Winds blowing across the usually dry, sandy riverbed of Rio Salado formed dunes along this part of the Rio Grande Valley. The Rio Grande, just southwest here, follows a massive geological trench shaped millions of years ago when huge blocks of land sank as the earth in central New Mexico slowly began pulling apart. The resulting Rio Grande Rift uplifted Los Piños Mountains to the east. Northwest are the Sierra Ladrones. Rocks on their 9,000-foot crests are similar to those buried four miles beneath these dunes.