Extensive settlement began in the Valley , primarily by Hispanic farmers and ranchers from New Mexico , in the 1850s after the construction of Fort Massachusetts by the U.S. Army for protection against the Utes , who previously barred settlers . Settlers built a church in the village that is now called San Luis and dedicated it on the Feast of Saint Louis , June 21 , 1851 . The history of the post-war U.S. military presence in the Valley is preserved at Fort Garland and other sites in the Valley , which became part of the Territory of Colorado in 1861 . The lands in the valley were surveyed by the United States using the New Mexico Meridian and Baseline , unlike the rest of Colorado . The valley was one of eight candidate sites to detonate the first atomic bomb when White Sands Proving Ground was selected for the Trinity ( nuclear test ) . Today , the Valley has the largest native Hispanic population in Colorado ; many families are directly descended from the original New Mexican settlers . The original Ute population was confined to the Southern Ute and Ute Mountain Indian reservations in the late 19th century
Trinity ( nuclear test )
Trinity (nuclear test)
San Luis Valley