The Great Sand Dunes of Colorado were formed over millions of years through erosion and sediment deposition. Around 60-70 million years ago, erosion exposed underlying rock layers in the region. Between 40-50 million years ago, mudstones and sediments were deposited, beginning to form the features of the San Luis Valley. Around 18 million years ago, the valley continued dropping due to rifting, creating more space to accumulate sediments. Between 5-15 million years ago, glaciers deposited sand and sediments. Starting around 440,000 years ago in the Pleistocene, dune formation began in the San Luis Valley as glaciers melted and deposited large quantities of sediments. Today, the dunes continue to be fed by sediments transported by