The Eastern Highlands encompass a sequence of mountains in the south crowned by Mount Kosciuszko and volcanic plugs, ash domes and flow remainders further north. Volcanic activity was extensive and there are enormous areas of related river sands and gravels in the mid-Shoalhaven valley, which are in the Cenozoic. Mount Canobolas was a chief volcano in diameter, now weather-beaten to disclose more than fifty leftover vents, plugs, dykes, and trachyte domes.