Williamsdale is a village on the New South Wales - Australian Capital Territory border in Australia on the Monaro Highway. The postcode is 2620.
To the east of Williamsdale is the Williamsdale Volcanics. These are made up from blue-green crystal tuff. The crystals are sized from 0.3 mm to 1 mm and are embedded in a fine matrix. Quartz crystals make up 25%, plagioclase 5%, alkali feldspar 10%, biotite 5-10% which is altered to chlorite, epidote and leucoxene.
Middle Silurian Colinton Volcanics foliated dacite and tuff is under Williamsdale. A roughly north south band of these acid volcanics extends to the north along the Monaro Highway and then follows the Cooma road to Cottondale. South the volcanics also follow the Monaro Highway in a more complex band through Colinton past Michelago and at least as far south as Bredbo.
To the west of Williamsdale is an outcrop of Upper Silurian Laidlaw Volcanics dark grey rhyodacitic and dacitic crystal tuff. This extends to the west as far as Angle Crossing. This band extends to the north north west as far as Mount Stromlo. The outcrop finishes a couple of kilometres south, but it is also found to the east, with outcrop from Fernleigh in the north, via Burra, and south to Michelago.
On the west side of the Murrumbidgee River around Angle Crossing can be found the Bransby Beds or Goosoon Beds. These beds contain tuff and dacite with some sediments. They form a narrow band extending to the south as far as Cooma following on the east side of the Murrumbidgee River.