Hikers in the Windows Section of Arches National Park. A window is a large arch framing a distant view. The Entrada Sandstone is the massive rock layer on the top part of the arch. NPS photo/Chris Wonderly.
The state of Utah is famous worldwide for its five spectacular national parks. But what often goes unsaid and/or unrecognized is that all five parks in Utah are fundamentally geologic places. In fact, they all became part of the national park system because of their geology. In the arid southwest, scenery and the underlying geology have direct connections. The view from almost any location in these five parks is dominated by exposed bedrock, featuring cliffs, spires, canyon walls, hoodoos, and rock domes, plus windows and arches, all made of bare rock.
The links between geology and the establishment of these areas as national parks is also evident through their names. Two of the parks (Arches and Canyonlands) are named for the abundance of the characteristic geologic landforms in them. Two more (Bryce Canyon and Capitol Reef) are named for specific geologic landforms. And the geologic aspect (canyon) of the park name is implied at Zion.
These five parks share some geologic features with one another, but also have their own distinctive features. The most prominent similarity is that the parks are erosional landscapes carved into sedimentary rock layers (formations to geologists). They are all located on the Colorado Plateau, the Four Corners region with overall high elevation but great topographic relief formed by the rapid incision (downcutting) by the Colorado River and its tributaries.
Fluvial erosion powered by the rivers cutting into the landscape is what drives landscape evolution in the five parks. The Colorado River joins its major tributary (the Green) in the heart of Canyonlands. Arches National Park borders the Colorado River, and the Fremont and Virgin rivers, both tributaries of the Colorado, flow through Capitol Reef and Zion respectively. Bryce Canyon’s amphitheaters containing its iconic hoodoos result from headward erosion (erosion in the upstream direction) of yet another tributary, the Paria.
The Waterpocket Fold, a 100-mile long upwarping, has tilted Capitol Reef National Park’s rock layers from their original horizontal position. The ridge formed by this upwarping is the reef of the park name. The Navajo Sandstone makes the white sandstone domes above the highest cliffs. The Wingate Sandstone makes the prominent red cliffs here like it also does in Canyonlands. NPS photo/Chris Roundtree.
Yet the parks scenery wasn’t carved by just the rivers. All contain networks of smaller tributaries, most that only flow during rare flash flood events. Still they are major sculptors of the landscape. Rockfall, rock avalanches, and landslides also play a prominent role in shaping the land in these parks. Evidence of past rockfall is evident in the boulders littering the slopes below every cliff and canyon wall.
Mass wasting (the technical term for gravity-driven erosional processes) is a particularly important process at Bryce Canyon because the Claron Formation is particularly soft and erodes rapidly. The intricately carved spire-like hoodoos also result from slight differences in the hardness of each of its thin layers, as well as networks of vertical fractures called joints that allow water to seep into the rock. Landslides have also had an important role in shaping Zion. A large mass wasting event nearly 5,000 years ago actually dammed the Virgin River creating a lake in the part of Zion Canyon where the lodge now is and where millions of people visit each year.
Sandstone is the major rock type in four of the national parks in Utah. Sandstone is a scenery-maker. It is strong enough to form cliffs and rock domes, and competent enough to support spans to create windows and arches. Most of the sandstones exposed on the Colorado Plateau are beautiful shades of reds to oranges that give the parks their beautiful palettes.
Some of the sandstone layers, like the Navajo Sandstone, are present in multiple parks. The Navajo Sandstone is the “rock star” of both Zion and Capitol Reef but it has more of a supporting role in Arches and Canyonlands. The Navajo Sandstone is much thicker in Zion and Capitol Reef, at 2,000 and 1,000 feet thick respectively, compared to just a few hundred feet thick in Canyonlands and Arches.
Arches National Park features a distinctive rock layer that is only present in a small area of southeastern Utah. The subdivision of the Entrada Sandstone known as the Slick Rock Member hosts most of Arches’ eponymous rock openings. One of the important rock layers in Canyonlands is also restricted to a small geographic area. The White Rim Sandstone, visible from the major overlooks of the Island in the Sky District, pinches out to the north and isn’t even present in the immediate Moab area.
Entire books exist that cover the geology of each national park, meaning that this column in Moab Happenings can only serve as a short introduction. But the hope is that it can help build appreciation that any visit to Arches, Bryce Canyon, Canyonlands, Capitol Reef, and Zion is, at least in part, part of a geology-appreciation tour.
The Green River Overlook provides an excellent perspective on the landscape of rock that is Canyonlands National Park. The White Rim Sandstone makes the prominent bench about halfway to the river. The Wingate Sandstone makes the distant cliffs. NPS photo.
Bryce Canyon National Park is a great place to learn about geology! Erosion happens particularly rapidly at Bryce Canyon because the Claron Formation is so soft. NPS photo/Peter Densmore.
View of the layering in the Navajo Sandstone in Zion National Park. The Navajo was deposited in a vast field of sand dunes. This type of layering (cross-bedding) forms parallel to dune surfaces as they migrate with the winds. NPS photo/Jason Burton.
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A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.