Geologically, the Sunda Shelf is a south-eastern extension of the continental shelf of Mainland Southeast Asia. Major landmasses on the shelf include the Bali, Borneo, Java, Madura, and Sumatra, as well as their surrounding smaller islands. It covers an area of approximately 1.85 million km2. Sea depths over the shelf rarely exceed 50 metres and extensive areas are less than 20 metres resulting in strong bottom friction and strong tidal friction. Steep undersea gradients separate the Sunda Shelf from the Philippines, Sulawesi, and the Lesser Sunda Islands. Definition Biogeographically, Sundaland is a term for the region of Southeastern Asia which encompasses these areas of the Asian continental shelf that were exposed during the last ice age. Sundaland included the Malay Peninsula on the Asian mainland, as well as the large islands of Borneo, Java, and Sumatra, and their surrounding islands. The same steep undersea gradients that mark the eastern boundary of Sundaland are identified biogeographically by the Wallace Line, identified by Alfred Russel Wallace, which marks the eastern boundary of Asia's land mammal fauna, and is the boundary between the Indomalayan and Australasian realms. The shelf has resulted from millennia of volcanic activity and erosion of the Asian continental mass, and the build up and consolidation of debris along the margins as sea levels rose and fell. The seas between the islands cover relatively stable ancient peneplains that are characterised by low seismicity, low isostatic gravity anomalies and no active volcanoes with the exception of Sumatra, Java, and Bali, which while connected to the Sunda Shelf, belong geologically to the young Sunda Arc orogenic system (i.e., the Sunda Mountain System). During glacial periods, the sea level falls, and great expanses of the Sunda Shelf are exposed as a marshy plain. The rise of sea level during a meltwater pulse 14,600 to 14,300 years Before Present was as much as 16 meters within 300 years. Present sea levels submerge a number of Pleistocene paleo river systems that drained much of Sundaland during the last glacial maximum 18,000 to 20,000 years ago. To the east of the Sunda Shelf is the Sahul Shelf. Separating these two regions of shallow seas is Wallacea, which encompasses Sulawesi and the thousands of smaller islands making up Nusa Tenggara and Maluku. Within Wallacea lie some of the deepest seas in the world, with depths of up to 7,000 metres. Passing between Bali and Lombok, and Borneo