The lake is believed to be more than a million years old. Originally, small streams drained its catchment into the Yass River, but then the Lake George Escarpment rose due to major crustal movement along a strong fault line, blocking this drainage and forming the lake.[1] Lake George has in previous Ice Ages been much larger and deeper.[citation needed]
The thickness of sediment beneath the lake exceeds 250 metres (820 ft), according to a Bureau of Mineral Resources Canberra drilling programme in the 1982/83 summer. The oldest sediments, which lie some distance above the bedrock, were dated at 3–5 million years using spore and pollen analysis and magnetic