The northern coastal region of Central Java has a narrow lowland. In the Brebes area, it is 40 km wide from the coast, while in Semarang, it is only 4 km wide. This plain continues with the depression of Semarang-Rembang in the east. Mount Muria at the end of the Ice Age (around 10,000 years BC) was a separate island from Java, which eventually fused because of alluvial deposits from flowing rivers. The city of Demak during the era of the Demak Sultanate was on the edge of the sea and became a thriving port. This sedimentation process is still ongoing on the coast of Semarang.