Tharwa, Australian Capital Territory

Deakin Volcanics rhyodacite modified to green colour

(postcode 2620) is a small village within the Australian Capital Territory, 35 kilometres (22 mi) south of Canberra, the capital city ofAustralia. At the 2006 census, Tharwa had a population of 109. The village is located on the banks of the Murrumbidgee River and at the junction of Tidbinbilla and Naas Roads, and Tharwa Drive. The main public buildings are a general store, a preschool and primary school (now closed), Saint Edmund’s Church, a cemetery, a community hall and tennis courts. The annual Tharwa Fair was hosted by the school, and was held in May until 2006. The Tharwa Fair is now organised by Tharwa Preschool.

History

Tharwa is the oldest official settlement in the Australian Capital Territory, proclaimed a settlement in 1862. Tharwa was named after the Aboriginalword for Mount Tennent, a nearby mountain peak which is part of Namadgi National Park. Mount Tennent was named after John Tennant, who was one the earliest and best-known bushrangers in the region. John Tennant lived in a hideout on the mountain behind Tharwa from which he raided local homesteads 1827–1828, before being arrested and transported to Norfolk Island. Tharwa Bridge, opened on 27 March 1895, crosses the Murrumbidgee River. Tharwa Primary School was opened soon after, in 1899. The Tharwa township narrowly avoided being burnt in the 2003 Canberra bushfires. The Tharwa community is currently

Deakin Volcanics red tuff

facing two further challenges: closures and repairs to Tharwa Bridge due to extensive rot in its supporting timbers discovered in 2005 and the 2006-07 Australian Capital Territory budgetannouncement of its plans to close the Tharwa preschool and primary school at the end of 2006. Tharwa Bridge was reopened for light traffic (less than 5 tonnes) in August 2008. Timber to further strengthen the bridge is currently being cured and work to reopen the brifge to heavy traffic should be complete by 2011.

Geology

Tharwa is in a different geological structural unit than the rest of Canberra, being on the Cotter Horst. The village itself is built on Tharwa Adamellite. This adamellite is coarsely foliated and contains biotite mica. It has been dated at 423 ±6 million years old. This places it in the upper Silurian age. The outcrop area is extended to the north north west to Freshford, and includes Castle Hill. It goes as far to the west as Sawyer’s Gully. To the south it goes close to Angle Crossing, and on the east side is bounded by the Murrumbidgee Fault. The Tharwa Adamellite is part of the Murrumbidgee Batholith. The latitude and longitude of Tharwa is 35°31’00S 149°04’00E. The geoid is 19.356 meters above the theoretical ellipsoid shape of the earth at Tharwa. The astronomical measurement of the position on the Earth’s surface is only very slightly distorted by a non vertical gravitational field 0.3″ to north and 0.6″ to the west. Magnetic declination at Tharwa is 11.817 deg east, total field strength is 43108 nT and magnetic inclination is -

66.031 degrees; as at 1 March 2006. Declination is increasing by 0.004 degrees per year. Inclination is increasing by 0.016 degrees per year (as in the rest of Canberra).

Geology of the Australian Capital Territory

Adaminaby Beds outcrop south of the ACT

Includes rocks dating from the Ordovician around 480 million years ago, whilst most rocks are from theSilurian. During the Ordovician period the region—along with most of eastern Australia—was part of the ocean floor. The area contains the Pittman Formationconsisting largely of Quartz-rich sandstone, siltstone and shale; the Adaminaby Beds and the Acton Shale. Most of the younger rocks are pyroclastic deposits from explosive volcanic eruptions, but the Yarralumla Formation is a sedimentary mudstone/siltstoneformation that was formed around 425 million years ago. In the 1840s fossils of brachiopods and trilobites from the Silurian period were discovered at Woolshed Creek near Duntroon by the Reverend William Branwhite Clarke. At the time these were the oldest fossils discovered in Australia, though this record has since been far surpassed. Other specific geological places of interest include the State Circle cutting and the Deakin anitcline.

