Field Notes for the Richmond–Kurrajong Area, Australia

© Tas Walker  May 05

This field excursion

This field guide covers four sites in the vicinity of Bells Line of Road from Richmond to Kurrajong Heights. This field trip is suitable for a family, a school class or a youth group and would take about 3–4 hours to complete. This field trip will help you 1. know how to read geological maps; 2. understand how geologists work; 3. learn some common geological terms; 4. understand how to work out a geological cross-section; 5. understand how to work out a geological history of an area.

This geological field trip will help you understand the basic geological history of eastern Australia. Although you will only cover a small area, the geological sequence is typical of the area from Newcastle to Wollongong and west to the Blue Mountains. There are some geological sequences that you will not see, including the basement rocks, granitic intrusions, and some of the basalt caps in the area. Nevertheless, the general sequence in this area will give you a good background to the geology of a large part of eastern Australia.

This geological field trip will also help you consider some of the evidence that points to the global Flood of Noah as recorded in the Bible. You will be able to contrast different ways of interpreting the geological evidence, namely, the conventional 'uniformitarian' method and the biblical framework. You will be able to see the strengths and weaknesses of each method and decide which makes the best sense of the evidence in the field.

Geological Map

The simplified geological map here covers the area from Richmond (bottom right) to Kurrajong Heights (top left) along Bells Line of Road. The different colours represent the different geological formations (kinds of rock) that are exposed at the surface. These are the geological formations that you would walk over as you walk over the ground, and that you would see exposed in any road cuts on the way.

The legend at the right of the map gives names of the different geological formations. The legend is arranged in the same order as the rocks are found in the field. That is, the box at the bottom (representing the Burralow Formation) lies under all the other formations, and the box at the top (representing the undifferentiated Quaternary) sits on top of all the other rocks.

Geologists construct geological maps by exploring the countryside and marking on the map each observed rock formation. In many areas it is not possible to observe the kinds of rocks at the surface because of soil and vegetation. Geologists complete the maps in these areas by using information from other areas and using their judgment.

Geologists often determine which rock formation sits on top by observing it (where they contact, or 'the contact' as it is called) directly in the field. Other times, they can work it out as they are completing the map.

The four sites typical of the area that are described here are marked on the map.

As you drive around the area, try to work out where you are on the geological map, and try to match up the rocks you can see (e.g. in road cuttings) with the rocks that are shown on the map. It won't take long for you to be able to recognize whether the rocks are part of the 'Hawkesbury Sandstone' or part of the overlying 'Ashfield Shale' (which is part of the Wianamatta Group).

Geological Cross Section

A simplified cross-section of the geology of the area is shown here. It represents what you would see if you could take a cut from the north-west (near Kurrajong Heights) to the south-east (near Richmond). The vertical scale is greatly exaggerated.

Contrary to what you may think, geologists do not work out these cross-sections by drilling into the ground. They work them out primarily from the geological map.

Because the cross-section is an interpretation, different geologists often have different ideas about what the rocks are doing underground. Usually this is not particularly important, but sometimes it can be quite costly, e.g., if the miners dig in the wrong place in a mine.

The Regional Setting

The Continental Setting

Navigation Map

A simple navigational map is available. Click the icon to the right. It is not difficult to find the four locations and you can visit them in any order. Descriptions of the sites are given in each of the four sections.

The Landscapes

As you drive around the area, notice the shape of the landscapes (this is called geomorphology—geo = earth, morph = shape, logy = study).

Make these observations:

• Around the Hawkesbury River near Richmond the landscape is flat and broad.
• As you move higher to the grazing lands around the Grose River the landscape starts to become hilly and the landscape has a rolling appearance. Creeks flow in wide gentle valleys.
• At high elevations (around Kurrajong–Kurrajong Heights) the land is cut by narrow, steep gullies.

Think about the processes that cut the landscape.

• From a biblical perspective, the basic landscape shape was cut while the floodwaters receded over some six month period.
• Initially the water level was above the whole continent, and the water flowed in broad sheets.
• As the water level dropped the flow would have concentrated into very wide channels, much larger than the rivers we see flowing today.
• Eventually the flow would have approached the size of the rivers we see today.
• After the Flood, the climate in Australia is believed to have been much wetter than today.
• After the flood the landscape would not have had vegetation immediately.
• There have been over 4000 years since the flood when erosion has carried on much like we see it happening today.
• Australia was not covered by ice after the Flood as parts of Europe and North America were.

Site 1. Contact Between Hawkesbury Sandstone and Ashfield Shale

As you drive up Bells Line of Road, turn right into Hermitage Road. Along this road turn right into Diamond Hill Road.

