These do not match the age of zircons from the interior of the Australian Shield, so the source of the sediments is from another continent. The other evidence from the Ordovician sediments are chemical composition indicates granitic source, with the absence of feldspar. Secondly the fine grained nature shows the sediments have been transported a long way from their ultimate, source, and could have been second cycle, derived from sedimentary rocks. At the time these sediments were deposited the location was at least 3,000 km from the Australian continent. Underneath there is no Proterozoic continental basement, instead these sediments are lying on top of oceanic crust. Acton Shale is a grey to black thinly laminated silceous shale containing graptolites. It is generally leached, and frequently silicified. The colour of the beds alternates between grey and black, but where weathered, it changes to whitish grey. The graptolites appear on the bedding planes as black films. In Canberra, the Acton Shale appears in several outcrops in Acton, in two bands through Aranda, through Bruce near the Calvary Hospital, on the Bruce Ridge behind Lyneham. Also another band starts under the University of Canberra in Belconnen, and heads north east through Lawson and Giralang and folded and faulted into several bands in Crace on Gungahlin Hill. Another band is found on the west side of Queanbeyan, extending north to Dundee, and south around the east side of Jerrabomberra Hill. Acton shale is only preserved in the cores of synclines, being eroded from uplifed parts. Brachiopods, conodonts and sponges fossils are rarely found. The beds are up to 60 metres thick and appear high in the Pittman Formation. The age range is Gisbornian to Bolindian of the Ordovician period. Lower beds contains fossils like Dicranograptus nicholsoni. Upper beds contain Climacograptus bicornbis, Chastatus, C tubuliferis, Dicellograptus elegans, and Dicranograptus hians which are late Eastonian in age. The sediments making the Acton Shale were deposited in the ocean in a reducing environment starved of oxygen, and lacking fresh sediment. The State Circle Shale was named by Öpik in 1958.