The municipality is generally flat, and swampy near the coast, but hilly in the far north, where the Yan Mountains intrude into northern Tianjin. Extensive tidal flats occur on the coastal plain adjacent to the city.[27] The highest point in the municipality is Jiuding Peak (九顶山) in Ji County on the northern border with Hebei, at an altitude of 1,078.5 m (3,538 ft).