As you enter Diamond Hill Road observe the small road cut on the right. At first you will see blocky sandstone exposed. This is the top of the Hawkesbury Sandstone. As the road rises you can see that you also move up the strata. As you move along the road notice that the strata change from blocky sandstone to layered strata of sandstone and siltstone. You are now moving into the Ashfield Shale, which is part of the Wianamatta Group. As you move further along the road the sandstone disappears and the strata are wholly shale. Notice that the shale is finer grained and has a sort of horizontal bedding. You can stop at the top of the low hill to inspect the rocks in the cutting. Notice that the rocks have several series of joints in different orientations. These were formed when the rocks were gently stressed as the formations were uplifted.

The lower blocky sandstone is the very top part of the formation called the Hawkesbury Sandstone. We will look closer at this at Site 3. The upper layer is part of a formation called the Ashfield Shale.

Observations and questions:

Check the grains in the three kinds of rock (The Hawkesbury Sandstone, The sandy layer of the Ashfield Shale, and the darker fine layer of the Ashfield Shale:

• What are they made of? Quartz, rock fragments, etc.
• What size are they? (1-2 mm = very coarse sand; 0.5-1 mm = coarse sand; 0.25-0.5 mm = medium sand; 1/8-1/4 mm = fine sand; 1/16-1/8 mm = very fine sand)
• Are they all about the same size (well sorted) or is there a range of sizes (poorly sorted)? Are the grains rounded or angular?
• What is holding the rock together? Clay, quartz, red iron cement, etc.
• Is the rock well cemented, or easily broken (friable)?

For the Ashfield Shale, check the way the grains and the layers relate together (i.e., its fabric):

• Is any bedding visible in the sandy layers?
• Is any bedding visible in the dark siltstone layers?
• Do the sandy layers and the silty layers parallel over big distances or do they merge with each other?
• Is the Ashfield Shale level, at an angle, or upside down from when it was deposited?

Check the Contact between the Ashfield Shale and the Hawkesbury Sandstone:

• Are the beds above and below the contact all parallel to each other or are they at an angle?
• Is the contact visible?
• Is the contact straight or is it there any evidence that the top of the lower strata have been eroded?
• Is there any evidence of a soil layer on the top of the lower strata at the contact?

Check the landscape:

• The gullies in the area, how were they eroded? When, during the Flood, or after the Flood?

Site 2. The Diamond Hill Diatreme

About 1.3 km along Diamond Hill Road, around a bend and alongside a dip in the road, on the left there is a large, overgrown, vacant lot. This is the location of one of many known diatremes in the Penrith area.

A ‘diatreme’ is a vertical ‘pipe-like’ volcanic tube filled with volcanic debris. They have also been called volcanic necks. Diatremes are believed to be the remnants of a short-lived explosive vent. All evidence of the surface volcanic features have been eroded away (Some estimates suggest that 4 km of strata have been eroded. This may be correct, but you need always ask how that was calculated.)

The Diamond Hill Diatreme is composed of broken, dark, fine-grained volcanic rock (breccia=angular rocks) and unbroken dark, fine-grained rock (basalt—known as an alkaline, olivine basalt. You may see some olivine crystals in the rock—they tend to be round like a large pin-head and look like green or yellow broken glass.).

Have a look at the rocks under the undergrowth. This shows that geologists do not always have spectacular rock exposures to examine. Often the rocks are hidden by vegetation, and not much is visible.

Observations and questions:

• Notice that the rocks are broken.
• Are they smooth or angular?
• Is there any banding in the rocks, or are they homogeneous?
• Are there any minerals visible in the rocks?
• Are the rocks fresh or are they starting to weather?
• Observe the shape of the land surface in the area. Is there any indication that this was once a volcano?

When were these rocks formed? Before the sandstone was deposited or after? Before the landscape was eroded or after?

Site 3. Hawkesbury Sandstone

As you drive up Bells Line of Road to Kurrajong Heights, notice the rocks in the deep road cuts. (Note, it is too dangerous to stop!) In places you will see the blocky, sandy exposures of the Hawkesbury Sandstone. In other places you will see the thin, almost-horizontal bands of the Ashfield Shale. Notice that the strata sloping at about the same angle as the road. All the land to the west has been lifted up to form the Blue Mountains and the strata have bent in the area as a result. This bent, hinge-like, geologic structure is known as a monocline. Notice on the geologic map that to the west of Site 4 the strata have broken to form a vertical fault. You can see the monocline and the fault in the geological cross-section above.

