Surrounded by ocean, Xiaomen Island has long been subject to marine erosion by tides and waves, resulting in diverse landforms. The Whale Cave is an easily accessible arch made by marine abrasion. It has been created by eons of waves striking against the cliffside. If you duck under it, you’ll hear the ocean’s roar.

Exposed rock strata are visible on the rock wall beside the trail circling the island. From top to bottom, you can see siliceous basalt columnar joints, peat layers, siltstone layers, and sulfur layers, the vestiges of a volcanic crater. Like the Whale Cave, it has a rare and unusual landscape, making it the ideal teaching material for understanding marine abrasion and geological history. That is why Xiaomen Island has been designated as a natural monument.

Xiaomen Island is also noted for the Xiaomen Island Strata, which is divided into an upper and lower layer based on geological age analysis. The upper layer consists of limestone crust, cemented by shell detritus, quartz grains, and tiny foraminifera; the lower layer comprises ferrous quartz sandstone interbedded with shale. Weathering has exposed the above stratification. Because of the clear stratification, this area is named the Xiaomen Island Strata, highlighting the extraordinary geological evolution of this island.