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The Xiaomenyu is surrounded by sea on all sides and has been long shaped by wave and tidal influences, creating various sea-eroded landforms. Among these, the "Whale Cave" (Jingui Dong) is the easily accessible and observable sea arch in Penghu. Its formation occurred as waves eroded through the coastline, forming a hollowed-through structure, hence it is termed a sea arch. Walking inside the cave, visitors can hear impressive waves. Moreover, along the coastal trail, clearly exposed rock layers reveal the "Volcanic Crater Lake Remnants" on cliff faces. From top to bottom, they include siliceous basalt columnar joints, peat layer, siltstone layer, and sulfur layer. Like the Whale Cave, these features are rare and unique. They serve as excellent field examples for studying coastal erosion evolution and geological development, hence both were announced as Natural Monuments of Xiaomen. Another notable feature in the Xiaomen area is the "Xiaomenyu Formation," which geological analyses divide into upper and lower layers. The upper layer is limestone, formed by the cementation of biological shell remains, quartz grains, and microscopic foraminifera. The lower layer consists of alternating iron-rich quartz sandstone and siltstone layers, clearly exposed through weathering. Because Xiaomen region displays the most complete exposure of this geological characteristic, it was named the "Xiaomenyu Formation."