Friday Fractal XVIII

Fractals are like landscapes. From a simple process, be it a formula or continental drift, one area can be strikingly different from another. This is true for my favorite type of fractal, a Julia set, "colored" with a bit of fractal Brownian motion:

Or the ripples on the Great Sand Dunes:

Stone, Steam and Sand: A Geologic Photo Tour of Southwest Colorado, Part III

In the first part of this series, I described the formation of the San Juan Mountains, and then a bit of the more recent history of the lands to the west of the range. The valley east of the San Juans had similar initial conditions, but produced a very different outcome. To begin, let's go back about 19 million years, after the volcanic activity building the San Juans began to settle down. The effects of one continental plate sliding under another were (and still are) cropping up. East of the San Juans, rather than bulging, the ground stretched and began to split. At this point in geologic history, the Rio Grande Rift was born. Beginning in New Mexico (about 30 million years ago, around the same time as the volcanic activity in the San Juans started) the rift split the Rocky Mountains down the middle. The rift stretched northward, where it continues today somewhere in Southern Wyoming. When the rift finally reached southern Colorado, the rifting created a wide valley and thrust new peaks upwards along the eastern edge: The Sangre de Christo Mountains and the San Luis Valley.

With rising peaks on each side, and the valley itself sinking deeper with the growing rift, the area was destined for isolation. Volcanic flows filled in the lower gaps, leaving a closed basin. Eventually, glaciation came and scoured the land. When things began to warm up and melt, about 12,000 years ago, there was no other place for the runoff to go. Water flowed into the valley, where it eventually evaporated in playa lakes, leaving silt and sand behind. Then came the wind. Blowing from the southwest, the sand was pushed into the northwest corner of the valley, where it settled at the foot of the Sangre de Christo Mountains, forming dunes.

The National Park Service offers a cool animation showing the formation of the rift valley and dunes. Click here to check it out.

Often, the winds over the dunes reversed. Blowing through gaps in the Sangre de Christos, the wind blew down, creating reversing dunes. Another NPS animation shows how the many of the dunes were shaped, here. (Other dunes have a more complex "star" formation, from winds from multiple directions.) Between the reversing winds and closed basin, the Great Sand Dunes became the tallest and sturdiest dunes in the world. Here is a video of sand blowing across ripples in the dunes, filmed and narrated by my husband, Alan:

Two creeks, (Sand Creek and Medano Creek,) one on each side of the dune field, continue to refresh the dunes, bringing in new sediment each spring, as the snow melts from the peaks of the Sangre De Christo mountains. The water flows across the sands, building and braking miniature dunes in wave-like surges, before disappearing beneath the sands for the winter. (This is an excuse to point out another interesting animation, here, explaining this unique hydrological phenomenon.)

At a glance, the Dunes seem inhospitable to life, be it plant or animal. With a closer look, you can see the incredible diversity of life, clinging to the various obscure niches, from alpine tundra to dunes beneath. Here, a few lady beetles mate on a branch of lemon scurfpea, one of the endemic plants growing on the dunes:

For more on the ecology of the area, check out the Sand Sheet Wonders page on the NPS site.

Humans have likely always been around the dunes, as well. At the same time as the glaciers receded and the dunes began to form, hunter-gatherers followed large game into the valley. Later, groups of southern Ute and Jicarilla Apaches described the dunes in oral histories. Written history of the dunes did not begin until Zebulon Pike came across the Sangre de Christo mountains. On January 28th, 1807, he wrote the following passage in his journal:

"After marching some miles, we discovered...at the foot of the White Mountains [today's Sangre de Cristos] which we were then descending, sandy hills...When we encamped, I ascended one of the largest hills of sand, and with my glass could discover a large river [the Rio Grande]...The Sand-hills extended up and down the foot of the White Mountains about 15 miles, and appeared to be about 5 miles in width. Their appearance was exactly that of a sea in storm, except as to color, not the least sign of vegetation existing thereon." (Via the National Park Service.)

If you've come this far, and you still want to play in the sand, but can't visit southern Colorado, check out this strange but cool toy from DofI:

Hell of Sand
(reviewed on Jay's Casual Gaming late last year.)

Historical images via the Western History Photos collection at the Denver Public Library.All other images belong to the author, fractals made by the author using ChaosPro.