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The geology of the Australian Capital Territory includes rocks dating from the Ordovician around 480 million years ago, whilst most rocks are from the Silurian. During the Ordovician period the region—along with most of eastern Australia—was part of the ocean floor. The area contains the ''Pittman Formation'' consisting 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/siltstone formation 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.〔ACT Heritage Council: ''(Woolshed Creek Fossil Site )'' 2004〕 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 anticline. 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. ... 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 Murrumbidgee Batholith 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.〔Plate tectonic processes in the southwest Pacific: a spatial and temporal context Barry Drummond, B. L. N. Kennett, R. J. Korsch, B. R. Goleby & P. A. Symonds Penrose Conference March 1999〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Geology of the Australian Capital Territory」の詳細全文を読む スポンサード リンク
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