Deposition occurred in a shallow marine environment with a delta. The sea level was relative higher during this depositional phase compared with the earlier and later subaerial volcanic deposit periods. Its age is early Ludlow. This has been determined by shelly marine fossils, and a tonalite intrusion southwest of Red Hill with age determined as 417±8 Mya. Outcrops occur on Red Hill (the hill) and throughout Deakin, Australian Capital Territory and in southern Yarralumla, and also Hughes, Australian Capital Territory. The formation extends from Red Hill and Woden in the South to Yarralumla and Lake Burley Griffin in the north. The formation is evidence of the last major period when eastern Australia was still covered by shallow seas. It shows fossil evidence of trilobites, coral and primitive crinoids.