THE GEOLOGY OF MT WILSON: THE WATERFALL TRACK BLUE MOUNTAINS NSW AUSTRALIA

THE GEOLOGY OF MT WILSON: THE WATERFALL TRACK BLUE MOUNTAINS NSW AUSTRALIA

The Waterfall Track at Mt Wilson (northern Blue Mountains) takes the walker down through rain forest into a valley where the basalt which makes up the upper part of the Mountain can be seen to lie above older sandstone. This is the situation all over this basalt capped mountain, as it is with others in the area, but this spot is perhaps the most accessible and the most studied of all.

You can see the area on my video (here) but it is one of those places you can easily visit yourself if you have a reasonable degree of fitness. Leave it as you find it – no specimens please!

Examine the extract from the 1:250 000 Sydney geological map. You will see that in this part of the Blue Mountains the geology is fairly simple. Comparatively flat lying basalt (Tv) occupies the highest ground (as you can easily see from viewpoints like Anvil Rock at Blackheath and Wynne’s Rocks at Mt Wilson). Beneath the basalt is the normal series of Triassic age sedimentary rocks seen throughout the central Sydney Basin: the Wianamatta Group (TRwl), the Hawkesbury Sandstone (TRh)and the Narrabeen Group (TRn).

Interpretation of the strata beneath Mt Banks

The Wianamatta Group of silty sediments is not present beneath the basalt at Mt Wilson whereas it is at Mt Tomah and Mt Irvine. The Hawkesbury Sandstone, likewise is present immediately beneath the Wianamatta Group in those same places but is not shown as being present at Mt Wilson. NOTE: see Additional Information at the end of this Blog. However, in the publication “Layers of Time”, the Hawkesbury Sandstone is stated to be present on Mt Banks beneath the basalt (p. 28) and is shown as such in the accompanying geological section. The Sydney 1:250 000 map does not show the Hawkesbury Sandstone as being present on Mt Banks  (bottom 

Mt Banks from Wynne's Rocks

left of map extract above , spot height 3474’).

I suspect that the Hawkesbury Sandstone is present, albeit a thin layer of it, in places around the edge of the Mt Wilson basalt, partly because of the considerable difference in elevation between the mountain and the general level of the tableland around it, and by analogy with Mt Banks.

You can download a copy of “Layers of Time” by going to DIGS (see my Blog entry on how to do this here). The reference number is GS1998/519. The Waterfall track is discussed on page 29. Another reference (old, but still good reading) is Rev J Milne Curran’s “Geology of Sydney” (1898 and 99). 

Columnar basalt at the waterfall

f

Sandstone beneath the basalt

Joseph Carne has something to say about Mt Wilson in Memoir 6 of the Geological Survey of NSW (1908) titled “The Geology and Mineral Resources of the Western Coalfield”. His main subject is, of course, the coalfield itself; however on pages 132-4 he quotes an article by AG Hamilton from the Linnean Society (1899) which is specifically about Waterfall

Sandstone at Wynne's Rocks

Creek at Mt Wilson. At the end of this Blog you will find the text of the most relevant passage in this book.You can download a copy of the entire work from DIGS (reference number Geology Memoir 06).

I need to make one further comment on the geology of Mt Wilson. Carne points out that there are a number of places in the area where sediments of Tertiary age lie beneath the basalt but above the older Triassic rocks, which represent the land surface at the time the basalt was poured out. There must have been streams flowing across this old land surface and it is the alluvium from these stream beds and flood plains which is preserved in a few places. Mt Tootie is one of these, Airly Mountain is another. In the case of Airly Mountain, gold, diamonds and sapphires are present in the old gravel. As the base of the basalt at Mt Wilson is not level (as with all the volcanic residuals), it is possible that such a deep lead is present there. About the only way to follow this up would be by examining the sediments in the streams draining the mountain. To do so will require a lot of bush-bashing. Perhaps Bowen’s Creek and the Wollongambe River carry some of these dense minerals in their alluvium, which might have come from such deep leads, whether still present in part on these peaks or now completely eroded away. 

