The average altitude of Denmark is about 30 m (98 ft), and the highest point, Yding Skovhoj in southeastern Jutland, is only 173 m (568 ft). In parts of Jutland, along the southern coast of the island of Lolland, and in a few other areas, the coast is protected by dikes. All of Denmark proper (except for the extreme southeast of the island of Bornholm, which is rocky) consists of a glacial deposit over a chalk base. The surface comprises small hills, moors, ridges, hilly islands, raised sea bottoms, and, on the west coast, downs and marshes. There are many small rivers and inland seas. Good natural harbors are provided by the many fjords and bays.