It formed as sea levels rose at the end of the last ice age. The almost uniform flatness of the sea bottom and the presence of drainage channels (traceable to the mouths of island rivers) indicate that the Sunda Shelf was once a stable, dry, low-relief land area (peneplain) above which were left standing a few monadnocks (granite hills that by virtue of their resistance to erosion form the present islands).
On the North. By the Southern limit of the South China Sea [Lucipara Point ( ) thence to Tanjong Nanka, the Southwest extremity of Bangka Island, through this island to Tanjong Berikat the Eastern point ( ), on to Tanjong Djemang ( ) in Billiton, along the North coast of this island to Tanjong Boeroeng Mandi ( ) and thence a line to Tanjong Sambar ( ) the Southwest extreme of Borneo], the South coast of Borneo and the Southern limit of Makassar Strait [By a line from the Southwestern extreme of Celebes ( ), through the Southern point of Tana Keke, to the Southern extreme of Laoet ( ) thence up the West coast of that island to Tanjong Kiwi and thence across to Tanjong Petang, Borneo ( ) at the Southern end of Laoet Strait].
On the East. By the Western limit of Flores Sea [A line from Tg Sarokaja ( ) to the Western Paternoster island ( ) thence to the Northeastern Postiljon Island ( ) and to the West point of Laikang Bay, Celebes].
On the South. By the Northern and Northwestern limits of Bali Sea [A line from the Western Paternoster Island to the East point of Sepandjang and thence through this island to the West point of Gedeh Bay on the South coast of Kangean ( ). A line from the West point of Gedeh Bay, Kangean Island, to Tg Sedano, the Northeast extreme of Java and down the East coast to Tg Bantenan, the Southeast extreme of the island], the North and West coasts of Java to Java Hoofd ( ) its Western point, and thence a line to Vlakke Hoek ( ) the Southern extreme of Sumatra.
On the West. The East coast of Sumatra between Vlakke Hoek and Lucipara Point ( ).
The southern section of the seafloor has long been recognized as geologically similar to northern Java, where oil fields occur and extend under the sea. Prospects are also favourable for oil fields in the waters off southeast Kalimantan. As the site of successful exploration for petroleum and natural gas, the Java Sea has become the basis of Indonesia's export program.