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State Circle Cutting, State Ccl, Parkes, ACT, Australia

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List Commonwealth Heritage List
Class Natural
Legal Status Listed place (03/06/2005)
Place ID 105733
Place File No 8/01/000/0041
Summary Statement of Significance
State Circle Cutting, along with Capital Hill unconformity beneath Parliament House, is ranked by the Geological Society of Australia as being of high significance as an outstanding exposure of an important folding event. State Circle Cutting is an important teaching locality for the interpretation of the early geology of the Canberra region, and the site is also of geological interest in interpreting the geological history of adjacent areas in eastern Australia.
 
Official Values
Criterion A Processes
The unconformity at the State Circle Cutting is a significant geological feature, and along with the nearby Capital Hill feature, provide keys to the interpretation of the ancient geological landscape in the Canberra region.  It is one of only two sites in the ACT listed by the Geological Society of Australia as being of international significance (Cochrane & Joyce 1986, Owen et al. 1988).
 
Other geological features at the site include sandstone rafts, ripple marks and a pallid zone, all of which are important indicators of the varied environments that existed in the region during the Ordovician (approx. 460 - 440 million years ago) and Silurian (approx. 440 – 420 million years ago) geological periods. Structural features, such as folds and faults, point to the nature of the deformation of these rocks. (Mayer 1995, Owen 1987)
 
The relationships revealed by its excavation in the early 1970’s led to a major re-assessment of Ordovician and Silurian geology of the Canberra region (Crook et al. 1973, Owen 1987), and led to the recognition of the Quidongan Movement as a significant tectonic event in southeast New South Wales which resulted in a major mid-Silurian unconformity (Owen 1987).
Criterion B Rarity
The place is one of the few sites that exposes the Early Silurian unconformity. The State Circle Cutting, along with the Capital Hill outcrop beneath Parliament House, represent the only two known sites in the Canberra region which clearly expose the Early Silurian unconformity. As the rock sequences that underlie the unconformity surface are different at each of these two localities, both sites display unique geological exposures. (Mayer 1995)
Criterion C Research
Perhaps the greatest importance of the State Circle Road Cutting lies in its value as a teaching site. The site lends itself to teaching on account of the excellent clarity of the geological features. The site provides the observer with learning opportunities that range from the most simple geological concepts to aspects of a complex nature. This, together with its easy accessibility, makes it an ideal site for observation of geological features and the study of geological history. It is a perfect illustration of features and concepts taught at tertiary level and it serves as a reference to geological scientists. (Mayer 1995)
 
The State Circle Cutting is likely to provide further information that will enhance the understanding of the area’s natural history through continued research of its strata and the fossils they contain. (Mayer 1995)
Criterion D Characteristic values
The site is a geological benchmark site for the Early to Mid - Silurian age of the Canberra region. (Mayer 1995, Owen 1987), and the site is a notable example of place that provides evidence of ancient geological landscapes and the habitats of now extinct faunas (Mayer 1995)
Criterion H Significant people
The place is associated with the works of A.A. Opik, who was one of the pioneers of geological mapping and the interpretation of geological history in the Canberra region (Mayer 1995).
Description
The State Circle Cutting is located immediately adjacent to traffic lanes in State Circle at Capital Hill between Commonwealth Avenue and Kings Avenue. The road cutting is approximately 320 metres in length. The exposure clearly shows the unconformable contact between the older State Circle Shale, and the younger Camp Hill Sandstone. A number of faults are present in the cutting, and some minor folds can also be seen. The gently folded Camp Hill Sandstone overlies the State Circle Shale, which is slumped and contorted.
 
Geological context:
 
430 million years ago a large deep sea covered the Canberra area. Fine grained, silty sediments were deposited in deep water. When compacted they became the rocks of the State Circle Shale. An older sequence of sandstones broke up on an unstable slope and slumped down to the seafloor as large blocks. These blocks can be seen today in the roadcut as pink coloured sandstone rafts surrounded by the lighter brown, finer grained rocks of the State Circle Shale. 425 million years ago the rocks of the State Circle Shale became strongly folded by forces acting within the earth and were uplifted above sea level.
 
The Canberra area then became dry land. Erosion then wore down the land and shaped the ancient land surface which we can still see in the roadcut as an unconformity. Again the sea flooded the ancient land surface and the sediments of the Camp Hill Sandstone were deposited in shallow water.

After the deposition of the Camp Hill Sandstone on the eroded landsurface, some 420 million years ago, the sequence of rocks was gently folded and uplifted to form dry land again. The sea retreated from the Canberra region and has never returned since. The uplifted land was then eroded down to its present level.
 
Following the uplift and mild folding of the land a number of fractures or faults developed along which the rocks of the State Circle Shale and the Camp Hill Sandstone were displaced.
 
Specific features in the cutting:
 
The State Circle Shale here is formed of mainly siltstone and very fine sandstone which has been strongly contorted by slumping. Marine fossil graptolites were found during excavation of the cutting, the most common species being Monograptus exiguous, which confirms the deposits were laid down in a deep oceanic environment, and they also help to indicate the age of the sediments. The age of these rocks has been estimated at approximately 430 million years old, which places them in the Early Silurian Period.
 
