The hills of Mount Faber, Telok Blangah and Kent Ridge form the Southern Ridges. The geology of the ridges mostly comprises sandstone and mudstone, as a result of the historical use and abandonment of the area, particularly Kent Ridge, as rubber plantations. The soils here are now mostly sedimentary with a lack nutrients for plant growth, and most of the plant species here are that of secondary forests. With an elevation of up to 70 metres (at Mount Faber), the ridges provide a rather steep hike for trekkers. In fact, these high points along the ridges around Pasir Panjang acted as fortresses for the British army to defend Singapore against the Japanese invasion during the Second World War.