The contemporary relief of Bulgaria is a result of continuous geological evolution. The Bulgarian lands were often submerged by ancient seas and lakes, some land layers rose others sank. Volcanic eruptions were common both on land and in the water basins. All three main groups of rocks, magmatic, sedimentary and metamorphic, are found in the country. The oldest rock formations in Bulgaria date from the Precambrian period than 500 million years ago. During the Archean, Proterozoic and Paleozoic eras (4.0 billion to 252 million years ago) the magmatic rocks were formed. Throughout most of that period the only land areas were Rila, Pirin and the western Rhodope Mountains. The Mesozoic era (252 million to 66 million years ago) saw the beginning of the Alpine orogeny that has formed the mountain ranges of the Alpide belt, including the Balkan Mountains and Sredna Gora. The Cenozoic era (since 66 million years ago) is characterized with active tectonic processes, the definitive formation of the Balkan Mountains, the formation of grabens and horsts in Rila, Pirin and Kraishte region.