FIELD GEOLOGY OF THE SHOALHAVEN DISTRICT (PART 4)
Chapter 2: The Kiama District
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| 3A and 3B Nowra HS Science excursion to Bombo quarries 1965 November |
The introduction of the railway and the subsequent demand for ballast led to extensive quarrying of the local volcanic rocks. Kiama became known as the “Blue Metal Metropolis” and there is abundant evidence around the town of this industry. In recent years quarrying has declined, but an increase in tourism and the expansion of industry around Wollongong has ensured the continued growth of the area.
The traveller approaching the Illawarra district from Sydney will observe three prominent topographical features, especially from a vantage point such as Mt Keira. The first of these is the distant hump of Saddleback Mountain at the tip of the Barren Grounds plateau. The other two (Red Point and Bass Point) project seawards from an otherwise smooth coastline. All three are made up of resistant volcanic rocks, not seen elsewhere on the south coast.
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| Spheroidal weathering in the Westley Park Sandstone at Gerroa August 1963 |
On the coast and lower slopes, the Berry Formation, part of the marine Shoalhaven Group, is dominant. The Gerringong Volcanics, whose stratigraphy was outlined in Chapter 1, is that part of the Berry Formation with which much of this chapter is concerned.
Excursion 2a
Shell Harbour Rail-Kiama-Gerringong (18 miles)
All the rocks seen on this excursion are of volcanic origin, being either flows, dykes, or tuffaceous sediments. Fine examples of volcanic features are seen and there are a number of places where marine fossils may be collected.
Route: Prince’s Highway from Shell Harbour railway station to Omega, then Seven Mile Beach Road to Gerroa. Various side roads lead to places of interest along the coast.
Stop 1. The Dunmore Quarry is being worked west of Shell Harbour station. Request permission before entering.
The rock being quarried is the Bumbo Latite, a rock which will be seen again at numerous points south. The rock itself is greyish, with a tendency towards brown on weathered surfaces. It is characterised by phenocrysts of plagioclase felspar up to ½ inch long, showing excellent multiple twinning. The latite differs considerably from basalt, notably in the higher percentage of potassium felspar it contains. Olivine is comparatively rare.
Two flows may be observed in the quarry, the upper being less vesicular than the lower. In some places a small thickness of sediment lies between the two flows.
Columnar jointing is the most prominent feature visible, and this feature is due to the contraction of the flow on cooling. Cavities within the latite may contain zeolite minerals, calcite or quartz.
Return to the highway and proceed south to the Minnamurra River bridge.
Stop 2. The Bumbo Latite is well exposed in the road cutting. The river is typical of many small coastal streams. Tidal influence extends for miles upstream; in this case for half of the total length of the river.
Proceed ½ mile to the road leading left to Minnamurra. Take this road and subsequent roads to the right.
Stop 3. Old quarries expose the Bumbo Latite. Along the waterfront there are extensive shell and pebble beaches. Small pieces of petrified wood and agate may be collected here.
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| Stack Island Minnamurra January 1953 |
At Minnamurra Point there is a fine view of Stack Island, formerly an extension of this hill. A basalt dyke cuts right across the tip of the point. Similar dykes occur south along the coast and are associated with veins of calcite and chalcedony.
Return to the highway. Further outcrops of the Bumbo Flow are seen for the next 1 ½ miles. The road descends to Bombo and extensive operating quarries are seen on the right. Turn left just beyond the railway siding and proceed under the main railway line to Bombo Beach. The track may be followed for 200 yards to the old Bombo quarries.
Stop 4. At this point the junction between the Bumbo Flow and the
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| The Bumbo Flow resting on the Kiama Sandstone at Bombo November 1965 |
underlying Kiama Sandstone may be studied. There is a zone of coarse agglomerate up to 6 feet thick between the two, containing blocks of sandstone and igneous rocks. Small springs issue from this zone.
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| Agglomerate base of Bumbo Flow Bombo Point November 1965 |
Climb up into the main quarry area. Here, as at Dunmore, the
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| Latite columns at Bombo Quarry August 1982 |
typical features of the flow may be examined. Piles of agglomerate frequently contain masses of black, creamy, or clear calcite. Patches of red haematite are also common. At the northern end of the quarry there are several basalt dykes, which contain fragments of granite (xenoliths) brought up from deep within the earth’s crust.
In fine weather at low tide the rock platform may be safely followed back to the beach. A weathered out dyke and large erratic blocks of igneous rock in the sandstone are seen en route.
Return to the highway and proceed into Kiama. Stop past the railway bridge where the road turns right.
Stop 5. Kiama Harbour was formerly a busy blue metal port. Walk north around the shore towards the baths. At first there are outcrops of the Blow Hole Flow, then volcanic agglomerate, similar to that at Bombo, and finally cliffs of Kiama Sandstone. The rock platform may be followed for some distance and a number of weathered out dykes seen.
Proceed to Blow Hole Point.
Stop 6. The Blow Hole Flow differs from the Bumbo Flow in that it
is less porphyritic and more vesicular. Cavities within the rock frequently contain well crystallised minerals.
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| The Blow Hole Flow at Blow Hole Point March 1963 |
The genesis of the Blow Hole is well known, and will not be dealt with here. Climb over the rocks well to the right of the opening. A narrow dyke of dark basaltic composition may be followed down to the sea. The columnar nature of the flow is very clear at the mouth of the Blow Hole and the dyke which led to its formation may be seen if the seas are calm.
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| Dyke cutting through the Blow Hole Flow at Blow Hole Point March 1963 |
Return to the highway and proceed south for ¾ mile. Turn left instead of crossing the railway line. Keep straight ahead to the top of the hill beyond the beach. Turn right, then left and follow the road until it descends to a small bay where the Little Blow Hole is situated.
