Great Sand Dunes National Park and Preserve
September 12, 2017
Photographer: Ray Boren
Summary Author: Ray Boren
Captured in a curve of a subrange of Colorado’s Rocky Mountains, and within the San Luis Valley, Great Sand Dunes National Park and Preserve’s sinuous dune field ridges and summits — some topping 750 ft (229 m), and therefore deemed the tallest in the United States — appear to echo the alpine peaks of the Sangre de Cristo Mountains, which are behind them in the photograph above, taken on the evening of May 20, 2017 (top photo).Most of the sand, which also underlies more stable sage and grass covered slopes and plains nearby, eroded from the San Juan Mountains, 65 mi (105 km) to the west. The high San Luis Valley (7,550 ft/2,300 m), traversed by the Rio Grande River, also had a Pliocene and Pleistocene great lake, today called Lake Alamosa. The ancient lake played an important role in accumulating and depositing sediments, including sand, geologists say. Blocked on the east by the Sangre de Cristo range (from the Spanish for “Blood of Christ”), which also contributes alluvium and opposing down-slope breezes, sand has been shaped and stacked, up and up and up, for perhaps 440,000 years, when Lake Alamosa disappeared, according to the National Park Service.
In addition, streams descending from the Sangre de Cristo — notably Medano Creek (bottom photo) play an intriguing role in the dune field’s existence and maintenance. In concert with the wind, the creeks corral the dunes and capture escaping sand, transporting grains west and south before vanishing into the sand sheet and valley floor. Then the fugitive surface sand is blown back into the dune field. It is fascinating to watch Medano’s shifty creek bed: The flowing water, especially in spring and summer, is constantly building mini-dams of sand. In a process called surge flow, the sand dams intermittently break, sending unusual wavelets rippling and rushing down the shallow stream.
Photo Details: Top - Camera Model: NIKON D3200; Lens: AF-S VR Zoom-Nikkor 70-300mm f/4.5-5.6G IF-ED; Focal Length: 70mm (35mm equivalent: 105mm); Aperture: ƒ/11.0; Exposure Time: 0.0031 s (1/320); ISO equiv: 400. Bottom - same except: Aperture: ƒ/10.0; Exposure Time: 0.0050 s (1/200).