The Pittman Formation was described originally by Öpik in 1958 who named it after the Pittman Valley, southeast of Aranda, Australian Capital Territory. It is entirely of Ordovician age and about 800 metres thick near Canberra, but at Captains Flat it is over 1,200 metres thick. The lower levels are greywacke, exposed east of Queanbeyan and north are very thick and heavily overturned and thrusted. At Etheridge Creek, the type locality is a repeating pattern of sandstone, micaceous sandy shale, mudstone, black argillaceous and radiolarian chert. In the sandstone beds there are occurrences of graded bedding, clay pellets, and current bedding. Fossils of graptolites, radiolarians, conodonts, and occasionally brachiopods and sponges are found. The geological formation east of Queanbeyan used to be known as the Muriarra Formation. This alternates between sandstone with a high quartz content and mica, and phyllite. Radiolarian chert is found in the central section. The railway forms the border between Queanbeyan in NSW and Oaks Estate in the ACT. West of the Queanbeyan railway station is a cutting where folding has overturned the beds, with axes dipping to the east at 50 degrees. 300 metres of thickness is exposed in this cutting. Llanvirnian age (Pygodus serrus conodont zone). Fossils found include Phyllograptus anna, Trigonograptus ensiformus, Pterograptus, Didymograptus, Isograptus, Hallograptus from the Darriwillian age. Near the top of the formation are fossils Dicellograptus sextans, D divaricatus, D salopiensis, which are Gisbornian.