It literally was a basin more than 190 million years ago and was open to the sea. it stretched from around Port Stephens in the north, inland to where Lithgow is today and south down the coast to around Nowra! Then over that 190 million year period the basin silted up forming sandstone and shale formations. The underlying vegetation and marine life fossilized and compressed to become the valuable coal seams being mined in the Sydney basin today. Then sections of the basin buckled around 12 million years ago to become the Blue Mountains. Weren't we lucky to have such reserves of energy to provide cheap and reliable electricity. Where would we have been without such a coal benefactor?
