The transition zone of mountains and valleys stretches between the Balkan Range to the north and the Macedonian-Rhodopean Massif to the south. The transition zone occupies and area of 30 807 m². It has a diversified relief of 31 mountains along the Sredna Gora Mountains with a length of 285 km and highest peak - the Bogdan Peak (1604 m). Apart from that it has numerous valleys between the mountains and vast plains, one of which is the Upper Thracian Plain. The plain occupies an area of 6 000 m ² and an average altitude of 165 m. It is the biggest plain in Bulgaria and on the Balkan Peninsula.