Geology of the Australian Capital Territory

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.^[1]

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.^[2] 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.^[3][4]

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. ...^[5]

There is little limestone evident at the surface in the district, but 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.