A Tour of Moab’s Best Geological Joints
In Praise of the Humble Rock Fracture by Allyson Mathis
Vertical joints in the Wingate Sandstone in Canyonlands National Park. The vertical fractures are much more prominent than the horizontal layering or bedding of the sandstone itself.
Canyon country is made of rock; it is a place where the Earth is laid bare. Tall canyon walls, soaring cliffs, imposing buttes, and singular spires and monuments all have been carved from exposed bedrock. One of the most important factors that has shaped the spectacular scenery surrounding Moab is not a characteristic of the rocks themselves, which are predominantly sandstones; e.g., sedimentary rocks made of sand grains that have been cemented together. Instead, much of the geomorphology (e.g., shape of the land) of southeastern Utah results from simple fractures in those rocks, particularly in combination with the erosion that has occurred along them. The Fins and Things 4x4 route in Sands Flats Recreation Area owes its existence to joints. Dashed lines show the trace of some of the joints found in the Navajo Sandstone. Deep erosion along the more prominent joints makes the fin boundaries.
A rock fracture is known as a joint in geology. They are among the most common types of structures found in rocks and their impact on the development of a landscape can be profound. In the Moab area, the presence of joints can dictate the routes of hiking trails, create unique challenges for four-wheel-drive enthusiasts, produce great concentrations of rare features like natural arches, and help shape cliffs, needles, and fins. The influence of joints on landscapes is not limited to the Moab area; joints have also shaped some of America’s best-known places, like Half Dome in Yosemite and the iconic Devils Tower in Wyoming.
Joints often control how and where erosion shape the land. They often occur as sets of parallel fractures that formed in response to stresses within the Earth that caused the rocks to brittlely crack. The stresses may be caused by a variety of forces, such as folding or faulting of the rock layers. (A fault is a fracture along which rocks have moved or have been displaced relative to one another.) In the case of volcanic rocks, joints can form due to the contraction caused by the cooling and solidification of molten rock.
In canyon country, the predominate joints are those in vertical sets. This type of joint is most noticeable in the Wingate Sandstone, which is the rock layer that forms the cliffs of Canyonlands and above Moab. Not only do joints provide passageways for water to flow into the rock hastening weathering and erosion, but they are places where a rock fall may be initiated. These rock falls sculpt the characteristic red-brown Wingate cliffs that tower above much of the surrounding desert.
As planes of weakness, joints channel water flow on top of and within rocks. The presence of water always hastens erosion, so joints become loci where erosion is concentrated. This means that erosion occurs preferentially along joints, widening and deepening them so that elongated rock ribs called fins may be left between them. The presence of rock fins is one of the hallmarks of Arches National Park, especially in Devils Garden. These narrow rock fins are in turn susceptible to weathering and erosion poking holes in them, forming natural arches.
A section of Canyonlands National Park gets its name from the rock needles created by erosion along two sets of joints that intersect each other at nearly right angles. Weathering and erosion along these intersecting joint sets formed rounded rock spires in a manner akin to the formation of Arches’ rock fins. Erosion along joints may even widen them sufficiently that they become passageways where hikers can travel through rocks instead of just on top of them.
The humble rock fracture is also responsible for other extraordinary landmarks in the Moab area, two of which are found in the Island in the Sky District of Canyonlands. Half Dome near the Green River Overlook looks like a smaller red-rock cousin of the much more famous Half Dome in Yosemite. And Black Crack in the Canyonlands backcountry is a particularly beguiling fracture that is deep and long and appears mysterious.
Like its namesake in Yosemite National Park, the sheer front face of Half Dome in Canyonlands was formed by a joint.
Black Crack in Canyonlands National Park.
The Joint Trail in the Needles District.
The Needles District in Canyonlands National Park owes its characteristic scenery to joints.
Google Earth image showing the parallel joints in the sandstone of the Devils Garden area in Arches National Park. Erosion along these joints created the elongated rock fins and a multitude of natural arches in those fins.
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.
To learn more about Moab’s geology, visit the Geology Happenings archive online at https://www.moabhappenings.com/Archives/000archiveindex.htm#geology
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.