Away from the Shagang Village or Beiliao Village in Huxi Township, there are two quadrangular mesa islets on the sea surface about 4 kilometers to the northeast of Beiliao Village. The columnar joints of the surrounding basalt cliffs are very impressive. These islets connect to each other during the ebb. They were originally a lava plateau formed 120 million years ago and then cut into a larger and a smaller islet by the strong marine abrasion. The basalt on these two islets contains olivine masses, indicating that the lava came from deeper underground stratum.
Sea caves and sea grooves can be found on the east, south and west sides of the larger Jishan Islet. There is a shallow beach on its southeast side. The smaller Jishan Islet is a typical quadrangular basalt islet. The columnar basalts in different heights stand irregularly in the sea like a super-large pipe organ stands erect in the blue water under the azure sky. The hexagonal joints have visible edges and corners and sound sonorously. The smaller Jishan Islet is a good place for people to enjoy basalts and birdwatching.
Most roseate terns in the north sea area breed at Jishan Islet in summer. About 1,000 to 2,000 roseate terns select the islet as their habitat. When conducting bird banding research in recent years, the Wild Bird Society of Penghu identified the birds from the northeast of Australia regarding the flags tied on the legs of roseate terns. Likely, roseate terns flying from Penghu are identified in Australia during the bird banding in winter. Even the roseate terns released by the Wild Bird Society of Penghu 10 years ago flew back to Penghu for breeding again. It indicates the roseate terns fly a far distance back and forth between Penghu and Australia in spring and autumn every year.
Protecting the islets is the best way to protect wild animals. With the development of tourism, Jishan Islet has become a new point of interest in the east sea area. The interaction between the basalts and roseate terns forms a harmonious natural scene.