillside above. 
Stop 2. The monzonite forms part of a large intrusion, which outcrops north, west, and south of Milton. The intrusion is evidently a multiple laccolith and this is part of its northern outcrop. 
The rock itself is typically porphyritic, the phenocrysts being of andesine or labradorite felspar. 
A short distance further on, the road descends to a tributary of Bunnair Creek.  
Stop 3. A small inlier of Ordovician slate occurs just upstream from the bridge and may be reached by following the creek or by turning left off the road 200 yards up the next hill. 
Continue up the hill. Turn right along the forestry road (Calgaroo Road) 500 yards further on. Stop where the road reaches the bottom of a hill a short distance from the Porter’s Creek road. 
Stop 4. Bunnair Creek is a short distance below the road on the left. Several exposures of Coal Measures sediments occur in the creek bed, associated with an inlier of slate. The lowest outcrop contains several seams of coal, up to 15 inches thick. Upstream, a second exposure in the creek bed is surrounded by slate, but contains no coal. A third exposure lies even further up, in the bed of the left hand branch. This locality can also be reached from the Porter’s Creek road, ¾ mile from Calgaroo Road. Coal and carbonaceous shale form the bulk of this small outcrop. 
The discontinuous nature of these Coal Measures exposures suggests that they formed in scattered small basins only and that minable coal is not likely to exist. It is possible that the basins may have been depressions in a glaciated plain, rather like the peat bogs of Canada and other areas recently covered by ice. A rise in sea level eventually submerged the whole plain and the marine sediments of the Shoalhaven Group began to form. It appears that this submergence began first in the south and west of Milton before gradually covering all the low lying ground. 
Continue towards the plateau. The road gradually ascends through the Conjola Formation and then rises more sharply through the Wandrawandian Siltstone. Several enormous blocks of Nowra Sandstone lie on their sides by the road and finally the Sandstone is reached at a point where the gradient of the road is quite steep. Stop at the crest of the hill, just past a large isolated block of sandstone on the right. 
Stop 5. This locality is correctly called The Pointer Gap but the name ‘Porter’s Gap’ has come into use in confusion with Porter’s Creek, a mile or so to the west. 
The road builders excavated a large quantity of fossiliferous sandstone and siltstone. Many fossils may still be found on the rubble-strewn slope below the road. Because of the unstable nature of the rock pile, this place is not recommended as a collecting ground. Some superb marine fossils were collected here during the original excavations. 
The Nowra Sandstone may be studied here to advantage. It shows the usual cross bedding towards the top, but the siltstone towards the base is not always seen elsewhere. It is possible that the prominent siltstone outcrop alongside the road corresponds to exposures towards the base of the Sandstone at Nowra, Parma Creek, and Yerriyong. Further sandstone underlies it before the Wandrawandian Siltstone is met with a little lower down the mountain. 
A little past Pointer Gap, the road to the Rotary Lookout branches off to the left (see Excursion 11a, Stop 7 for details). Turn left at the next branch. The other road leads to the Devil’s Pinches and Tianjara (Excursion 11a). A little over 1 mile further south, the road swings to the right towards the dam. At this point, turn left along a track which goes almost to the cliff edge a short distance away. 
Stop 6. Little Forest Trig., elevation 1,832 feet, affords extensive views and particularly points out the fertility of the Milton district. In the far south may be seen Durras Mountain with Mt Dromedary near the horizon. Immediately below lies the dairying country based on the Milton Monzonite. The roughly circular nature of the outcrop around Milton is best seen from here.  
The cliff may be descended 500 yards south of the Trig. station. A vehicular track leads from the Trig. station track. It goes to the southern edge of the Little Forest Plateau, 2 miles further south, where the view of the Pigeon House from the cliff is probably the best available. The deep gorges of Jindelara and Pigeon House Creeks add to the grandeur of the scene. 
Proceed to Porter’s Creek Dam. 
Porter's Creek Dam Under Construction March 1967
The water of Porter’s Creek is fed through a tunnel into a pipeline which then descends to supply Milton and Ulladulla. The dam wall is 50 feet high and 812 feet wide. It is built into solid sandstone and is unusual in that it stands within a few hundred yards of a 150 foot cliff. 
Walk downstream along the right bank of the creek. The view of the falls and Mount Talaterang in the background is well worth the effort. The cliff is of typical Nowra Sandstone and there is little chance of descending it in this vicinity. 
Mt Talaterang April 1968
Mount Talaterang, elevation 2,585 feet, is an extensive outlier of the Nowra Sandstone and Berry Formation. There is a thickness of at least 500 feet of cliff forming Berry sandstones on the highest parts of the mountain. It is possible to ascend Talaterang from the northern end of the mountain, but the climb is very arduous. From Porter’s Creek, it would be necessary to skirt the cliff top for 5 miles before the ridge leading to the mountain is reached. A better route would be via the Tianjara road to a point ½ mile south of the First Devil’s Pinch, then in a south westerly direction. From the summit, the land drops over 2,000 feet into Pigeon House Creek in less than a mile. The Clyde River canyon and The Castle group of monoliths are prominent in the west.  
The Pigeon House from Little Forest August 1966
Questions 
1. Discuss the nature and possible origin of the Clyde Coal Measures. 
2. Give an account of the Milton intrusion as seen on this excursion. 
3. Why was the Milton water supply dam built on Porter’s Creek rather than on one of the more accessible coastal streams, such as Conjola Creek? 
4. What is the possible origin of the Tianjara mesas?  
5. Compare the Berry Formation sandstone with the Nowra Sandstone. 

Song Studies. Bible studies based on hymns and songs
Shoalhaven District Geology.

Comments

Post a Comment

Popular posts from this blog

Image
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,...
Read more
Image
FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT  NSW AUSTRALIA (Part 1)   John G Paix 1970 (Reviewed 2013) ©  Australian Field Geology Club visit to the Cambewarra Range February 1965   A Word or Two of Explanation This book was written in the years 1968-1970. At the close of my time as a Science teacher at Nowra High School in 1967, I undertook to write a report titled " The Geology of the Shoalhaven Shire " for the then Shoalhaven Shire Council, which was published by them early in 1968.  It soon became apparent that there was much additional information, especially relevant to the Geology excursions I had conducted during my 5 years in Nowra, which was not included in that book.  What you see in front of you today was prepared for publication over the next three years and I fully expected to see it published in 1970. For reasons that do not matter at this distance in time, this never happened. I was left with a typed, proof-read copy, all the maps, drawings, p...
Read more
Image
FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT (PART 23) CHAPTER 15: THE CLYDE RIVER VALLEY T aking its rise at an elevation of 2,500 feet near Sassafras, this small river flow south and descends by way of a great canyon to a level of 200 feet at Yadboro. It does this in less than 20 miles, yet it is a further 40 miles to the sea downstream. This is a peculiar result of the dip of the Shoalhaven group sediments. At one time, these must have extended further south than at present. When the area was uplifted, it was tilted to the north, so that the ancestral Clyde River had to erode its bed against this trend.  This undoubtedly led to the formation of many waterfalls and the evolution of the canyon upstream from Yadboro.  The southern edge of the sandstone scarp presents a bold face and is broken into numerous outliers. Some such as The Castle and the Pigeon House are well known to bushwalkers, but there are many others. They all have a similar structure, being capped by cliffs of the...
Read more