Lake George is an ancient lake, 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. Lake George has in previous Ice Ages been much larger and deeper.
The thickness of sediment beneath the lake exceeds 250 metres, according to a Bureau of Mineral Resources Canberra drilling program 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 reversal stratigraphy