At the end of Warks Hill Road, Kurrajong Heights, we walk down a bush track to observe an excellent outcrop of Hawkesbury Sandstone.

The ‘sand’ grains in the Hawkesbury Sandstone are mostly quartz particles with very minor amounts rock fragments, clay pellets, feldspar and mica. The sand is held together in a clay matrix cemented by quartz and siderite (iron carbonate). Near the surface the iron mineral weathers and stains the sandstone with reddish rings, called ‘Liesegang’ bands.

The Hawkesbury Sandstone covers most of the Sydney Basin, and is clearly visible from lookouts over Sydney Harbour, in road cuttings around Sydney, and in gorges in the lower Blue Mountains. It can exceed 200 m in thickness.

Observations and questions:

Check the grains:

• What are they made of? Quartz, rock fragments, etc.
• What size are they? (1–2 mm = very coarse sand; 0.5–1 mm = coarse sand; 0.25–0.5 mm = medium sand; 1/8–1/4 mm = fine sand; 1/16–1/8 mm = very fine sand)
• Are they all about the same size (well sorted) or is there a range of sizes (poorly sorted)? Are the grains rounded or angular?
• What is holding the rock together? Clay, quartz, red iron cement, etc.
• Is the rock well cemented, or easily broken (friable)?

Check the way the grains relate together (i.e., its fabric):

• Check the shape and size of the beds (these are called cross beds)
• Check how some layers in a bed have been cut by layers from a later bed.
• Check the pattern of larger grains embedded in lenses in the sand
• Check the size of the beds, and their angle.
• Check the way all the beds relate together.
• Is the Hawkesbury Sandstone level, at an angle, or upside down from when it was deposited?

Check the landscape:

• The steep gully alongside, how was it eroded? When, during the Flood, or after the flood?
• Down the track about 50 m there is a good view of a larger landscape. How did that landscape get carved? Was it during the Flood or after?

Over the years different scientists have tried to compare the Hawkesbury Sandstone with something we observe happening on the earth today.

• 1844    Under the ocean (marine)
• 1880    Partly under glaciers (a partly glacial environment)
• 1883    A desert (aeolian)
• 1883    Part desert and part lake
• 1920    Freshwater lake
• 1964    River
• 1969    Marine tidal delta
• 1975    A wide flat river

  For further discussion about the depositional environment of the Hawkesbury Sandstone and how it provides compelling evidence for Noah's Flood see here.

Site 4. The Lowlands Formation—Adjacent to the Grose River

The Lowlands Formation is the name given to the sediment that forms a broad terrace next to the Hawkesbury River. The formation is much smaller than the Hawkesbury Sandstone—it has a maximum thickness of about 12 metres, and covers only some 48 square kilometres (e.g., 4 km x 12 km).

A river flows through this area, a modern geological process.

Observations and questions:

Is it sedimentary, metamorphic or igneous unit?

Check the pieces of rock (clasts):

• Check the size.
• Are they all about the same size (well sorted) or is there a range of sizes (poorly sorted)?
• Are the clasts rounded or angular?
• Have they been transported a long way or a short way? Were they transported quickly or slowly?
• What are they made of? What sort of rocks can you identify?
• Where did the pieces of rock come from?
• Is the formation cemented together or is it unconsolidated?
• Is there any evidence of life in the rocks?

Think about the geological processes:

• Is the river carrying any sediment?
• Is sediment accumulating in the area? Or is sediment being eroded from the area? Or is it simply being transported through?
• Was the Lowlands Formation deposited by this river flowing very much like we see it flowing these days, or would it have required a much faster flow?

REFERENCES

1. Jones, D.C., and Clark, N.R., (Eds.), Geology of the Penrith 1:100,000 Sheet 9030, New South Wales Geological Survey, Sydney, 1991.
2. Clark, N.R., and Jones, D.C, (Eds.), Penrith 1:100,000 Geological Sheet 9030, New South Wales Geological Survey, Sydney, 1991.
3. Wright, D, and Bell, P., Practical Guide to Rocks and Minerals, Chancellor Press, London, 1985.
4. Mayer, W., A Field Guide to Australian Rocks, Minerals and Gemstones, Ure Smith Press, Sydney, 1976.
5. Allen, D., Sediment transport and the Genesis Flood—Case studies including the Hawkesbury Sandstone, Sydney,TJ 10(3):358–378, 1996.
6. Walker, T.B., A biblical geologic model, In: Walsh, R.E. (ed.), The Third International Conference on Creationism, Creation Science Fellowship, Pittsburgh, pp. 581–592, 1994.

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