Yarralumla is located on the Yarralumla Formation which is a mudstone/siltstone formation that was formed around 425 million years ago during the Silurian Period. The formation extends from Red Hill and Woden in the South to Lake Burley Griffin in to the north, passing under the suburb of Yarralumla. The formation is evidence of the last major marine sedimentary period when eastern Australia was still covered by shallow seas. It shows fossil evidence of trilobites, coral and primitive crinoids. The Yarralumla brickworks quarry and the Deakin anticline are places where the formation is exposed and easily studied.