|
 Not to imply anything about the virtue and good character of geologists*, but places in Utah with the word Devil in their name tend to be the type of places that geologists love. Instead of being sinister, these spots typically are fanciful places where bedrock is exposed and that have unusual and striking landforms.
Most of these sites in Utah are found in canyon country. In southeastern Utah alone, there is Devils Canyon, Devils Lane, Devils Pocket, Devils Monument, and two Devils Gardens.
Devils Garden in Arches National Park may be the best known of these devilish locales. Arches’ Devils Garden is a landscape dominated by rock: rock fins, towers and spires and with an impressive collection of natural arches, all colored in a beautiful spectrum from orangish red, to tan, yellow, and off white. The name allegedly originates from the red rock fins and spires looking like tongues of flames.
 |
| The Devils Garden area. Aerial photograph by Carsten Stegar. CC BY-SA 4.0 Deed. |
Despite its name, Devils Garden entices people from all around the world to walk among and experience its rocks. It is one of the premier areas of Arches. Located at the end of the 18-mile park road, it offers a campground, a picnic area, and a network of trails. A hiker who walks the main trail and all the spur trails will cover 7.9 miles and visit seven natural arches. One of the highlights is Landscape Arch, the longest arch in North America. Its span reaches 306 feet and soars 88 feet above the ground, although the arch itself is thin, with the narrowest spot only six feet thick.
Although its moniker suggests a supernatural origin, Devils Garden is the result of earthly geologic events and forces that have sculpted all of canyon country. What makes Devils Garden different than other places near Moab is its unique combination of rock layers (formations to geologists), location on the edge of a major salt anticline (an upward fold formed by flowing salt), and prominent sets of fractures in the bedrock called joints. Together, these parameters have yielded a set of parallel fins (elongated rock ridges with nearly vertical sides). Because they are narrow and subject to further weathering and erosion along each side, fins are a frequent precursor to arch formation.
 |
| Devils Garden is located on the northeast side of the Salt Valley anticline where the rock layers have rolled over into the collapsed anticline. This folding created sets of parallel joints. Erosional along these joints formed the elongated rock ridges known as fins. |
Two rock layers are exposed in Devils Garden. The main rock that makes the fins is the red-orange Entrada Sandstone, with the Moab Member (Curtis Formation) making a white cap on some of the taller features. The Entrada Sandstone is massive and able to form vertical and even overhanging slopes. But it is also weakly cemented by calcium carbonate, which can dissolve in rain and groundwater that is naturally slightly acidic. Both of these characteristics are essential to its role as the principle arch-forming rock layer in the park.
Devils Garden is just northeast of Salt Valley salt anticline, a geologic upwarping formed by salt flowage in the subsurface. This part of southeastern Utah is underlain by thick deposits of salt from an ancient hypersaline sea. Under the pressure of the overlying rock layers, salt can easily flow like a plastic or putty. When it nears the surface, groundwater dissolves the salt, causing the top part of the fold to collapse. Although salt flows ductilely, rocks like sandstone crack and fracture. The main set of these vertical fractures (joints) is nearly parallel to the axis of the salt anticline.
Jointing in the Entrada Sandstone and the Moab Member is particularly well developed along the northeastern limb of the Salt Valley anticline between the Fiery Furnace and Devils  Garden. In this area, the rock layers have rolled over or warped into the valley formed by the collapsed salt anticline. The flexing above the roll over causes the joints to open up as the top part is stretched. This slight bit of stretching makes the joints an even better conduit for groundwater, enlarging them. Eventually, rock fins can be left standing between joints.
After a fin has formed, weathering and erosion continue to attack it from each side. It is thought that several different factors may contribute to the perforation of a fin to form a natural arch. Slight variations in the amount or composition of the cement holding the Entrada’s sand grains together, small lenses of softer rock types within a fin, or zones of fracture concentrations all may contribute, as well as areas where groundwater may preferentially flow or accumulate.
Once an arch forms, it will ultimately succumb to the same forces that formed it. Landscape Arch experienced a major rock fall in 1991. Wall Arch, which was nearby, completely collapsed in 2008.
These rock falls and arch collapses show the Devils Garden is an active geologic landscape. And with its stunning scenery, maybe instead it is a garden of earthly delights—and not just one for the geologists.
*Colorado must have a better opinion of its geologists as Garden of the Gods is a landscape of red rock pinnacles and spires a bit like Devils Garden.
 |
| A hiker on top of a fin on the way to Double O Arch. |
|
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.