Canyonlands National Park

Geology
Gorp.com

Canyonlands National Park is a showcase of geology. While this area has diverse ecosystems and rich history, it is geologic processes that have played such an important role in shaping Canyonlands. The arid climate and sparse vegetation allow the exposure of large expanses of bare rock and the great canyons of the Colorado and Green Rivers reveal 300 million years of geologic history.

Geologic History

Canyonlands is located within a geologic region called the Colorado Plateau. It is a great section of continental crust that has endured millions of years of rock building and erosion.

Advancing and retreating oceans left thick deposits of beach sands and marine limestones. Great river systems moved tons of sediment from ancient eroding mountain ranges such as the Ancestral Rockies (forerunners of today's Rocky Mountains) and deposited that sediment in low-lying areas. Buried sediment became solid rock as pressure from overlying layers and filtering water cemented them.

After millions of years of predominantly rock-building processes, the erosion that continues today began. Roughly ten million years ago, plates in the Earth's crust moved so the western edge of the continent began to rise. The slowly rising land mass, including the Colorado Plateau, became higher and therefore more susceptible to erosion. Newly elevated highlands captured rain and snowfall and gave birth to the Colorado River system. The uplifting land caused rivers to down-cut faster, entrenching themselves in solid rock. The results are the 2,000-foot deep canyons of the Colorado and Green Rivers cutting through the heart of Canyonlands.

Countless Canyons

The origins of the thousands of small canyons found in the area may be puzzling at first glance. Most of the year, time seems to stand still and the process of erosion is imperceptible. To witness the occasional rock fall or landslide is exceptional luck. But, some of the erosive processes are sudden and violent. Sparse vegetation and an abundance of exposed rock make Canyonlands especially vulnerable to flash flooding. Thunderstorms drop huge amounts of rain locally. With little soil and vegetation to hold the water, runoff is fast. It quickly collects in gullies and small washes, magnifying its power as water funnels into the canyons. The erosive power of the debris and sediment-laden water is tremendous. Flash floods are continually scouring and deepening the canyons.

Needles and Spires

Three hundred million years ago this area was a great low-lying basin partially open to the sea. Periodically, the basin would fill with salty sea water. An arid climate evaporated the water until layers of salts were left. These cycles of flooding, evaporation, and salt deposition continued until thick layers of gypsum, halite or rock salt, sylvite, and dolomite formed, creating the Paradox Formation. As more rock layers were deposited over these very soft salts, pressure was increased. The salts began to flow with the consistency of toothpaste away from the areas of highest pressure and toward the areas of lowest pressure. It is now accumulated in great masses where it pushes upward and bends the overlying rock.

In the Needles District, great systems of parallel cracks formed as overlying rock slid toward the Colorado River on the relatively slippery salt. From high vantage points, a cracked checkerboard landscape is visible. Over time, rainwater and snow penetrate through weak joints. The cracks widen and erosion accelerates with increased surface area until only thin fins and 'needles' of rock remain.

Soft rocks that would normally form slopes can also become spires if they have a layer of erosion-resistant caprock on top. A good place to see caprock spires is Monument Basin, visible from Grandview Point in Island in the Sky.

Profile of a Cliff Face

Most cliffs of Canyonlands show classic profiles that can be seen throughout the southwest. Some of Canyonlands' rocks are massive layers of uniform sand that have become cemented. They are the cliff formers such as the Wingate and Cedar Mesa Sandstones. Gravity is an important partner, contributing to erosion as much as the infrequent rains. The softer, underlying rock layers erode more quickly. This undercuts the harder, upper layers and they break off in huge slabs. Sometimes they form beautiful arched alcoves and, more frequently, the falling rock leaves vertical cliffs.

Poorly cemented and softer layers of sand and shale tend to be slope formers such as the Organ Rock Shale and the Chinle Formation. Their surfaces are usually covered with loose sediment and are often littered with fallen rock slabs from overlying cliffs.

Areas of Geologic Interest

Canyonlands is one of the best places in the world to see classic landforms and the result of geologic processes. Much of our current understanding of the principles of geology come from this area when, in the late 1800s, geologists first studied the Colorado Plateau, reading the history of the Earth from one of its most exciting chapters.

Upheaval Dome, Island in the Sky - ancient salt dome or meteorite impact crater?

Joint Trail, Needles - walk the shady narrows of a deeply eroded joint system.

Confluence Overlook, Needles - a viewpoint of the confluence of the Green and Colorado Rivers.

Colorado and Green River Canyons - to see a cross section of the park, take a trip by canoe, raft, or jetboat through the canyons that created this landscape.


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