Xiao Men Natural Monument Introduction
Xiaomendue is surrounded by the sea, shaped by long-term wave and tidal actions that create a variety of marine erosion landforms. Among them, the "Whale Cave" is an easily accessible and observable sea arch in Penghu, formed by the erosion of seawater that has hollowed out the sea cliff, hence its name as a sea arch. Inside the cave, one can hear the impressive sound of the waves. Additionally, the rock walls along the circumnavigation trail expose distinct and complete rock layers, consisting of silicon-rich basalt columnar joints, peat layers, siltstone layers, and sulfur layers from top to bottom, known as the "Volcanic Lake Remains." Like the Whale Cave, it boasts rarity and uniqueness, making it an excellent field study material for understanding marine erosion processes and geological evolution, and is thus designated as a natural monument of Xiaomendue. The Xiaomendue area is also famous for the "Xiaomendue Formation," which can be divided into upper and lower layers based on geological age analysis. The upper layer is shell limestone formed from biological shell debris, quartz grains, and tiny foraminifera, while the lower layer consists of alternating iron-rich quartz sandstone or sandy mudstone, which reveals clear rock layers due to weathering. This geological feature is most complete in the Xiaomendue area, leading to its naming as the "Xiaomendue Formation."
