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The Geologic History of Moab

The geology of Utah is very complex, and unique in the world. Rock layers are exposed that represent over 500 million years of geologic changes. The Colorado Plateau (which includes the Moab area) has remained relatively undisturbed until recent times. To discuss the geology of Moab, I will first recount the conditions that led to the deposit of the different rock layers, then the more recent events that created the Moab valley. Sure it's complicated. But think of how you'll impress your biking buddies when you know this stuff. - Bruce Argyle
Pennsylvanian Period: Deposition of the Paradox Formation salt. During the Pennsylvanian Period (320-285 million years ago), much of Utah was covered by ocean. A small arm of the Ancestral Rockies penetrated into Utah from the east, north of the Moab area. Extending south from the tip of these highlands was an area that was occasionally submerged, occasionally "high and dry." The Moab region was a gigantic deep "sinkhole," called the Paradox Basin. 
From time to time, the Paradox Basin would be flooded with ocean water as sea levels rose (or the land bridge subsided). A layer of limestone would form in this new ocean bay. Then the Paradox Basin would be cut off from the ocean and would dry out, leaving evaporated salts, capped with shale. Multiple such cycles occurred over millions of years, leaving over 1000 feet of salt. No rock of the Pennsylvanian Period is exposed at the surface in the Moab area, but the shape of the land has been affected by the Paradox Formation salt (more on this later).

Potash (a salt of potassium) is mined from Paradox Formation deposits deep under the Colorado River. The plant can be seen from here at the overlook at the end of the Amassa Back trail.

Permian Period: Canyonlands and Monument Valley strata. The Ancestral Rockies continued to erode during the Permian Period (285-245 million years ago), and were essentially gone by the Triassic Period. In the Permian, the Paradox Basin had completely filled, and the Moab area was a region of near-ocean dunes (the ocean lay to the west). During this time, sandstone was deposited in southeast Utah, such as the DeChelly sandstone that forms the spires of Monument Valley, the Cutler formation sandstone that forms the needles and arches of Canyonlands, and the White Rim and Organ Rock sandstone of Canyonlands. (No rock of the Permian Period is seen in the Moab valley.)
Triassic Period: Muddy floodplains give way to sand dunes. By the Triassic Period (245-205 million years ago), the Ancestral Rockies were eroded away. The southeast corner of the state was a large flat floodplain, with the seashore moving back and forth over huge mudflats. The Moenkopi formation, then the Chinle formation, were formed of layers of mud, with occasional layers of harder sandstone. 
In the later Triassic, the sea retreated to the west, and the area again became a giant desert (similar to today's Sahara). The Wingate sandstone was deposited by these sand dunes, then the harder Kayenta on top of it. Wingate sandstone forms the high cliffs west of Moab, and the spires of Castle Valley, visible from the Porcupine Rim trail.

On the Gemini Bridges trail, the "skirts" of the cliffs are Chinle formation shales and sandstones. The tall cliffs are Wingate sandstone. Note the rim of hard Kayenta sandstone that protects the cliffs (upper left).

Kayenta has a denser matrix between the grains of sand, so it resists erosion. The Kayenta is a ledge-former, making it a good way to get to the top of the Moab Rim. Once the Kayenta erodes away, the underlying Wingate crumbles quickly.

The salt-dome anticline (which will be discussed below) has tilted this bed of Kayenta, providing a "road" up the Moab Rim trail.

Jurassic Period: The rising of western Utah changes Moab from sand dunes to river floodplain. The Jurassic Period (205-140 million years ago) started with the southern part of Utah covered by deep sand dunes. This formed the famous Navajo sandstone, seen on the Slickrock trail and at the top of the Moab Rim and Poison Spider Mesa trails. 
Because of the salt-dome anticline (which will be discussed below), the Navajo sandstone lies at different levels on either side of the Moab Valley. On the west, for example at the top of the Moab Rim, it's high on top of the cliffs. On the east at Slickrock, it's just above the valley floor.

In this view from the Moab Rim trail, we see rolling domes of Navajo sandstone.

