Black Mountain
Black Mountain is a small mountain with an elevation of 812 metres (2,664 ft) AHD that is situated to the west of the Canberra CBD on the northern shore of Lake Burley Griffin in the Australian Capital Territory. Black Mountain is protected from development by the Canberra Nature Park and is predominantly covered in native bushland and is a haven to native wildlife.
Black Mountain is unique in its structure of sandstone and inter-bedded siltstone with evident layering, folds, distortions and plunges. The formation of Black Mountain extends to the State Circle area and is flanked by the Deakin Fault and the Acton Fault. Mt Ainslie and Black Mountain are the two most prominent peaks in Canberra and despite their close proximity have very different histories and form.
The Geology of Black Mountain
Black Mountain Sandstone is deposited on top of State Circle Shale. It is made up from thick beds of grey quartz sandstone mostly, but has some beds included of siltstone and grey shale. The grain size is fine to medium. It was originally named by Opik after the mountain; Black Mountain where it is found. Originally it was believed to be Ordovician, but is actually from the Silurian period, late Llandoverian epoch. Some of the slopes of Black mountain are covered in fanglomerate. The deposition was in a marine proximal turbidite fan, with the turbidity current flowing to the east. There are no fossils, but there is some sedimentary structure including plane, cross or convolute laminations, load casts, slump units and flute moulds.
The bulk of Black Mountain consists of the white quartz Black Mountain Sandstone. On the south east slopes and North West there are exposures of State Circle shale. The Black Mountain Peninsula contains mudstone in the north and greywake from the Ordovician age Pittman Formation in the south.
State Circle Shale and Black Mountain Sandstone deposited in a marine environment as turbidites. The source of the Black Mountain sand was near by from the west, from the Wagga Wagga area. The Canberra area was on the proto-Canberra-Yass Shelf. East of Canberra deep water of the Monaro Basin remained in the Captains Flat area.
A second unconformity occurred after the Black Mountain Sandstone was uplifted and eroded at the end of the early Silurian. This was called the Quidongan Deformation. The Canberra formation was deposited in shallow water with limestone, and shale forming. There were some small volcanic activities at this stage with dacite and ashtone layers included.
The road up to the summit has many cuts into the mountain that detail the varying rock types and changing structures as you ascend. The stages look at the differing makeup of the mountain and are also located where reasonable parking is available.
Stage 1
The orange bands of rock have a light coloured seam approximately 300mm wide. What is its consistency?
Stage 2
Look at the visible seams in the rocks. Describe how they move?
What is the colour of the rocks at this point?
Stage 3
What is the thickness of the visible “lighter” band?
Stage 4
Are quartz bands visible in the rocks?
Stage 5
At this point you are looking down the North face. What do the visible rocks look like?
Please submit your answers as soon as you can. It's ok to post prior, I will check your findings and respond if there are issues. Feel free to add a photo of yourself at the site if you want too. Group visits are encouraged. I hope you enjoy the experience, this is a very special area.