The Gunnison River carved Black Canyon through a 50-mile block of rock called the Gunnison Uplift, whose edge rises in the slopes above. The Cimarron River cut through its walls and into the canyon, creating a geographic doorway to the river. Ute Indians, trappers, and explorers passed this way before the coming of the trains. In 1882 the Denver & Rio Grande Railroad spent some 15 months laying track down the canyon from Gunnison to reach the mining towns in the San Juan Mountains to the south. Cimarron began as an unruly camp and grew quickly, and work crews housed there dealt with debris and hazards in Black Canyon to keep open one of the most challenging stretches of rails in Colorado. By 1887, Cimarron's roundhouse held the extra engines needed to pull trains over Cerro Summit to the west.