Perth is on the Swan River, named for the native black swans by Willem de Vlamingh, captain of a Dutch expedition and namer of WA's Rottnest Island, who discovered the birds while exploring the area in 1697. This water body was known by Aboriginal inhabitants as "Derbarl Yerrigan". The city centre and most of the suburbs are on the sandy and relatively flat Swan Coastal Plain, which lies between the Darling Scarp and the Indian Ocean. The soils of this area are quite infertile.