Great Sand Dunes

Waking up at Great Sand Dunes Lodge near the park entrance.

From here, the first daylight makes it look unreal.

The dunes appear almost pink in the early light.

Surprising some deer on my way back to the lodge lol.

The very first sun ray hitting the dunes.

Through the breaking apart and movement (rifting) of large surface plates on Earth’s surface, the Sangre de Cristo Mountains were uplifted in the rotation of a large plate. Fossils from the bottom of an ancient sea are now preserved in high layers of rock in the Sangre de Cristos. The San Juan Mountains were created through extended and dramatic volcanic activity. With these two mountain ranges in place, the San Luis Valley was born.

Sediments from both mountain ranges filled the deep chasm of the valley, along with huge amounts of water from melting glaciers and rain. The presence of larger rocks along Medano Creek at the base of the dunes, elsewhere on the valley floor, and in buried deposits indicates that some of the sediment has been washed down in torrential flash-flood events.

In 2002, geologists discovered lakebed deposits on hills in the southern part of the valley, confirming theories of a huge lake that once covered much of the San Luis Valley floor. They named this body of water “Lake Alamosa” after the largest town in the valley.

Lake Alamosa suddenly receded after its extreme water pressure broke through volcanic deposits in the southern end of the valley. The water then drained through the Rio Grande River, likely forming the steep Rio Grande Gorge near Taos, New Mexico.

Smaller lakes still covered the valley floor, including two broad lakes in the northeastern side of the valley. Large amounts of sediment from the volcanic San Juan Mountains continued to wash down into these lakes, along with some sand from the Sangre de Cristo Mountains.

Dramatic natural climate change later significantly reduced these lakes, leaving behind the sand sheet. Remnants of these lakes are still found today, in the form of Sabkha Wetlands.

Sand that was left behind after these lakes receded blew with the predominant southwest winds toward a low curve in the Sangre de Cristo Mountains. The wind funnels toward three mountain passes here – Mosca, Medano, and Music Passes – and the sand accumulates in this natural pocket.

The winds blow from the valley floor toward the mountains, but during storms the winds blow back toward the valley. These opposing wind directions cause the dunes to grow vertically.

Looking down the dunes into the Medano Flats.

Two mountain streams, Medano and Sand Creeks, also capture sand from the mountain side of the dunefield and carry it around the dunes and back to the valley floor. The creeks then disappear into the sand sheet, and the sand blows back into the dunefield.

This combination of opposing winds, a huge supply of sand from the valley floor, and the sand recycling action of the creeks, are all part of the reason that these are the tallest dunes in North America.

Currently, there is enough vegetation on the valley floor that there is little sand blowing into the main dunefield from the valley. However, even today there are still some small parabolic dunes that originate in the sand sheet and migrate across grasslands, joining the main dunefield.

At other times, some of these migrating dunes become covered by grasses and shrubs and stop migrating. Thus, the dunes system is currently fairly stable.