Complex sentence: His route took him through the Chuska Mountains to the Hopi mesas in what is now Arizona, then north towards Utah, reaching Oljeto Creek in what is now San Juan County, Utah.
Split into simpler sentences: His route took him through the Chuska Mountains to the Hopi mesas in what is now Arizona. The expedition went north towards Utah, reaching Oljeto Creek in what is now San Juan County, Utah.