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The Geology of the  San Rafael Swell

Because people with brains ride mountain bikes, 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 Sandstone.
As the 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 great experiences.