Italy contain their own complex features, as does mainland Europe itself, where the relief contains many plateaus, river valleys, and basins that complicate the general trend. Iceland and the British Isles are special cases. The former is of North Atlantic volcanic formation, while the latter consist of upland areas once joined to the mainland until cut off by rising sea levels. Peninsula of peninsulas Europe is sometimes called a "peninsula of peninsulas", to draw attention to the fact that Europe is a relatively small, elongated appendage to Asia, and that a large part of Europe is made up of peninsulas. A prehistoric perspective would include Britain and Ireland as the core of a further very significant European peninsula prior to the post-glacial rise in sea-levels. Partial list of European peninsulas Absheron peninsula Balkan peninsula Peloponnese Chalkidiki Istria Gallipoli Brittany Cotentin Peninsula Crimea Fennoscandian Peninsula Kola Peninsula Scandinavian Peninsula Iberian Peninsula Italian Peninsula Jutland Kanin Peninsula Geology Europe's most significant geological feature is the dichotomy between the highlands and mountains of Southern Europe and a vast, partially underwater, northern plain ranging from Great Britain in the west to the Ural Mountains in the east. These two halves are separated by the mountain chains of the Pyrenees and the Alps/Carpathians. The northern plains are delimited in the west by the Scandinavian mountains and the mountainous parts of the British Isles. The major shallow water bodies submerging parts of the northern plains are the Celtic Sea, the North Sea, the Baltic Sea complex, and the Barents Sea. The northern plain contains the old geological continent of Baltica, and so may be regarded as the "main continent", while peripheral highlands and mountainous regions in south and west constitute fragments from various other geological continents. The geology of Europe is hugely varied and complex, and gives rise to the wide variety of landscapes found across the continent, from the Scottish Highlands to the rolling plains of Hungary. Population Figures for the population of Europe vary according to which definition of European boundaries is used. The population within the standard physical geographical boundaries was 701 million in 2005 according to the United Nations. In 2000 the population was 857 million, using a definition which includes the whole of the transcontinental countries of Russia and Turkey. Population growth is comparatively slow, and median age comparatively high in relation to the world's other continents. Rivers The most important