On top of the Navajo is the orange Entrada sandstone, fine-grained and an excellent arch former. Entrada is seen at Bartlett Wash and Tusher Canyon. It forms the arches in Arches National Park, just north of Moab.

With apologies to the fans of the Navajo sandstone of Slickrock, I find the Entrada to be a much more interesting riding surface. Here at Bartlett Wash, rolling bowls of Entrada create a huge playground.

Above the Entrada is the Curtis formation, a hard white sandstone.

During the late Triassic and Jurassic, western Utah was rising in altitude. But the eastern half was sinking. Utah's "dividing point" for the past 500 million years has been the Wasatch line -- the line of faults running down the Wasatch Front and into the Hurricane Fault of southwestern Utah. With slumping of the land east of the Wasatch line, the sea advanced temporarily into eastern Utah.

At Tusher Canyon, the white Curtis Formation sandstone forms cliffs above the ridged orange Entrada.

As the area east of the Wasatch line filled with sediment, eastern Utah became a river floodplain (with rivers running east out of western Utah and meandering towards the Gulf of Mexico). This is the Morrison Formation, a soft shale with pastel colors. The Morrison Formation can be seen along the highway north of Moab.

The Morrison Formation is host to both dinosaur fossils and to deposits of uranium and vanadium. (The dinosaur quarries near Vernal and Price are in this formation.)

The late Triassic, the Jurassic, and the Cretaceous Periods are the age of dinosaurs. In floodplain and mud deposits, such as here on the Klondike Bluffs trail, dinosaur footprints can be seen.

Cretaceous Period: Seas reclaim eastern Utah. For most of the Cretaceous (140-65 million years ago), western Utah remained a highland, while the Gulf of Mexico spread through the middle of the continent, covering eastern Utah. During this time, shales were deposited. The gray Mancos shale dominates the landscape all the way from Price until Highway 191 begins dropping down into the Moab area. The sea retreated towards the end of the Cretaceous, as the western half of the continent began to uplift. The Mancos shale, and the Cretaceous-era Dakota sandstone underneath it, eroded away from Moab area as the salt dome elevated the land surface (see below).
Tertiary Period, Event 1: The Great Basin subsides while the western continent is uplifted. The Colorado establishes its present course. During the Tertiary (65 million years ago), the Rockies and Uinta Mountains began to rise, and the entire western continent was uplifted. (At the Wasatch line, the Great Basin area began to slump and pull away relative to eastern Utah.) 
The Colorado River found a new course towards the Pacific, and captured the flow of the Green River. As the continent rose up, the Colorado cut deeply into the layers of underlying rock, because it was trapped in its present channel.

The meandering course of the Colorado (see the photo at the top) and the goosenecks of the San Juan river suggest that this was once a slow-moving stream in a near-level plain. As the western US rose up, the river began to cut deeply into its established channel. Behind this biker at Slickrock, the black cliffs in Navajo sandstone drop quickly to the river.

Tertiary Period, Event 2: The Paradox Salt forms a dome, creating an anticline. Under great pressure, salt can flow like a glacier. Salt of the Paradox Formation was squeezed into a dome in the Moab area, elevating the rocks above it. A "hump" of strata is called an anticline. As the dome gradually rose up, the Colorado cut down through the rising rock to maintain its course.
Cracks formed in the rock over the dome of salt. These are the fissures that allowed formation of fins and arches in Arches National Park and the Slickrock area.

At this viewpoint over Arches National Park from the end of the Klondike Bluffs trail, the effect of the vertical rock fractures are seen.

Tertiary Period, Event 3: The Moab Fault allows salt to erode, collapsing the center of the anticline. A fault line runs down each side of the Moab valley. These faults join near the entrance to Arches. Perhaps because of water running down along the fault line, salt was dissolved away deep under the rock of Moab. The overlying slab sunk down, creating a "collapsed anticline" that is the Spanish and Moab valley. 
Knowing the geologic history of Moab makes identification of the various rock layers fun! It's almost as exciting as roadkill identification, and less odorous.

A short distance northeast of the Moab valley, the Moenkopi and Chinle formation shales and sandstones (from the early Triassic) form terraced skirts around the Wingate sandstone spires in this look over Castle Valley.