Xiaomen Natural Monument Introduction
Xiaomen Islet is surrounded by the sea, and over time, it has been shaped by waves and tides, creating diverse coastal erosion landforms. Among them, the "Whale Cave" is an accessible and observable sea-eroded arch in Penghu. It was formed by the erosion of waves that penetrated the cliff, creating a hollow space, hence it is called a sea-eroded arch. Inside the cave, visitors can also hear the impressive sound of crashing waves. Additionally, the rock wall along the island path reveals distinct and complete rock layers. From top to bottom, these layers include siliceous basalt columnar joints, peat layers, siltstone layers, and sulfur layers, which together are known as the "Crater Lake Remains." Like the Whale Cave, the Crater Lake Remains possess rarity and uniqueness, making them excellent field materials for understanding the processes of coastal erosion and geological evolution; thus, they have both been designated as natural monuments of Xiaomen. The Xiaomen area is also famous for the "Xiaomen Islet Formation," which geologic analysis divides into upper and lower layers. The upper layer consists of shell limestone formed from biogenic shell debris, quartz grains, and tiny foraminifera, while the lower layer consists of interbedded iron-rich quartz sandstone or sandy mudstone, which are clearly exposed due to weathering. Because of such geological characteristics, the Xiaomen area showcases these features most completely, leading to its naming as the "Xiaomen Islet Formation."
