FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT (PART 8): THE CAMBEWARRA RANGE AND EXCURSION 4a
Chapter 4: The Cambewarra Range
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| Sunrise from Cambewarra Lookout November 1965 |
Cambewarra Lookout is renowned for its superb views of the rich dairying country below. The aboriginal word ‘Cambewarra’ is believed to mean ‘smoke coming from mountain’. Many people have therefore assumed that the mountain on which the lookout is situated is an extinct volcano, and its shape seems to support this belief. In fact, this shape is due to a small capping of sandstone, this having preserved the mountain’s flat top. The word probably refers to the fact that mist frequently pours over gaps in the range from Kangaroo Valley, giving the impression of smoke. Also contrary to common belief, the lookout is actually situated on Good Dog Mountain; Cambewarra Mountain is south of the Moss Vale road.
When viewed from below, these mountains give an appearance of
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| Cliffs of Hawkesbury Sandstone. Broughton Head (Berry Mountain) from Broger's Creek May 1964 |
solidity and permanence, but from their summits they appear in their true light – a remnant of the Robertson Plateau cut off from it by the broad sweep of Kangaroo Valley. In places the crest of the range is more than two miles across, narrowing to only a few yards in others. The geology of the range is very similar to that seen further north at Kiama and the Barren Grounds. The backbone of the whole mass is its capping of Hawkesbury Sandstone, the top of which lies at an altitude of about 2,000 feet throughout. The highest points lie on Cambewarra Mountain (2,050 feet).
Erosion has removed much of this capping, and sizable residuals of it are found only at Broughton Head (Berry Mountain), Kangaroo Mountain, Cambewarra Mountain and Mt Scanzi, with several smaller ones. The gaps between these residuals provide access into Kangaroo Valley, and roads enter the valley through four of them. It is impossible to consider the geology of the Cambewarra Range without reference to Kangaroo Valley, and much of the information outlined below should be referred to when considering that district.
Beneath the Hawkesbury Sandstone lies the Illawarra Coal Measures. The Shoalhaven region lay near the southern end of the Sydney Basin when the Coal Measures were being deposited and they thicken from south to north – from 40 feet at Cambewarra Mountain to 500 feet at Woodhill. Mineable coal is not present, but inferior coal and outcrops of lithic sandstone and reddish shale are common.
Most of the range is carved out of the Berry Formation. This includes a considerable thickness of volcanic rocks, which are considered in detail in Chapter 2.
From top to bottom, the Berry Formation is subdivided in this area as follows:
Cambewarra Flow 500 feet}
Broughton Sandstone 500 feet} (Gerringong Volcanics) and Berry Siltstone 800 feet
This comparatively simple arrangement is complicated by the fact that the volcanic nature of the Formation increases from south to north, towards the Kiama district. Thus, at Woodhill, at least two other flows occur, while south of Good Dog Mountain the name Budgong Sandstone replaces the Broughton Sandstone as its volcanic fraction ceases to be dominant.
Beneath the Berry Formation lie older sediments of the Shoalhaven Group. These are only seen in lower Kangaroo Valley and Budgong Creek.
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| Hawkesbury and Budgong Sandstones on the eastern side of Kangaroo Valley. November 1967 |
Excursion 4a
Berry-Woodhill (17 miles return)
This excursion shows the following features:
The stratigraphy of the Berry Formation and Illawarra Coal Measures
Permian marine and nonmarine fossils
Coastal landforms and a young deep V-shaped valley.
Route: Berry-Broughton Vale Road-Broughton Vale and return-Woodhill Gap-upper Broger’s Creek-Berry
The Broughton Vale road leaves the Prince’s Highway at Broughton Mill Creek. For the first mile, the road crosses recent alluvial deposits. Both Broughton Creek and Broughton Mill Creek are subject to extensive flooding. Tidal influence extends up the creeks almost as far as the highway, below which are broad flats criss-crossed by flood drains.
Turn right for Broughton Vale 1 ½ miles from the highway and proceed to Broughton Mill Creek.
Stop 1. The creek carries gravel which gives a fair indication of the rock types found upstream. Such types as vesicular and porphyritic latite, coal, tuff and sandstone are common. Careful search will also turn up pieces of agate, jasper, and petrified wood. The rock outcrops on the hill nearby are of the Broughton Sandstone. The view to the north shows clearly sandstone cliffs of Triassic and Permian age.
Return to the Woodhill road and turn right towards the range.
Stop 2. Weathered siltstone of the Berry Formation is seen in cuttings on the left of the road one mile further on. Bedding planes and vertical joints are prominent.
