The northern chain of mountains begins near the Mexican border with the Cuchumatanes range, then stretches east through the Chuacús and Chamá sierras, down to the Santa Cruz and Minas sierras, near the Caribbean Sea. The northern and southern mountains are separated by the Motagua valley, where the Motagua river and its tributaries drains from the highlands into the Caribbean being navigable in its lower end, where it forms the boundary with Honduras.