Nigeria's most expansive topographical region is that of the valleys of the Niger and Benue river valleys (which merge and form a Y-shape). To the southwest of the Niger is a "rugged" highland. To the southeast of the Benue are hills and mountains, which form the Mambilla Plateau, the highest plateau in Nigeria. This plateau extends through the border with Cameroon, where the montane land is part of the Bamenda Highlands of Cameroon.