Geologically, the Balkan Mountains are a mountain chain of fold mountains, a young part of the Alp-Himalayan chain that stretches across most of Europe and Asia. It can be divided into two parts: the main Balkan Chain and the Pre-Balkans (Fore-Balkan) to the north, which intrude slightly into the Danubian Plain. To the south, the mountains border the Sub-Balkan valleys - a row of 11 valleys running from the Bulgarian border with Serbia east to the Black Sea, separating the Balkan mountains from a chain of other mountains known as Srednogorie which includes Vitosha and Sredna Gora.