Fort Union National Monument protects remnants of the Southwest's largest frontier fort, including earthworks, archeological deposits, and stabilized adobe ruins. In addition, a network of Santa Fe Trail ruts is still visible on the surrounding prairie. These cultural resources are part of a landscape developed over the last 100 million years, including deposition of sandstone and shale in the Western Interior Seaway, which extended from the Arctic to the Tropics, covering the entire west-central part of the North American continent.