Spotted Wolf Canyon cuts through the San Rafael Reef, a 30-mile-long stone barrier and sawtooth ridge on the edge of the San Rafael Swell, where for centuries only the most intrepid travelers found passage through narrow slot canyons to the Green River crossing. Early Spanish explorers detoured 20 miles north to avoid the wall, and in 1883 the Denver and Rio Grand Railroad likewise avoided breaching the Reef, laying track just east of its sheer walls on the route north to Salt Lake City. In 1957, as the nation’s interstate highway system was expanded, I-70 was engineered to bisect the San Rafael Swell at this canyon, where before construction began in October 1967 workers could stand and touch both walls. Engineers and surveyors worked with body harnesses and ropes as high as 400 feet above the canyon floor, and crews excavated 3.5 million cubic yards of rock at a cost of $4.5 million for eight miles of road. On November 5, 1970, the Utah Department of Transportation opened the 70-mile section from Fremont Junction to Green River for two-lane traffic, and two more lanes were added in the mid-1980s.