Canon City, Colorado
October 20, 2020
I was at Great Sand Dunes National Park on Sunday, October 18, 2020.
Pretty impressive formation of sand. It is a darker sand - grayish. It was wonderful to see how popular the dunes are, especially with kids and their boards. I talked to one gentleman who had brought is 16-year old granddaughter to the dunes for the day - from Denver which he said was four hours away. Walking on deep, loose sand at 8,000 feet will test your aerobic conditioning. Let's say that almost a month of driving has not increased mine. ☹️
Here's the Wikipedia article (abbreviated) on the park:
Great Sand Dunes National Park and Preserve is an American national park that conserves an area of large sand dunes up to 750 feet talll on the eastern edge of the San Luis Valley, and an adjacent national preserve in the Sangre de Cristo Range, in south-central Colorado, United States. The park was originally designated Great Sand Dunes National Monument on March 17, 1932, by President Herbert Hoover. The original boundaries protected an area of 35,528 acres. A boundary change and redesignation as a national park and preserve was authorized on November 22, 2000, and then established by an act of Congress on September 24, 2004. The park encompasses 107,342 acres while the preserve protects an additional 41,686 acres (for a total of 149,028 acres). The recreational visitor total was 527,546 in 2019.
The park contains the tallest sand dunes in North America. The dunes cover an area of about 30 sq mi and are estimated to contain over 1.2 cubic miles of sand Sediments from the surrounding mountains filled the valley over geologic time periods. After lakes within the valley receded, exposed sand was blown by the predominant southwest winds toward the Sangre de Cristos, eventually forming the dunefield over an estimated tens of thousands of years. The four primary components of the Great Sand Dunes system are the mountain watershed, the dunefield, the sand sheet, and the sabkha. Ecosystems within the mountain watershed include alpine tundra, subalpine forests, montane woodlands, and riparian zones.
Evidence of human habitation in the San Luis Valley dates back about 11,000 years. The first historic peoples to inhabit the area were the Southern Ute Tribe; Apaches and Navajo also have cultural connections in the area. In the late 17th century, Diego de Vargas, a Spanish governor of Santa Fe de Nuevo México, became the first European on record to enter the San Luis Valley. Juan Bautista de Anza, Zebulon Pike, John C. Frémont, and John Gunnison all travelled through and explored parts of the region in the 18th and 19th centuries. The explorers were soon followed by settlers who ranched, farmed and mined in the valley starting in the late 19th century. The park was first established as a national monument in 1932 to protect it from gold mining and the potential of a concrete manufacturing business.
Visitors must walk across the wide and shallow Medano Creek to reach the dunes in spring and summer. The creek typically has a peak flow from late May to early June. From July to April, it is usually no more than a few inches deep, if there is any water at all. Hiking is permitted throughout the dunes with the warning that the sand surface temperature may reach 150 °F in summer. Sandboarding and sandsledding are popular activities, both done on specially designed equipment that can be rented just outside the park entrance or in Alamosa. Visitors with street-legal four-wheel drive vehicles may continue past the end of the park's main road to Medano Pass on 22 miles of unpaved road, crossing the stream bed of Medano Creek nine times and traversing 4 miles of deep sand.Hunting is permitted in the preserve in the autumn, but prohibited within national park boundaries at all times. The preserve encompasses nearly all of the mountainous areas north and east of the dunefield, up to the ridgeline of the Sangre de Cristos.
Geology
Creation of the San Luis Valley began when the Sangre de Cristo Range was uplifted in the rotation of a large tectonic plate. The San Juan Mountains to the west of the valley were created through extended and dramatic volcanic activity. The San Luis Valley encompasses the area between the two mountain ranges and is roughly the size of the state of Connecticut. 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 floods.
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. The body of water was named 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, 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 Range. Dramatic natural climate change later significantly reduced these lakes, leaving behind the sand sheet. Remnants of these lakes still exist in the form of sabkha wetlands.
Sand that was left behind after the lakes receded blew with the predominant southwest winds toward a low curve in the Sangre de Cristo Range. The wind funnels toward three mountain passes—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. 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. Barchan and transverse dunes form near these creeks. The 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..
