million years, an ice sheet entered the area of Poland eight times, bringing along with it changes of reaches of the river. In warmer periods, when the ice sheet retreated, the Vistula deepened and widened its valley. The river took its present shape within the last 14,000 years, after the complete recession of the Scandinavian ice sheet from the area. At present, along with the Vistula valley, erosion of the banks and collecting of new deposits are still occurring. As the principal river of Poland, the Vistula is also in the centre of Europe. Three principal geographical and geological land masses of the continent meet in its river basin: the Eastern European Plain, Western Europe, and the Alpine zone to which the Alps and the Carpathians belong. The Vistula begins in the Carpathian mountains. The run and character of the river were shaped by ice sheets flowing down from the Scandinavian peninsula. The last ice sheet entered the area of Poland about 20,000 years ago. During periods of warmer weather, the ancient Vistula, "Pra-Wisła", searched for the shortest way to the sea—thousands of years ago it flowed into the North Sea somewhere at the latitude of contemporary Scotland. The climate of the Vistula valley, its plants, animals, and its very character changed considerably during the process of glacial retreat. Navigation The Vistula is navigable from the Baltic Sea to Bydgoszcz (where the Bydgoszcz Canal joins the river). The Vistula can accommodate modest river vessels of CEMT class II. Farther upstream the river depth lessens. Although a project was undertaken to increase the traffic-carrying capacity of the river upstream of Warsaw by building a number of locks in and around Kraków, this project was not extended further, so that navigability of the Vistula remains limited. The potential of the river would increase considerably if a restoration of the east–west connection via the Narew–Bug–Mukhovets–Pripyat–Dnieper waterways were considered. The shifting economic importance of parts of Europe may make this option more likely. The Vistula is the northern part of the proposed E40 waterway, continuing eastward into the Bug River, linking the Baltic Sea to the Black Sea. Historical relevance Large parts of the Vistula Basin were occupied by the Iron Age Lusatian and Przeworsk cultures in the first millennium BC. Genetic analysis indicates that there has been an unbroken genetic continuity of the inhabitants over the last 3,500 years. The Vistula Basin along