The Camp Hill Sandstone, which is approximately 425-420 million years old, is comprised of fine to coarse quartz sandstone, interbedded with siltstone and silty mudstone. The unit is fossiliferous, with poorly preserved brachiopods, corals and trilobites found during the excavation work.
 
Sandstone rafts: The presence of large slabs or rafts of sandstone, which are now completely enclosed within the finer grained shale, probably originated when a large packet of sandstone and siltstone layers, resting on a sloping oceanic surface, started to slide towards the deeper parts of the ocean basin. As the sequence of sediment layers tumbled downslope, the sandstone beds broke up into slabs of various sized and mixed with the finer grained sediments.
 
Pallid zone: The uppermost 20 to 50cm thick horizon of the State Circle Shale has a pale, almost white colour, which supports the argument that the plane of the unconformity represented an ancient land surface exposed to weathering.
 
Ripple marks: The unconformity in the State Circle Cutting marks a geologically short time of just a few million years. This is the time that elapsed between the elevation of the State Circle Shale from the floor of the ocean, its transformation into a hilly land of severely deformed rocks, and its subsequent wearing down by erosion to a low-lying area that could then be reclaimed by the sea.
 
It was in this younger sea that the Camp Hill Sandstone was deposited. Ripple marks have been preserved on the top surfaces of some of the sandstone layers, and fossils, particularly brachiopod shells of the genus Rhipidium, as well as specimens of corals and trilobites, have been found in the Camp Hill Sandstone. The presence of ripple marks and these fossils indicates that the sea was a shallow one.
 
History Not Available
Condition and Integrity
The original State Circle Cutting, excavated between 1969 and 1971, produced cuts at two levels. The lower of these forms a steep, almost vertical rock face up to 6 meters high. Above this another rock face rose less steeply to a height  of up to 7 meters. The cutting of the upper level was partly removed during the construction of the two bridges and roads for the new Parliament House. Subsequent to this work, the remaining part of the upper level cutting was planted with low, dense shrubs as part of the landscaping of the area surrounding the new Parliament House. Only the lower part of the original cutting remains exposed today.
 
In the 33 years since its excavation, the cutting has suffered relatively little damage. However, a combination of natural weathering, the vibrations created by the high volume traffic flow on State Circle, and, to a lesser extent, the removal of rock samples by the public, has resulted in the partial destabilization of the outcrop. The accumulation of loose material at the base of the rock face provides evidence for this.
 
Location
The exposed rock face on the northern side of State Circle between Commonwealth Avenue and Kings Avenue, Parkes.
Bibliography
      Abel, R.S., 1991. Geology of the Canberra 1:100 000 Sheet Area, New South Wales and Australian Capital Territory. Bureau of Mineral Resources, Australia.

Cochrane, R.M. and Joyce, E.B. (1986) Geological Features of National and International Signficance in Australia. A report prepared for the Australian Heritage Commission. Geological Soc. of Australia, University of Melbourne, Victoria.
      Crook, K.A.W., Bein, J., Hughes, R.J., and Scott, P.A., 1973. Ordovician and Silurian history of the southeastern part of the Lachlan Geosyncline. Journal of Geological Society of Australia v 20, 113-144.

Henderson, G.A.M., 1973. Geology of the Capital Hill area, Canberra A.C.T. Bureau of Mineral Resources, Australia, Record 1973/35 (unpublished).

Henderson, G.A.M. 1981. Geology of Canberra, Queanbeyan and Environs - notes to accompany the 1980 1: 50 000 geological map. Bureau of Mineral Resources, Australia

Henderson, G.A.M. and Strusz, D.L., 1982. Rocks and Fossils Around Canberra. AGPS, Canberra.

Mayer, W. 1995. Nomination by the National Trust ACT of the State Circle Cutting and Capital Hill Unconformity to the Register of the National Estate.

Mayer, W., 1996. Images in Stone – A Guide to the Building Stones of Parliament House. AGPS Press, Canberra.

Opik, A.A., 1958. The Geology of the Canberra City district. Bureau of Mineral Resources, Australia, Bulletin 32

Opik, A.A., 1971. The Silurian of Canberra. Journal of the Geological Society of Australia, 17, 231-232

Owen, M. 1987. Geological Monuments in the Australian Capital Territory. A report prepared for the Australian Heritage Commission. Geological Society of Australia, Commonwealth Territories Division, Monuments Subcommittee.

Owen M., Senior D., Owen J. & Hodgson J. (1988). Geological Monuments
in the Australian Capital Territory. Geological Society of Australia.
      Strusz, D.L. and Henderson, G.A.M. 1971. Canberra City ACT 1:50 000 Geological Map. Bureau of Mineral Resources, Australia, Explanatory Notes.

 Strusz, D.L. and Jenkins, C.J., 1982. The Stratigraphic implications of the MONOGRAPTUS EXIGUUS from Camp Hill, Canberra, ACT. BMR Journal of Australian Geology and geophysics, 7, 78-9.

Townley, K.A. and Veevers, J.J., revised by Strusz, D.L., 1974. ROCKS AND FOSSILS AROUND CANBERRA. Bureau of Mineral Resources, Australia.
 

Report Produced  Mon Jul 14 04:00:19 2014