Norway is formed of some of the oldest rocks in the world. It is dominated by mountain masses, with only one-fifth of its total area less than 150 m (500 ft) above sea level. The average altitude is 500 m (1,640 ft). The Glittertinden (2,472 m/8,110 ft, including a glacier at the summit) and Galdhøpiggen (2,469 m/8,100 ft), both in the Jotunheimen, are the highest points in Europe north of the Alpine-Carpathian mountain range. The principal river, the Glåma, 611 km (380 mi) long, flows through the timbered southeast. Much of Norway has been scraped by ice, and there are 1,700 glaciers totaling some 3,400 sq km (1,310 sq mi). In the Lista and Jaeren regions in the far south, extensive glacial deposits form agricultural lowlands. Excellent harbors are provided by the almost numberless fjords, deeply indented bays of scenic beauty that are never closed by ice and penetrate the mainland as far as 182 km (113 mi). Along many coastal stretches is a chain of islands known as the skjærgård.