The Datun Volcanic Group began to erupt, forming a higher volcanic terrain. The changes in the terrain caused the ancient Danshui River to flow northward into the Taiwan Strait, and the Linkou alluvial fan stopped developing until about 400,000 years ago. The collapse caused Linkou to rise to a platform, while Taipei began to sink to form a basin. The Taipei Basin is the second largest basin in Taiwan and the most densely populated area in Taiwan.