The Third Fort Union (1862 - 1891) was built in the traditional "territorial" style, and was constructed using native resources such as clay, stone, and lumber. The walls were constructed of adobe brick that stood on stone foundations, and were coated with plaster fired in limekilns. They were adorned with red bricks brought to New Mexico along the Santa Fe Trail.
The Third Fort Union
Other materials, such as tools, nails, window glass, fire bricks, and roofing tin were transported along the Santa Fe Trail from Fort Leavenworth. As beautiful as the fort appeared, it had been hastily constructed. Faulty roofing meant constant leaks onto the adobe structures, making repairs a frequent necessity.
The Post at Fort Union was made to accommodate four companies, both cavalry and infantry; and the Fort Union Quartermaster Depot was equipped to supply all other New Mexico forts. As the central supply hub for all New Mexico posts, the Fort Union depot was larger than the Post, and it employed more men, mostly civilians.
Fort Union stayed in operation until the arrival of the Atchison, Topeka, and Santa Fe Railroad in 1879 slowly put an end to the Santa Fe Trail. The railroad ended one era and opened another in the Southwest. Fort Union lingered on for another ten years, and as the buildings deteriorated the end was in sight. The post, along with the trail, had outlived its usefulness. On February 18, 1891, the Las Vegas Optic reported that "The last few days have told a terrible tale at Fort Union. Four days ago everything was in running order, now everything is upside down and inside out...The soldiers are busy packing government and private property."
After its abandonment in 1891 the once majestic buildings fell into ruins and faded into the pages of history. Fort Union National Monument was established in 1956, after a group called Fort Union Inc., and others, advocated for its protection and preservation. Today, the Fort Union ruins stand as a reminder of its unique history and the vital role it played in the development of the Southwest.