Stop 7. The sea has eroded along a joint and removed one of the columns to form the Little Blow Hole. This is a good place to examine these columns. They tend to be perpendicular to the cooling surface, in this case the base of the flow, which was itself irregular. Vesicles in the latite are elongated in the direction in which the lava was moving. Like the Bumbo Flow at Dunmore, the presence of agglomerate suggests that the Blow Hole Flow may be a composite of two or more flows one above the other.
Follow the coast to the north. A large tunnel has been formed by the marine erosion of a dyke. Climb the hill beside the tunnel and cross over to the other side. Vughs in this vicinity contain agate, calcite and amethyst.
Return to the highway and 1 mile beyond the railway bridge turn left (Attunga Avenue, then Anembo Crescent). Follow these roads until they descend towards the little bay in the south.
Stop 8. Westley Park has been visited by students for more than a century. Avoid damaging any of its unusual geological features so that it will remain of interest to future generations.
The accompanying map shows the main features and will help to locate them. At A the Blow Hole Flow contains large vughs lined with quartz and calcite crystals, many of which have been destroyed. The pebble beach nearby is composed mainly of latite pebbles with an occasional piece of agate or chalcedony. Other rock types originated as pebbles in the Westley Park Sandstone.
The rock platform is approached by way of a weathered out dyke (B). Along the track may be found boulders of ultrabasic rock, rich in olivine. These are xenoliths derived from the dyke, evidently brought up from below by the magma.
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| Dyke cutting through the Blow Hole Flow May 1959 |
The Westley Park Sandstone is distinctly tuffaceous and in places rich in marine fossils, especially brachiopods. There are also numerous erratic blocks of igneous rock, similar to those seen at Bombo. Some geologists regard these blocks as volcanic bombs while others consider that they were dropped by melting icebergs. The junction with the overlying flow is fairly regular; otherwise it resembles outcrops seen previously.
At C the dyke may be clearly seen. On either side there is a zone of
hardened sandstone and joints are closer together here than away from the dyke. Likewise it shows joints parallel to the strike, and these are frequently filled by quartz, calcite, and chlorite. Another set of joints strikes across the dyke, at right angles to the cooling surface.
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| The Blow Hole Flow resting on Westley Park Sandstone May 1959 |
If the seas are high, it may be difficult to pass this point (D). Here the dyke has split in two and clearly shows those features seen at C. Beyond this point, the base of the flow continues to rise and marine fossils become more common. Notice how the rock platform continues to the south while northwards, where igneous rocks form the shore, there is no platform.
At E the Horizontal Blow Hole provides a spectacular example of marine erosion. Two dykes similar to those seen in other places have been eroded; one of them has formed a tunnel which the waves seal as they enter. When the water returns, the expansion of compressed air causes a violent expulsion of air and water.
Although walking becomes more difficult, the platform may be followed for 2 ½ miles to Werri Beach. Ten more dykes will be seen and there are opportunities to collect marine fossils.
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| Werri Beach and Gerringong January 1953 |
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| Dykes north of Gerringong May 1965 |
Turn left at Omega railway station. Pass through Gerringong and turn left to Boat Harbour just beyond the town.
Stop 9. In addition to its geological features, Boat Harbour provides a rock pool and launching ramp. The cliff is wholly made of tuffaceous Westley Park Sandstone, which displays spheroidal weathering exceptionally well. The pool itself is built where a dyke has weathered out. The dyke may be seen in the cliff above as a zone of clay. The platform may be followed northwards to Werri Beach. There is a good view of a small stack at Bare Bluff to the north.
Return to Seven Mile Beach Road and go south towards Gerroa. At the crest of the last hill before the descent, the road cutting has yielded, from weathered latite, excellent specimens of amethyst, quartz crystal, and onyx. The descent to the Crooked River shows the Blow Hole Flow and Westley Park Sandstone, both extensively weathered. In the distance, Berry Mountain may be seen. It is a prominent outlier of the Hawkesbury Sandstone.
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The Crooked River entrance Gerroa March 1963 |
Stop 10. Gerroa is renowned as a collecting area for Permian marine fossils, some of which are found nowhere else. It must be remembered that all fossils are irreplaceable and should only be removed for legitimate purposes. Unusual or perfect specimens ought to be made available to universities and museums for study and preservation.
Most of the features to be seen here are similar to those described elsewhere. The map locates points of interest.
An old sea cliff may be seen at A and is a relic of the days before Seven Mile Beach cut off the bay behind it. The tuff displays spheroidal weathering on a grand scale. On the hillside numerous small nodules of agate and occasional quartz crystals are scattered, evidently weathered out of the rock above.
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| Dyke cutting through the Westley Park Sandstone at Gerroa. March 1963 |
Fossils are abundant from this point on, both in the solid rock and in loose pieces. At C an aboriginal shell heap is seen. Another dyke, less obvious than the previous one, is seen at D. The same dyke is exposed at E, on the other side of Black Head. At this second exposure, masses
of white calcite near the dyke probably represent
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| The rock platform at Gerroa March 1963 |
fossiliferous material
which has dissolved and recrystallised.
Black Head (F) is an extensive area of tuff forming a prominent rock platform. There are examples here of fossil polyzoan colonies which are unique. They should not be disturbed. In places the rock is highly tuffaceous and crystals of augite up
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| Fossils in the Westley Park Sandstone at Gerroa March 1963 |
to ¼ inch in length are scattered through it.
At G, a storm beach composed of large blocks or rock has yielded fine specimens of fossil brachiopods. Note also the notch at the base of the cliff formed by wave action at an earlier period of higher sea level.
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| Wave-cut rock platform at Gerroa March 1963 |
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