The early European name for the district was “Limestone Plains”. In 1820, following the discovery of Lake George and the Yass River, Governor Lachlan Macquarie decided to send a party, with provisions for one month, to discover the Murrumbidgee River. Joseph Wild was accompanied by James Vaughan, a constable, and Charles Throsby Smith, a nephew of the explorer Charles Throsby. Detailed instructions had been given to the explorers by Charles Throsby, who had accompanied the Lake George exploration party earlier in the year. They were provided with acid to test for limestone. On 7 December 1820, Smith recorded in his journal:

“Came on to one of the plains we saw at 11 o’clock. At half past 1, came to a very extensive plain, fine Rich Soil and plenty of grass. Came to a Beautiful River plains that was running thro’ the plains in a S.W. direction, by the side of which we slept that night. When we made the Hut this evening, we saw several pieces of stone that had been burnt by all appearances. I then examined some of it, which proved to be limestone”

Construction of Tharwa Bridge 1893. Note the size of river before Tantangara Dam (1962) commenced diverting almost 50% of the river's flow.

There is, however, little limestone evident at the surface in the district. There is an outcrop at Acton, near the Museum of Australia, by the shores of Lake Burley Griffin. These formations became exposed when the ocean floor was raised by a major volcanic activity in the Devonian forming much of the east coast of Australia. Much of the western and southern parts of the Australian Capital Territory (ACT) are made from granite-like rocks. These are from the MurrumbidgeeBatholith intruding during the late Silurian or early Devonian times.

Tectonic context

Tectonics explains the large-scale structure of the Earth’s crust and its constituent rocks in terms of blocks moving along faults, uplifted into horsts or downthrown into grabens. The ACT is positioned on the Australian continent, which was once a part of the supercontinent Gondwana. The ACT is in the Tasmanides, the deformed rocks of the orogen that make up the core of the old mountain range that makes up the Australian continent east of the Tasman Line. These rocks are an addition onto the Proterozoic core of the continent. The Tasmanides are the result of compression, horizontal shortening, and vertical thickening of various “terranes” such as small continental fragments and volcanic island arcs that were plastered against the original continental margin as a result of plate tectonic movements. The Tasmanides also extended into Antarctica in the south and northern China on the north, as these continental units were attached to Australia at the time, in Gondwana. The ACT is part of the Eastern Lachlan Fold Belt, which is located on a terrane that is called the Benambra Terrane in Victoria, but the Molong-Monaro Terrane in New South Wales.

Australian Alps Walking Track

A sign marking the trail on the Bogong High Plains.

Is a long distance walking trail through the alpine areas of Victoria, New South Wales and ACT. It is 655km long, starting at Walhalla, Victoria and running through to Tharwa, ACT near Canberra. The track weaves mainly though Australian national parks, such asAlpine National Park and Kosciuszko National Park, though it is not exclusively restricted to national parks. It ascends many peaks including Mount Kosciuszko, Mount Bogong, and Bimberi Peak, the highest points in N.S.W., Victoria, and the A.C.T. respectively. The AAWT crosses exposed high plains including the Victorian Bogong High Plains and the Main Range in NSW. To walk the whole trail can take between 5 to 8 weeks. It has been signposted for part of its length in a tri-state agreement. However, most parts of the Track require hikers to have highly developed navigation skills. The Australian Alps Walking Track is an extension of the older Victorian Alpine Walking Track, established during the 1970s. The Victorian track was extended after many years of promotion by the Federation of Victorian Walking Clubs and various government departments. The NSW stretch of the walk is less imaginative then the Victorian section. Where the Victorian section typically follows spurs and ridges, the NSW section typically follows fire trails/tracks. The route recommended by John Siseman adds some interest to the NSW section of the walk.

Between Walhalla and Tharwa it passes through these National Parks:

• Baw Baw National Park
• Alpine National Park
• Kosciuszko National Park
• Namadgi National Park
• Brindabella National Park