Extract from "Geology and Mineral Resources of the Western Coalfield"

Additional Information 26th November 2014

 The Geological Map of the Western Coalfield (Southern Part), issued by the NSW Department of Mineral Resources in 1992 (DIGS Reference R00027991) clearly shows the Hawkesbury Sandstone (TRh)beneath the basalt under much of Mt Wilson, including the Waterfall track area. My apology for not checking this resource earlier.

https://johnsbluemountainsblog.blogspot.com/2013/12/links-to-all-blog-entries-and-relevant.html All Blue Mountains blogs and videos

https://johnsnewenglandminerals.blogspot.com/2013/12/list-of-all-blog-entries-to-date.html

All New England and other Geology blogs and videos

https://www.blogger.com/blog/posts/1325599670344725659?pli=1

Limestone Caves of NSW

https://www.blogger.com/blog/post/edit/1147003616874115974/2430103125095714141

Song Studies. Bible studies based on hymns and songs

https://www.blogger.com/blog/posts/1515863000684737715

Shoalhaven District Geology

ANVIL ROCK, BLACKHEATH BLUE MOUNTAINS NEW SOUTH WALES AUSTRALIA

ANVIL ROCK, BLACKHEATH 

BLUE MOUNTAINS NEW SOUTH WALES AUSTRALIA

Anvil Rock is located on the edge of the Grose River gorge, about 7 km north of Blackheath. Access is via Hat Hill Road to the Blue Mountains National Park; the road and track to the Rock are clearly signposted. 

Anvil Rock was named because of its shape by the then Blackheath Municipal Council in 1938 when the present road was constructed. Ten years later, the steel anvil which is now attached to the top of the rock was first put in place. You can read the story of how this was done, how it was lost for more than 30 years and how it was reattached in 2008 here .

The walk from the car park takes less than 15 minutes and there are great views of the Grose River gorge both from the track and from Anvil Rock itself. The day I was last there was clear and fine and it was easy to zoom in and photograph the high rise buildings in the centre of Sydney.

Mt Banks August 1964

The views from Anvil Rock are superb, and the centrepiece is Mt Banks, which lies immediately opposite. The annotated photo below comes from “Layers of Time: The Blue Mountains and Their Geology” (DIGS reference GS1998/519). See the Blog entry on “DOWNLOADING PDF DOCUMENTS FROM DIGS”. You can readily identify the main strata, from the basalt cap down through the Triassic and into the Permian below. This is one of the best places in the Blue Mountains to see such a stratigraphic column of the Sydney Basin sedimentary rocks.

For the Wild Walks description of the Anvil Rock walk click here .

Here is a link to a 1950’s photo of the rock by celebrated photographer Frank Hurley.

My Anvil Rock video may be found here . 

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https://johnsnewenglandminerals.blogspot.com/2013/12/list-of-all-blog-entries-to-date.html

All New England and other Geology blogs and videos

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Limestone Caves of NSW

https://www.blogger.com/blog/post/edit/1147003616874115974/2430103125095714141

Song Studies. Bible studies based on hymns and songs

https://www.blogger.com/blog/posts/1515863000684737715

Shoalhaven District Geology.

UNCONFORMITY AT SOUTH BOWENFELS NEAR LITHGOW NEW SOUTH WALES AUSTRALIA

UNCONFORMITY AT SOUTH BOWENFELS NEAR LITHGOW NEW SOUTH WALES AUSTRALIA

Aplite dyke intruding granite (very weathered)

This is the first purely geological entry I’ve written for this Blog. Most readers will be familiar with the basic geology of the western Blue Mountains. Essentially, there is a series of sedimentary rocks resting on an eroded surface of older rocks, both igneous and sedimentary. The rocks that we see from lookouts such as Echo Point or Govett’s Leap consist of Triassic sandstones and siltstones (the Narrabeen Group, which forms the cliffs) and the Permian Illawarra Coal Measures (nonmarine) and Shoalhaven Group (marine) which form the slopes and more level areas below.

Once Victoria Pass is descended into the Hartley Valley, there comes a point near Hartley where the Permian Shoalhaven Group sedimentary rocks gives way to the underlying granite, which occupies the lower ground. The sedimentary rocks and the granite are of quite different ages, Permian and Lower Carboniferous respectively, and the surface between them is called an unconformity (a time gap).

Shoalhaven Group conglomerate overlying weathered granite

At the very least, the time gap represented by this unconformity covers the time involved in forming the granite (as an intrusion into even older rocks), uplifting the earth’s crust and eroding away the overlying rocks and deeply weathering the now exposed granite. 

