The geographical history of Rottnest Island has been dominated by changes in sea level. These changes occurred either as sea water became trapped and released when ice sheets advanced and retreated, or as the land slowly rose and fell in response to changing stresses in the earth's crust.
It is believed that Rottnest Island was separated from the mainland 7,000 years ago. The sea level rose, cutting the Island off from the land mass, and it is now the largest in a chain of islands (which includes Garden and Carnac Islands) on the continental shelf opposite Perth. These islands all are formed of limestone rocks with a thin covering of sand. The limestone base of Rottnest Island has an effect on all life on the Island, including the types of plants which can grow on it, the species of animals which can feed upon the plants, and the extent to which humans can make use of the Island.
The limestone coral reef surrounding Rottnest grew approximately 100,000 years ago when the sea level was thought to be at least three metres higher than the present day. This reef system provides a home to much of Rottnest's marine life, as well as presenting a significant hazard for shipping.
The total land area of Rottnest is 1,900ha. The Island has six major habitats: coastal, salt lakes, brackish swamps, woodlands, heath and settled areas. Salt lakes occupy ten per cent of the area of Rottnest Island. Many of them - including Lake Baghdad, Lake Vincent, Herschel Lake, Garden Lake, Government House Lake and Serpentine Lake - are permanent and have surrounding beaches. Other lakes such as Pink Lake, Lake Sirius, Lake Negri and the twin Pearse Lakes may dry out in summer.
