Permission was granted from the Department of
Environment,Climate Change and Water to place this EarthCache
in the National Park. Please remember
the Geocaching motto of "Leave No Trace"
Please be careful while attempting this EarthCache.
Be sure to stay on the track as there are
unfenced cliffs in the vicinity. There is no need to cross any
safety fences or leave any walking tracks to obtain information or
take photos. The stairs can be very slippery when icy,
snowy or wet, so please take care.
It is recommended that you do this cache in fine weather, and carry
water, especially in hot weather. Even though it is a relatively
short walk to GZ, there is a significant stair climb involved and
the creek water is NOT safe to drink.
Because of issues that Oregon and Colorado GPS units have with
paperless caching, the questions are placed at the front. Make sure
to read the rest of the information though. It may even have some
answers.
To do this cache, park at the Wentworth Falls car park (near WP1),
and follow the Den Fenella track to GZ.
To log the cache, answer the following questions and post the
replies to me via my Geocaching profile page.
1. How many hanging valleys can you see between Wentworth Falls and
the tower on the cliff opposite?
2. Describe how the vegetation changed as you descended from the
car park to Ground zero. How would you account for these
changes?
3. How far has the hanging valley you are in been eroded? Hint:
subtract your altitude at GZ from the altitude at the car
park.
4. Post a photo of your party at Ground zero.
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The Blue Mountains are an elevated and dissected plateau formed
about 10 Ma (Million years Ago) when the region was uplifted in a
very slow and asymmetric manner. The resulting fold is called a
monocline, and stretches some 150 Km from North to South. There are
several monoclines that form the Blue Mountains. The most easily
observed is the Lapstone Monocline. This monocline is most easily
seen at Lapstone, where it is the subject of another EarthCache
(GC185XM), The Hawkesbury Lookout and at Kurrajong. The uplift was
extremely slow, with some of the largest rivers in the region being
able to keep pace with the rate. The Nepean River, in particular,
actually enters the Blue Mountains region and leaves again. Its
rate of erosion easily kept up with the rate of uplift. Lesser
rivers did not keep up. The Jamison Creek, in front of you, did
not, but starting at the larger Kedumba River, has worked its way
back upstream progressively eroding a channel through the blocky
Hawkesbury Sandstone. At present, the eroded channel ends, or
starts, depending on your viewpoint, at the Wentworth Falls to your
left.
Caves can be cut into softer sedimentary rocks at a cliff base.
These softer rocks are more susceptible to chemical weathering than
the more resistant sandstones above. Refer to Earth Cache GC1Y57X
for more information on these Permian age rocks. Once the cave is
undercut sufficiently, the cliff above is at risk of collapse. As
the sandstones of the Narrabeen Group of the upper Blue Mountains
are traversed by widely spaced vertical joints it is natural for
huge chunks to shear from cliff sides parallel to the original
cliff. This leaves fresh cream-coloured vertical rock faces and
rocky rubble (talus) below the cliff. This process drives the
vertical retreat of cliff faces at the Blue Mountains and gives the
area its special natural character and appearance.
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The amount of uplift involved in forming the Blue Mountains can
be realized in the fact that the Narabeen Group rocks here at
roughly 900m altitude are actually beneath the 200m thick
Hawkesbury Sandstone which can be seen at sea level at Long Reef
(GC218HB)
As the Jamison Creek has created its gorge, it has left a number of
lesser streams stranded far above it. Many of these are now dry
most of the time, and are an indicator that the climate was once
wetter than it is now. These are called hanging valleys. The cleft
you are in is one such, but is a bigger stream than most, and has
water most of the year. On the other side of the valley, you can
see a number of these hanging valleys.
View from GZ showing the typical erosion profile of the Blue
Mountains. There is a relic plateau on top coming to an abrupt edge
at a sandstone cliff. Below the cliff is a talus slope leading to
the valley floor. This talus is in most places covered by
vegetation and in turn, covers the softer rock that has undermined
the sandstone above.
Thanks again to Department of
Environment,Climate Change and Water for permission to
place this EarthCache in the Blue Mountains National Park
References and further reading:
Branagan, D. & Packham, G., Field Geology of New South Wales,
Science Press Sydney
Pickett, J & Alder, J. Layers of Time: The Blue Mountains and
their Geology, NSW Dept of Mineral Resources
Sedimentary Case Study: Blue Mountains NSW
http://www.vnc.qld.edu.au/enviro/bluemtns/blue.htm