During the Ordovician and Silurian Periods that part of the Lachlan Geosyncline lying between Yass, N.S.W., and the Victorian border changed from a deep-sea region with basic volcanics, volcanic graywackes, and cherts, to a shallow marine to subaerial platform dominated by acid volcanics, volcaniclastics, mudstones, and limestones. In the course of this development a quartz-graywacke distal flysch body spread throughout the region during the Late Ordovician. This was deformed during the Benambran Orogeny at the end of the Ordovician by gravity tectonics consequent upon the anatexis, metamorphism, folding, and uplift of the Wagga Metamorphic Belt to the west. The region remained deeply submerged after the Benambran Orogeny, and a series of submarine fans comprising a quartz graywacke proximal flysch was developed along its western margin during the late Llandoverian. Distal equivalents may be present, unrecognized, within the Ordovician flysch to the east. Early in the Wenlockian the Quidongan Orogeny, probably due to anatectic collapse of the sedimentary pile, has been recognized. It produced marked unconformities between early and late Silurian strata at Quidong and Canberra. It terminated flysch accumulation, and transformed much of the region into a low-lying land-mass. Marine conditions persisted in some areas, and acid volcanism, beginning near Yass, extended south and east during the remainder of the Wenlockian and Ludlovian. During this time a slight subsidence drowned the eastern half of the area, where late Silurian marine sediments and volcanics with stratiform ore deposits then accumulated, lying unconformably on the deformed Ordovician flysch.