Geology of the Great Sand Dunes National Park & Preserve

The Great Sand Dunes National Park & Preserve is home to the very tallest of sand dunes, but you’ll probably start wondering about where they came from and how they got there. What’s the story?

Well, the story of the Great Sand Dunes National Park & Preserve and how the great dunes were formed is one of those never ending stories – new research is making new discoveries by the year, but here’s the story so far . . .

The Sangre de Cristo Mountains (which border the park) were created by the movement and breaking apart of the plates on the earth’s surface. Evidence of this is shown as there are ancient, sea bed fossils preserved high up in the rock layers. Dramatic and extensive volcanic activity then created the San Juan Mountains, and within these two ranges was born the San Luis Valley, an extensive area the size of the entire state of Connecticut.  The sediments from both of these extensive mountain ranges filled up the valley, as well as water from rivers and mountain streams.

A huge lake once covered the entire San Luis Valley floor, evidence of this was discovered as recently as 2002 which confirmed earlier theories. This body of water – named Lake Alamosa – eventually receded due to climate change and due to the water eating through the volcanic deposits towards the south of the valley. Once this wall was breached the water could well have drained into the Rio Grande River. There are still some smaller lakes which have survived in the valley floor, although their sizes have also been significantly reduced due to changes in the climate.

As these lakes receded they left an awful lot of sand behind, the predominant southwesterly winds blowing it towards the relatively low curve of the Sangre de Cristo Mountains. The sand was funneled towards three mountain passes – Medano, Music and Mosca passes with the sand accumulating within the natural pocket. The dunes have grown vertically due to the opposing winds – generally blowing towards the mountains from the valley floor but blowing back towards the valley during the regular storms.

So that’s how come these dunes, the tallest sand dunes in North America got so tall. It’s all thanks to the opposing winds, the enormous supply of sand which was available on the valley floor, plus sand which is recycled due to the action of the various creeks.  The site of these giant sand dunes rising in the foreground of the alpine Sangre de Cristo mountains, with seasonal streams flowing through the sand is, you must admit, a rather unexpected and unusual sight.

Absolutely amazing.