Stop 3. Outcrops of Broughton Sandstone are frequent in road cuttings on the right. Spheroidal weathering is very obvious, while fossils to be found include small molluscs, brachiopods and fragments of polyzoa.
The view to the south is excellent. Immediately behind Nowra looms Nowra Hill, while Coolangatta is clearly seen against the sea to the east. In the far south, the characteristic flat top of Tianjara and the top of the Pigeon House are visible.
Stop 4. A roadside quarry on the right is cut into pink tuff, here equivalent to the Jamberoo Sandstone. It is somewhat weathered and contains numerous pebbles of igneous rocks. Few marine fossils are to be seen, but traces of plant fossils are found high in the quarry. Evidently the transition from marine to non marine conditions was very gradual and did not coincide with the close of volcanic activity. The fact that occasional marine fossils are found even above the Cambewarra Flow suggests that the sea periodically returned before retreating finally when Coal Measures sedimentation began. For this reason, the top of the Cambewarra Flow is taken to be the top of the Shoalhaven Group, since it marks a precise instant in time.
Boulders of igneous rock found in this quarry are derived from the overlying flow.
The line of boulders which can be seen on the southern side of the road, at a slightly lower level, represents the diminished outcrop of the Bumbo Flow. This is not readily seen where it crosses the road.
The steep climb which follows is best examined on the return trip. Cuttings on the right show good exposures of the Cambewarra Flow, the top of which is seen just below the concrete supports.
Stop 5. About 400 yards from the crest of the hill, tuffaceous Coal Measures sediments are exposed on the left. Some fine pieces of chalcedony and greenish chert may be found here and in the creeks below.
Stop 6. Woodhill Gap, with an altitude of 1,170 feet, is the lowest part of the Cambewarra Range and the lowest point on the coastal ranges between Stanwell Park and Nowra. It has provided access to Kangaroo Valley since 1820, but has never been well used because of the steep and difficult descent on the western side.
Note the steeply dipping strata at the road junction – this is due to slumping of the rock down the slope rather than any tectonic process. Climb the steep slope on the southwest (high) side of the road. The eroded slope is strewn with boulders of petrified wood, some weighing more than 50 pounds. They owe their origin to the replacement by silica of the cells in wood buried in the tuffaceous sediments. Fragments of coal and fossiliferous shale may also be found. The extensive view includes the broad sweep of Seven Mile Beach, the deep valley of Broger’s Creek, and the TV masts on Knight’s Hill. If the track is followed up for ½ mile, the foot of Broughton Head will be reached. This is a very prominent outlier of Hawkesbury Sandstone, which can be scaled with difficulty. The igneous rock found here is the Kangaroo Mountain Basanite, part of a sill of basaltic rock squeezed in between the Coal Measures and the sandstone. Occasional masses of brown chalcedony weighing 2 or 3 pounds are found here.
Return to the road junction. The bitumen road leads down into Kangaroo Valley, and is described in Excursion 5c.
Diversion.
The gravel road ahead leads into upper Broger’s Creek. The old building on the left formerly served as a school and church.
Just before the descent begins, a track on the right leads up to the Barren Ground. The sandstone at the top has eroded into weird shapes resembling tables and chairs and this locality is known as the Drawing Room (3 miles return). Basalt outcrops along this track are of the Bong Bong Sill.
Proceed up the road as far as conditions allow. Broger’s Creek is incised 1,400 feet below the adjoining plateau. Tuff is the common rock outcropping, while coal fragments are often found in the creek. About 2 miles beyond the first ford, good outcrops of coal can be seen in the bed of the creek.
Return to Woodhill Gap.
Stop 7. The top of the Cambewarra Flow, seen just beyond the concrete roadside supports, is rather vesicular and includes glassy phenocrysts in a dark groundmass. The flow is often called a trachyte, rather than a latite, and is noticeably paler then the Bumbo and Blow Hole Flows.
If this section is walked as far as the quarry at Stop 4, it will be seen that the flow becomes darker and coarser towards the base. This due to the settling out of the denser ferromagnesian minerals during crystallisation.
The Cambewarra Flow is about 500 feet thick on Cambewarra Mountain; it extends from Budgong Valley to Jamberoo. It appears to have been a submarine flow and in some areas is represented only by a bed of boulders, attesting to the fact that it was subjected to immediate erosion. Marine fossils are sometimes found associated with this boulder horizon, which marks the close of marine sedimentation on the South Coast.
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