Sufficient vegetation has grown on the valley floor that there is little sand blowing into the main dunefield from the valley; however, small parabolic dunes continue to originate in the sand sheet and migrate across grasslands, joining the main dunefield. Some of these migrating dunes become covered by grasses and shrubs and stop migrating. The dunes system is fairly stable as the opposing wind directions balance each other out over time. Also, the main dunefield is moist beneath a thin layer of dry surface sand. While the top few inches of sand are blown around during windstorms, the moist sand remains largely in place.
Scientists estimate that Lake Alamosa disappeared about 440,000 years ago, but the dunes themselves apparently originate from sand deposits from later, smaller lakes. A relatively new dating process, optically stimulated luminescence (OSL), is still in development. This method takes core samples of sand from deep within a dune, and attempts to measure how long quartz grains have been buried in the dark. If the deepest sand deposits can be accurately dated, the age of the dunes could be determined. Samples of sand from deep in the dunes have returned OSL dates varying between a few hundred years to tens of thousands of years old. The oldest dated deposits found so far would have accumulated in the late Pleistocene epoch, during the middle years of the current ice age's third stage.
The dunes contain dark areas which are deposits of magnetite, a mineral that has eroded out of the Sangre de Cristo Range. Magnetite is both attracted to a magnet and can be magnetized to become a magnet itself; it is the most magnetic mineral in nature. Magnetite is an oxide of iron which is heavier than the surrounding particles of sand. When overlying sand is removed by wind, magnetite deposits stay in place and are visible as dark patches in the dunefield.
Geography
Great Sand Dunes National Park and Preserve is located in Saguache and Alamosa Counties, Colorado at approximately 37.75° north latitude and 105.5° west longitude. The national park is located in the San Luis Valley while the national preserve is located to the east in an adjacent section of the Sangre de Cristo Range of the Rocky Mountains. Elevations range from 7,515 ft in the valley west of the dunes, to 13,604 ft at the summit of Tijeras Peak in the northern part of the preserve.
The dunes cover an area of about 30 sq mi while the surrounding relatively flat sand sheet which feeds the large dunes is actually the largest component of the entire dunes system, containing about 90% of all the sand in the park. The forested and often snowcapped mountains exceeding 13,000 ft to the east are the most prominent feature, towering over the high dunes. Other features include snow-fed creeks originating high in the mountains, and several alpine lakes. Two spring-fed creeks in the sand sheet along with a few small lakes in the valley's sabkha section southwest of the dunes create a wetland that nurtures wildlife.
The main dunefield measures roughly 4 mi east-to-west and 6 mi (north-to-south, with an adjacent 6 sq mi (area to the northwest called the Star Dune Complex, for a total of about 30 sq mi. The park and preserve together are approximately 15 mi (east-to-west at the widest point, and approximately 15 mi north-to-south, also at the widest point. The park encompasses 107,342 acres, while the preserve protects an additional 41,686 acres for a total of 149,028 acres.
The Rio Grande National Forest is located to the north and southeast while the remaining forested slopes directly to the east of the dunes were redesignated the Great Sand Dunes National Preserve. The San Isabel National Forest is located to the east of the preserve just beyond the ridge of the Sangre de Cristo Range. Private property abuts most of the southern boundary of the park. The San Luis Lakes State Wildlife Area lies adjacent to the southwestern corner of the park, while the Rio Grande flows through the valley farther to the southwest. The Baca National Wildlife Refuge lies adjacent to the west, and the slopes of the San Juan Mountains begin at the western edge of the valley. Private property of the Baca Grande subdivision of Crestone lies adjacent to the northwest.
The nearest city is Alamosa which is about 30 mi away by road to the southwest. The nearest towns are Crestone to the north, Mosca and Hooper to the west, Blanca and Fort Garland to the south, and Gardner to the east. Colorado Springs and Denver are located a few hours away by car to the northeast. The major roads through the San Luis Valley are U.S. Route 160 on an east–west alignment passing south of the park, and U.S. Route 285 on a north–south alignment passing west of the park and generally parallel to Colorado State Highway 17, which is the closer of the two north–south roadways.
Hey, wait a minute!