At this point in time the Sydney Basin began to form as the land subsided. The first sediments deposited in this vast subsiding area were the marine sandstones, conglomerates and siltstones of the Shoalhaven Group. These contain lots of pebbles (sometimes boulders) of the more resistant older local rocks. Most of these consist of hard quartzite, evidently derived from the nearby Upper Devonian Rydal Syncline. 

Detail of the unconformity

It’s not often you can put your finger on an unconformity like this, representing as it does perhaps 100 million years of time. The geological map is a small portion of one published in 1875 (see below). The excavation where the photographs were taken is in Portion 241. You can read the notes (21 and 22) which accompany the map. I have yet to find any pebbles containing Upper Devonian marine fossils nor any Permian marine fossils in the Shoalhaven Group sediments, but these are well known from other nearby localities. The cross section from the Sydney 1:250 000 geological map goes right through this spot and clearly shows the relationship between the various rocks.

A large quartzite cobble from the conglomerate.

 References: “Geological Map of the Districts of Hartley, Bowenfells, Wallerawang and Rydal” (Department of Mines, New South Wales, 1875). The DIGS reference is Heritage Map H0252. Geological Map of Sydney 3^rd edition 1966. The DIGS reference simply says “Map”, so try using the Report Identification Number R00019218 instead.

https://johnsbluemountainsblog.blogspot.com/2013/12/links-to-all-blog-entries-and-relevant.html All Blue Mountains blogs and videos

https://johnsnewenglandminerals.blogspot.com/2013/12/list-of-all-blog-entries-to-date.html

All New England and other Geology blogs and videos

https://www.blogger.com/blog/posts/1325599670344725659?pli=1

Limestone Caves of NSW

https://www.blogger.com/blog/post/edit/1147003616874115974/2430103125095714141

Song Studies. Bible studies based on hymns and songs

https://www.blogger.com/blog/posts/1515863000684737715

Shoalhaven District Geology.

 

DOWNLOADING PDF DOCUMENTS FROM DIGS

DOWNLOADING PDF DOCUMENTS FROM DIGS

PDF: Portable Document File

DIGS: Digital Imaging Geological Systems

In an earlier blog entry, I showed how to download useful books on the Blue Mountains from the National Library of Australia’s website Trove. (See entry dated 5^th August 2013).

Today’s entry shows you how to download material from DIGS which deal with the geology and mineral deposits of NSW, from the NSW Trade and Investment, Resources and Investment website. There is a vast amount of information available which formerly could only be obtained from libraries or by purchasing hard copies.

There are a few useful books you can download on the geology of the Blue Mountains which are worth having on hand in your computer or mobile device.

You will need a PDF reader on your computer to read the files. Adobe Acrobat  here and Foxit here are both freely available on the internet. I use the latter.

How to access DIGS. 

Click on this URL: http://digsopen.minerals.nsw.gov.au/#null which you should then bookmark. This is the entry point to DIGS. Sometimes this doesn’t work and no amount of retrying makes any difference. Try again tomorrow. 

Follow the prompts: Click on “Search DIGS”. This brings up the search menu. To make it easy at this point, here are the reference numbers for some documents you really should download if you have any interest at all in the geology of the Blue Mountains. They are the items in brackets after each title. Simply click on “Search” after entering the reference number at the top. You can select individual parts of each document or download the whole. 

Guide to the Sydney Basin (Mineral Bulletin 26). This covers a much greater area than the Blue Mountains, of course; there is a wealth of information here.

Geology of the Western Blue Mountains (Mineral Bulletin 20).

Geology and Mineral Resources of the Western Coalfield (Geology Memoir 06). A classic geological report in the old style.

Layers of Time: The Blue Mountains and Their Geology (GS1998/519). A modern treatment of the geology in an understandable form.

https://johnsbluemountainsblog.blogspot.com/2013/12/links-to-all-blog-entries-and-relevant.html All Blue Mountains blogs and videos

https://johnsnewenglandminerals.blogspot.com/2013/12/list-of-all-blog-entries-to-date.html

All New England and other Geology blogs and videos

https://www.blogger.com/blog/posts/1325599670344725659?pli=1

Limestone Caves of NSW

https://www.blogger.com/blog/post/edit/1147003616874115974/2430103125095714141

Song Studies. Bible studies based on hymns and songs

https://www.blogger.com/blog/posts/1515863000684737715

Shoalhaven District Geology.