Jedediah Smith is thought to have been the first American to explore the area in 1826. A fur trapper, Smith was soon followed by other pioneers traveling the Old Spanish Trail between Santa Fe and Los Angeles. The trail crossed the area on the eastern edge of Fort Irwin, between Salt Spring and the Mojave River. The Old Spanish Trail passed through Silurian Valley, then west through the Avawatz Mountains at Red Pass and beyond the playa of Red Pass Lake, through a gap between the Soda and Tiefort Mountains to Bitter Spring in a wash in the next valley. Bitter Spring was the only reliable watering and grazing place along the route. From Bitter Spring the trail led 18.75 miles southwest climbing Alvord Mountain to cross Impassable Pass to descend Spanish Canyon and cross the plains to the location of Fork of the Road on the north side of the Mojave River where it met the Mohave Trail.