About 440,000 years ago, a massive lake named Lake Alamosa covered the San Luis Valley. As it dried up, it left behind vast deposits of sand. This sand, and the disappearance of the lake played a role in the slow formation of these dunes. . 👏📸wandering.russo . . . #greatsanddunesnationalpark #nationalpark #coloradohistory #explorecolorado #ColoradoAdventure #adventureguides #optoutside #sanluisvalleycolorado #nationalparks #outboundfolk