Complex sentence: The main valley settlements around the perimeter of the massif are at an altitude ranging between and but, because of the massif's great overall height, a considerable proportion is permanently glaciated or snow-covered and is exposed to extreme arctic conditions.
Split into simpler sentences: The main valley settlements around the perimeter of the massif are at an altitude ranging between and. Because of the massif's great overall height, a considerable proportion is permanently glaciated or snow-covered and is exposed to extreme arctic conditions.