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.