The dunes were formed from sand and soil deposits of the Rio Grande and its tributaries, flowing through the San Luis Valley. Over the ages, glaciers feeding the river and the vast lake that existed upon the valley melted, and the waters evaporated. Westerly winds picked up sand particles from the lake and river flood plain. As the wind lost power before crossing the Sangre de Cristo Range, the sand was deposited on the east edge of the valley. This process continues, and the dunes are slowly growing. The wind changes the shape of the dunes daily. Scientist agree that the dunes are probably less than 440,000 years old. What? Less than 440,000 years old? Who gives stats like that? Are they giving themselves room for error by 400,000 years?
There are several streams flowing on the perimeter of the dunes. The streams erode the edge of the dune field, and sand is carried downstream. The water disappears into the ground, depositing sand on the surface. Winds pick up the deposits of sand, and blow them up onto the dune field once again.
Digging a couple inches into the dunes even at their peaks reveals wet sand. Part of the motivation of turning the Monument into a National Park was the extra protection of the water, which Colorado's cities and agriculture covet.
Many visitors (that would be use) to the site try to sled down the dunes. The Park Service provides hints as to the best time to sled (when the sand is wet) and which equipment works best. The Oasis store/restaurant/cabins/campground, located right outside the park will rent sand boards for you to sled down the dunes. They are $20 for all day. We decided we could share one.
It is very easy to experience the dune-building process. This is a very windy region, as many of our friends who have visited will attest, as on many days they will be pelted by sand and even small rocks when hiking on the dunes. We were fortunate in that it was not windy on the day we visited. Of course that made it hotter too. The wind carries sand and rocks from many miles away. While the dunes don't change location or size that often, there are still parabolic dunes that start in the sand sheet, the outer area around the dunes, and migrate towards the main dune field. Sometimes they join the main dune field, and sometimes they will get covered with grass and vegetation and remain where they are. The dunes are relatively stable; however, they change slightly with the seasons. The direction of the wind greatly affects the dune type. The winds normally go from southwest to northeast; however, during the late summer months, the wind direction reverses causing reversing dunes. This wind regime is part of the reason why the dunes are so tall.
Visitors anytime other than late fall through early spring are also advised to avoid bare feet or sandals, and stick with sturdy, closed footwear. While the sand looks alluring, its chocolate color absorbs heat. The daylight sand temperature can reach 140 °F (60 °C) and will burn bare feet. I can attest to that. I wore flip flops and as I walked on the soft sand, it covered my feet. The sand was burning my feet. Mr. W. gave me his shoes and he took off running. He would run as far as he could go and then he would put the sand board down and stand on it to let his feet cool off. When I caught up with him, he would start running again. Repeating the same procedure. What a guy!
One of the most unusual features of the park happens at Medano Creek, which borders the east side of the dunes and is located next to the Visitor Center and Bookstore. Because fresh sand continually falls in the creek, Medano Creek never finds a permanent and stable streambed. Small underwater sand dunes that act like dams continually form and break down, and so waders in the stream see surges—which look like waves—of water flowing downstream at intervals of anywhere from just a few seconds to a minute or more. In a high-water year, these surges can be as much as a foot in height, resembling ocean waves. Building sand castles with the creek sand is a popular visitor activity, and Skimboarding is a great activity for young people to do because only an inch or two of creek depth is needed. Zapata Falls is a waterfall on the southeast side of the park and requires a short hike through a small cave to access. We didn't make it to Zapata Falls. We are saving that for another time.
When we returned our board to the Oasis, we decided to have lunch in their restaurant. This was our view. I have never seen 6 hummingbirds eating out of the same feeder at the same time.
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