The Geology of the San Rafael Swell
Because people with brains ride
mountain bikes, UtahMountainBiking.com presumes you're interested in knowing
something about the world you ride on. We therefore offer this treatise on the local
geology. - Bruce Argyle
Located about 60 miles west of Moab, the San Rafael Swell offers interesting
eye-candy rock formations and some great mountain biking. The San Rafael's
cliffs, spires, and deep gorges are the result of a steep uplift of the rock
layers in this area.
||We'll begin the story of the San Rafael Swell in the Permian
Period (285 to 240 million years ago). This area was on the seashore, as
the remaining hills of the ancestral Rockies eroded away in the east. Sandstone was
deposited with crisp, cross-hatched layers as seashore dunes piled up, then were covered
by new sand.
|This sand forms the Coconino Sandstone, a rough irregular
stone with cross-hatching and variable hardness. As the Coconino erodes,
it forms small fins and ledges. This creates some of the most interesting
mountain biking in the San Rafael area. The Five Miles of Hell Trail offers highly technical riding on Coconino.
||As ocean covered the area, a layer of limestone covered the
Coconino Sandstone. This is the Kaibab Limestone. It's a thin layer here
(the Kiabab is much thicker towards the west, for example near St. George),
but it's hard enough to resist erosion. Because the limestone is more
resistant to erosion than the mudstone layers above it, it forms a base
for many of the flat open areas in the upper San Rafael.
As the Triassic Period (245 to 205 million years ago) began, the San Rafael
area was on the edge of a giant flat mud-plain. As the ocean moved back and
forth over the area. Layers of soft mud, mudstone, and sandstone were deposited.
|Jutting up from many of the "bench" areas high in
the San Rafael are castles skirted by softer clay and mudstone. This is
the Moenkopi Formation. The color of this formation varies from place to
place, and depends on oxidized minerals such as iron. The top of the butte in the photo is a
layer of limestone (called Sinbad Limestone) within the Moenkopi that acts
as a caprock. The skirts are the Black Dragon member of the Moenkopi
formation. This view is on the Red Trail.
||In the later Triassic Period, western Utah rose up out of
the ocean. Utah broke in half, with the western side rising high above sea
level, while the eastern side remained flat. The breakpoint was along the
"Wasatch Line," a weak spot in the crust that (still today) is
the site of the Wasatch Fault and Hurricane Fault lines. During the
transition time, a variable layer of sandstone and mudstone formed, called
the Chinle Formation.
|The Chinle Formation is the site of uranium and radium
deposits. These radioactive minerals were absorbed by superheated
groundwater from deep-seated lava deposits. Percolating through the
surrounding rock, the uranium replaced organic debris in ancient
streambeds within the Chinle Formation. The Temple Mountain Trail
passes by old uranium and radium mines in this rock layer.
||Eastern Utah dried out and formed a desert of shifting sand
dunes. First came the Wingate Sandstone, which forms towering cliffs. A
temporary wet spell created the Kayenta Sandstone, which forms layers and
tends to resist erosion, protecting the Wingate cliffs below it. On the Devil's
Racetrack Trail, you're riding on ledges created by the Kayenta
Jurassic Period started, desert dunes created the Navajo Sandstone. The
photo shows typical layers of the eastern San Rafael. The Moenkopi layer also
includes the yellow Black Dragon mudstone, separated from the brown by a
thin layer of Sinbad Limestone. This view is looking downhill on the Black
Dragon Wash Trail.
||Late in the Jurassic Period, the dunes gave way to a wet
floodplain. A narrow finger of ocean intruded north-to-south, following the
border between the western highlands and the plain of eastern Utah. This
created the Carmel Formation, an ocean-derived layer. The ocean retreated
again, leaving the floodplain to accumulate further layers of sandstone.
Many of the later rock layers in the Jurassic Period, as seen
in the Moab area, are NOT found in the eastern San Rafael. Something
happened (which we'll discuss below) that eroded away the Entrada,
Morrison, and Curtis Formations from most of this area.
||OK, now we've reviewed the layers from the bottom up:
Coconino Sandstone and Kiabab Limestone from the Permian Period, Moenkopi
and Chinle mudstones from the early Triassic Period, Wingate and Kayenta
Sandstone from the later Triassic, then Navajo from the early Jurassic.
||Around the end of the Jurassic Period, something pushed the
San Rafael area upward. Geologists think it was probably a pool of
upwelling magma that didn't quite make it to the surface. (This was about
the same time that the Rocky Mountains and Uinta Mountains were forming.) The rock layers
were pushed up like a giant blister.
|The steep eastern slope of the uplifted area is exposed on the
east flank of the San Rafael Swell, and is called the San Rafael Reef. This upsloping Navajo Sandstone forms an
impressive shark-tooth line of steep sandstone. In some places, this sandstone can be accessed from a
mountain bike, for example at Black Dragon Wash.
The western slope of the Swell is more gradual, eroding into canyons,
spires, and castles.
||In the Tertiary Period (beginning 65 million years ago), the
western side of the US was lifted up. The Great Basin fell and
stretched away from the Colorado Plateau. (This plateau includes the San
Rafael and Moab areas.) Powerful erosion began to wash the rock
layers away. Once the Cretaceous Period deposits were eroded, the original uplifted region
broke the surface again.
||As the "blister" broke, the reef edge was exposed.
Some highlands were protected by hard underlying rock layers, forming
large flat areas. Deep gorges were cut through
the harder rock as water drained from the uplifted areas.
| Where a small bit of hard rock was spared in a higher
layer, castles and spires were left standing. This occurs where Kayenta
protects the underlying Wingate, or when Navajo sandstone is capped by a
harder layer of Carmel. These spires are the Twin Priests, composed Navajo
sandstone along the Devil's Racetrack.
The result of the erosion is a dynamic and varied
landscape of open vistas, castles and spires, cliffs, deep gorges, and
beautiful rock -- today's San Rafael. Although remote from civilization,
and largely unknown by mountain bikers, the San Rafael Swell offers some