In the 1840s, not far from the Duntroon Station (here) fossils of brachiopods (info) and trilobites (info) from the Silurian period (info) were discovered at the Woolshed Creek by the Reverend William Branwhite Clarke (bio here). Although now not the case, at the time these were the oldest fossils discovered in Australia.
The actual location of the fossil site is called Pialligo. The name Pialligo has been used for the area since at least 1820. Pialligo was the name of the original land grant of Duntroon and is probably of Aboriginal origin. The fossils can be found in the bed of Woolshed Creek which is a small tributary of the Molonglo River.
I (re)located the fossil bed right next to a bridge with a plaque fixed on the pillions declaring it's historical importance. Several Steel Spikes of a different era protrude from the rock and I would say they were some form of anchor point (original crossing perhaps). Sadly any remnants of exposed fossils have been removed since the time I originally saw them. I would say between professional and amateur "fossil hounds" the resource has been brutalised for 40 years. The nature of the rock is "shale like" and I imagine it would contain more specimens within.
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The entry information in the ACT Heritage Register is (here)(pdf)