Friday, 20 March 2015

Geology Field Trip 2

I covered the first trip a few days back.  This was the medium trip (about 80km) going round the Tidbinbilla Loop.  These images cover some of the things we saw which interested me.

Our first stop, at Casuarina Sands. had us looking at the Winslade Fault, with the Murrumbidgee at the bottom of the Fault.  This one is unusual for a major fault in this area as it runs NW-SE rather than N-S.
The area where we were standing was the processing site for a Waterwatch Survey checking the state of the Murrumbidgee.  A related programme occurs in NSW, including downstream in Whiskers Creek.
In the background of the talk about rocks I could hear. and eventually see, a Restless Flycatcher doing its  scissor-grinder call.  This appallingly bad photo emphasises the white throat of this species.
We then moved up the road towards Urriara Station and looked down on the river.  (Our previous stop was close to the left end of the bridge.)  The image shows the river flowing North - up the Murrumbidgee Fault - and then chucking a turn when it hits the Winslade.
Going in to check the New! Bigger!! Cotter Dam we found a rather impressive set of warning notices.  I was intrigued about the need for  a warning about razor wire - it is pretty obvious.
A front on view of the dam wall.  A couple of interesting points were noted:
  • the current outflow is playing on concrete at the base of the dam, which seems like a recipe for erosion; and
  • there is a chain link fence across the river in front of the wall.  I wondered how long that would last in a serious river flow.  Noting the way fences get trashed when they cross small creeks I'd guess the answer is minutes rather than millenia.
According to a sign at the lookout nearby the wall is designed to hold up during an event with the water running 5m deep over the spillway.  This would have 5710 cubic metres per second going over, and an event like that is expected once every 4 million years so perhaps the fence has a bit of life in it yet.

A view of the exposed rock near the lip of the dam.
Down at base level these two colours of rock were adjacent.  However they are the same type of ignumbrite rock - presumably meaning the same type of mineralisation and formed at roughly the same time - but from different volcanic eruptions
This is the lookout, which presumably cost a heap (although considering the overall cost of the new dam probably a trivial proportion of the total),
There were a bunch of interpretive signs around  which is very good,  This one contrasts the old and new designs.
This is an extract from another sign,  It shows the aggregate annual inflow to the dam: the yellow line is the 30 average, and clearly shows the drought of the noughties: is a pity they haven't updated it to show the subsequent wet years.
On the subject of pities it is a shame that the many interesting and informative signs are not included on the ACTEW website.  The bright sunshine made photography difficult.

We next moved to the road in to the Tidbinbilla Tracking Station.  Here we looked across at the granite boulders exposed by erosion through the action of Paddy's River.  As we were about 40m above the River this was broadly equal to 8,000,000 years of erosion!
On the opposite side of the road a cutting had exposed some much more recent sediments.
The rounded pebbles are clearly the work of water, so this was presumably an historic course of the River.
These two images have caused much thought about how erosion happens in some areas as opposed to to  others,  When I have got my ideas a bit more organised another post may offer some commentary on this.

A worker turned up on his quad with Kelpie.  He commented to some of the group that she was a bit tired.
However, after a bike-borne blat across the paddock to some sheep the dog regained energy and got stuck into her work.
Here is some Autumn foliage  with a dish in the background.
As we had our lunch we reflected on how such a tool is made out of great big girders, but can be tuned to very fine levels of accuracy to pick up objects squillions of light-years away.

The car park was pretty full.  It turned out this was because it was "50 years ago today Bob Menzies got the dish to play" (sorry Sgt Pepper - you were only 20 years ago in 1967) and they were having an event (aka party) for former staff.
Moving on to Birrigai a flock of Australian King Parrots came to visit.
The design of the sign is interesting as I and at least one other initially thought 'cycle pathway' was something to do with bicycles rather than the "rock cycle".
An example of the informative signs and samples around the pathway.
There are a range of outdoor education possibilities here as detailed by the Principal.  Some of these are musical including this downpipe xylophone.  Note the Gibraltar Rocks in the background.
Our final stop was the car park at Tidbinbilla where we were serenaded by a Grey Butcherbird.
This is a view of the range between the Paddys River Valley and Murrumbidgee.  From Camelback (pictured it is possible to see a rocky slope going down to the latter river  based on tilted Ordovician rocks.  These are mainly metamorphic rocks of a range of types depending up whether the original sediment was mud, silt or sand.

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