in the Taiwan Strait area. The Sishuang islands () in Haidao Township, Xiapu County, Ningde, Fujian, China (PRC) are north-northwest of the township. The deepest point in the waters around the Matsu Islands is off the coast of Dongyin. After passing Liang Island (also referred as Wave Islet), ships riding from Nangan encounter stronger waves, which explains why Dongyin was originally called Tungyung (Dongyong). The waters around Dongyin host large schools of fish, and coral reefs, which makes Dongyin one of the best domestic fishing spots and the most densely populated conservation area for Larus crassirostris (). The geological formation of Dongyin is closely related to Mainland China, where the granite bedrock was formed by Yanshan Movement () dated to 100 – 200 million years ago. Thanks to its resistance to weathering, granite bedrock is seen above ground widespread in Dongyin. Although massive granite is tough, lacking internal structures, granite of poor primary permeability is easily eroded by sea water, rain, and gales, resulting in various coastal sights. The coastal line of Dongyin is maternally related to that in Fujian, drowned valleys () of Submergent Coastlines (). Caps and embayments, typical coastline traits, are prevalent. Caps feature cliff erosion, including sea caves, marine arches, sea stacks, sea trenches, and abrasion platforms, while sea caves are dominant. The less prevailing sedimentary landforms are mainly seen in the inlets and are characteristic of pebble beaches. Distinctive geological features On account of the geological distinctiveness of Dongyin, in “Natural Landscapes and Resources Survey in the Matsu Islands” (), conducted in 2000 by National Taiwan University Professor Ging-Chan Lin, twelve out of eighteen sights on the island were rated A. Among the 5 evaluation criteria, vividness, diversity, and scarcity best manifest the peculiarity of the island. Seeing as its various and geological formations, six features of Dongyin are introduced as below. Rocky coastlines The bedrock of Dongyin comprises mainly gray granite, characteristic of primitiveness and assorted texture. As a consequence, odd and jagged rocks are on display. Most of the hills on Dongyin are covered with grass, with granite underneath. The surrounding granite cliffs are connected with sea, waves, rocks, soil, grass, and sky, creating a wide range of colors, including navy blue, white, maize, and jade. Along the sea coast of Dongyin, the serial changes of colors can be taken in at a single glance. Suicide Cliff This is a towering abrasion cliff,