Georgia is largely surrounded by the Greater Caucasus and Lesser Caucasus mountains, which form part of a natural boundary between Eastern Europe and West Asia. Because the Europe–Asia boundary is essentially a "historical and cultural construct", Georgia's continental placement has varied greatly. Anaximander placed the boundary between Europe and Asia along the Phasis River, which effectively located northern parts of Georgia in Europe and the south in Asia, a convention also followed by Herodotus. According to one 18th century definition, which set the Kuma–Manych Depression as the continental boundary, Georgia and the entire Caucasus fell into Asia. However, yet another definition drew the line at Aras River, effectively placing all of Georgia in Europe.