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.
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| 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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