From 40-50 million years ago the entire Pearl River Delta was an ancient bay where a volcanic eruption ejected a large quantity of lava into the sea. On cooling, the molten rock formed a conical mountain; the embryonic form of Mount Xiqiao. Thereafter eruptions of lava continued and produced the current 72 peaks. After several million years of erosion by the sea, the area flattened and was gradually colonized by vegetation.