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
||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
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
||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
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.