Where It All Began
Here is how the Great Sand Dunes were made:
500 million years ago, during the Paleozoic Era, Great Sand Dunes National Park started to form. Sediments were deposited everywhere in the region by rivers and streams. Then, pressure inside the Earth pushed the land really high so that it ended up above sea level. The landform was called the "Laramide Orogeny" back then and it still has sedimentary rocks from 60 to 70 million years ago. In some places, the sediment is completely gone, showing off the metamorphic rock below it, and in other places, there is still much sedimentary rock remaining. The sedimentary rocks included shale, a rock that can break into many thin pieces, sandstone, which is made of sand and bits of quartz, and conglomerate, which is made of sand, clay, and bits of other minerals or rocks such as limestone. About 40 million years ago, deserts came and all of the lakes and rivers dried up. Then, about 34 million years ago, a volcano erupted in the San Juan Mountains, giving it a new layer of rock called the Conejos Formation. Nineteen million years ago, the Sangre De Cristo Mountains got taller. This caused rifting which widened the valley and made the floor drop. This process continues even today. About five million years ago, more volcanoes started erupting and that caused rivers to dam up in the San Luis Basin. Then, the Pleistocene Era started about 1.8 million years ago, and there was a huge change in the climate. This caused glaciers to form in mountain valleys. The glaciers had lots of rocks and ice, so twelve thousand years ago, when the glaciers finally melted, all of the rocks, silt, and gravel from the glaciers were carried by streams and rivers that were newly formed from the melted ice into the San Luis Valley. This process continues today.