The Guadalupe-Nipomo Dunes were formed by a combination of factors including beach sand which was blown inland by the wind and the Santa Maria River which brought sediment to the coast. Dune-building began 18,000 years ago with the Nipomo and Orcutt Mesas. This Dune System has the highest dunes on the entire western coastline of the United States. Among these, Mussel Rock Dune is the highest, measuring approximately . Another rare geographic treasure is Oso Flaco Lake, a freshwater lake located amid the Dunes.