 Cumberland Basin is the preferred physiographic and geological term for the low-lying plain of the Permian-Triassic Sydney Basin found between Sydney and the Blue Mountains, and it is a structural sub-basin of the Sydney Basin.The Cumberland Plain has an area of roughly .Shaping the geography of Sydney, it extends from north of Windsor in the north, to Picton in the south; and from the Nepean-Hawkesbury River in the west almost to Sydney City's Inner West in the east.