Great Sand Dunes National Park is a complex geologic story going back millions of years. Intense volcanic activity built the San Juan mountain range to the west of the modern day dunes and tectonic activity raised the Sangre de Cristo Mountains to the north and east. A rift, or place where the Earth’s crust is being pulled apart, further widened the valley to cover more land than the state of Connecticut. Over time, melting ice and snow carried sediments into the valley creating a massive lake known as Lake Alamosa. About 440,000 years ago, Lake Alamosa dried up and created a system of much smaller lakes which are believed to be the primary source of sand which later formed into dunes. after the lakes dried.. The sand flats, or sabkha, formed after the large lakes evaporated and the leftover minerals cemented the sand into hard, flat sheets. Seasonal rise and fall of the water table dissolves and resolidifies the sand back into hard sheets in an annual cycle that continuously recycles the sabkha. The majority of the supply of sediment (roughly 90%) to the dunes lies in the sand sheet that surrounds the main dune field on three sides. This sand makes up much of the grassland in the park and is also the birthplace of other dune types, such as nebkha dunes and parabolic dunes. The dune field itself lies in a pocket within the Sangre de Cristo Mountains because southwesterly winds push the sand into this natural entrapment. Wind from the northeast funnels through the Sangre de Christo Mountains and pushes against the opposing southwestern winds, which erodes additional sand from the mountain sides and brings it into the dune system. This back and forth switching of the wind direction builds dunes on top of each other instead of pushing them continually forward. Another source of sand into the dune system is sediment runoff from the surrounding mountain ranges. Medano Creek and Sand Creek continually capture windblown sand from the mountains, collect new sediments, and carry it back down to the base of the dunes.