Complex sentence: The major pre-Columbian population centres of the highlands were located in the largest highland valleys, such as the Valley of Guatemala and the Quetzaltenango Valley in the southern highlands, a belt of volcanic cones running parallel to the Pacific coast.
Split into simpler sentences: The major pre-Columbian population centres of the highlands were located in the largest highland valleys, such as the Valley of Guatemala and the Quetzaltenango Valley. In the southern highlands, a belt of volcanic cones running parallel to the Pacific coast.