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Sunday, 8 June 2014


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). You can download a copy from Trove (see also the Blog on this) by
Columnar basalt at the waterfall
following this link here.
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.

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