Wangan Township governs many islands, and bigger islands with people living there are mostly square mountain terrain. Wangan Island is showed cut square mountain terrain, and its outline is like a standing rabbit. The highest point is in the west coast and 53 meters high. The north and east coasts are very different. The north end is concave like rabbits’ ears in both east and west. There are Putai port and Tanmen port along east coast, and Mentzupien peninsula extending to southeast. Coasts surrounding the whole island are not common scenes and only appear in Chihketzu of northeast, and the 30~40 meters long coast from Tientai mountain of west to Chungshe. Chiangchunao Islet is also a square mountain terrain with 29 meters height, extending from northeast by east to southwest by west. The bays along the west cost of Tungchi Islet forms an isthmus with east coast that separates the islet into two small square mountain terrains in south and north. The highest point of northern small square mountain is 48 meters; the highest of the southern small square mountain is 39 meters.
Hsichiyu is an inclined small square mountain island, high in north and low in south. The 27 meters high in north tilt gradually to the south (by west), and sink in the sea near the south. Tienyuping is a 50~60 high square mountain which extends from northeast to southwest. According to the east small valley, they are divided into the North and South Mountain. Hsiyuping level ground is an islet in the shape of tetragon square mountain with the surfaces flat height of 40 meters. Both Big Cat Islet and Small Cat Islet are irregular, massy islands. Big Cat Islet is about 79 meters, and the highest in Penghu archipelago.

1.  Basalt
Except for Flower Islet, other islets are all composed of mezzanines of basalts that was formed by lava flows and of deposited rock. In the primary stage, basalt lava belongs to the poured rock of volcano under the sea. It has horizontal joints, and several layers of deposited rock that contain walrus fossils. Afterwards, lava continually pouring out has remarkable vertical pillar joints. The reason of the vertical pillar joints formation is that when lava rock poured out of the surface of the earth, it made basalt construct rapidly and form six-angled pillar because of quick cooling. Furthermore, because surface lava solidifies first and bottom lava later, so it composed rupture surfaces between top and down that are commonly said slate joints.
2.  Andesine
The terrain of Flower Islet is very different from others in Penghu. It is mainly the middle-aged Mesozoic andesine lava composed of porphyry, quartz porphyry and quartz dikes. The constructional age of these rocks are earlier; andesine constructs most parts of Flower Islet; quartz porphyry becomes thick dikes of meters that passes through Flower Islet; then quartz dikes invades. Andesine is in deep green color with hard and dense rock structure. Quartz porphyry is mostly under weathering that it becomes yellow and grayish. Quartz is not easy to be weathered so quartz granules remain and pile up in concave slopes so that it becomes quartz sand stratums.