There are three distinct belts lying parallel to the coast. The low coastal belt is about 40 km (25 mi) wide, with tidal creeks, shallow lagoons, and mangrove marshes. The land then rises to rolling hills, with elevations of 60–150 m (200–500 ft). The third belt, comprising the bulk of Liberia, is marked by abrupt changes of elevation in a series of low mountains and plateaus, less densely forested than the hilly region. The Nimba Mountains are near the Guinea frontier. The Wologizi Mountains reach a maximum of about 1,380 m (4,528 ft) with Mt. Wutuvi, the nation's highest point. Of the six principal rivers, all of which are at right angles to the coast and flow into the Atlantic Ocean, only the Farmington is of much commercial importance. Sandbars obstruct the mouths of all rivers, making entrance hazardous, and upstream there are rocky rapids.