Much of the island lies within 15 meters of sea level. The area of igneous rocks in the center of the island forms rounded hills and gentle spurs and valleys, over which rise the peaks of Bukit Timah (Singapore's highest point at 163 meters), Bukit Gombak (139 meters), Bukit Panjang (132 meters) and Bukit Batok (106 meters). To the west and southwest of the island, the sedimentary rocks give rise to a series of narrow ridges, including that of Pasir Laba, Pasir Panjang, and Mount Faber.
The coast if flat, but in a few places, cliffs come down to the sea. Considerable stretches of the coastline are not natural formations = having been significantly modified by reclamation work, the building of embankments and swamp clearance.
Singapore is drained by a number of small streams. The largest of these, Sungei Seletar, is about 15 kilometers long. The Singapore River is 3 km long, from Kim Seng Bridge to the river mouth. The valleys through the built-up areas are now drained by tree-lined concrete channels rather than by natural streams
Granite occurs in two separate masses. The larger one is found in the central and northern areas, he smaller one in parts of the northeast. Granite or igneous rocks underlie the Bukit Timah Nature Reserve and the Central Catchment Area in the center of the island.
The sedimentary rocks of the Jurong Formations skirt the central granite and form extensive areas in souther, southwestern and western Singapore. These variations of conglomerate, sandstone and shale are also found on the islands to the south and west.
The semi-hardened Old Alluvium in the east was deposited by an ancient river system, probably in the Pleistocene epoch, during a low stand of the sea. It masks older rocks beneath.
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