Geology of Indonesia by Wallace

Wallace found that the region comprising Sumatra, Java, Borneo, the Thai-Malay Peninsula and Indochina was part of a partially submerged continental mass. Sundaland was assembled during the Triassic Indonisian orogeny (251 to 228 million years ago) and formed an exposed landmass during Pleistocene landmass (1.8 million years ago).

We know nowadays In spite of its shallow seas, it is not a stable region. A dynamic model explains that the maintenance of relief and hence sediment supply over long time.