The northwestern extension, bounded by a dramatic escarpment known as "The Rim", extends much further into New Mexico, to near the Sacramento Mountains. The range is bounded on the north by Four Mile Canyon; on the east by the valley of the Pecos River; and on the west by Piñon Creek, Big Dog Canyon, Valley Canyon, Middle Dog Canyon, and West Dog Canyon. Much of the range is built from the ancient Capitán Reef that formed at the margins of a shallow sea during the Permian period. The Guadalupian epoch of the Permian period is named for these mountains, and the Capitanian age within this epoch is named for the Capitan reef. For details on the area's geology, see Delaware Basin. As the range is built up almost entirely of limestone, upland areas have little or no surface water. The only significant surface water is McKittrick Creek, in McKittrick Canyon, which emerges from the eastern side of the massif, just south of the New Mexico border. Elevations at the base of the range vary from above sea level on the western side to on the east. Several peaks on the southern end exceed .