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Geology of the area in Utah

The exposed geology of the Bryce Canyon area in Utah shows a record of deposition that covers the last part of the Cretaceous Period and the first half of the Cenozoic era in that part of North America. The ancient depositional environment of the region around what is now Bryce Canyon National Park varied from the warm shallow sea (called the Cretaceous Seaway) in which the Dakota Sandstone and the Tropic Shale were deposited to the cool streams and lakes that contributed sediment to the colorful Claron Formation that dominates the park's amphitheaters. Other formations were also formed but were mostly eroded following uplift from the Laramide orogeny which started around 70 million years ago (Mya). This event raised the Rocky Mountains far to the east and caused the retreat of the sea that covered the Bryce Canyon area. After Laramide mountain building came to an end, about 15 mya, a large part of western North America began to be stretched into the nearby Basin and Range topography. The greater Bryce area was uplifted as part of the High Plateaus by the same forces. Uplift of the Colorado Plateaus and the opening of the Gulf of California by 5 mya changed the drainage of the Colorado River and its tributaries, including the Paria River, which is eroding headward between two plateaus adjacent to the park. The uplift caused the formation of vertical joints which were later preferentially eroded to form the free standing pinnacles called hoodoos, badlands, and monoliths we see today. The formations exposed in the area of the park are part of the Grand Staircase. The oldest members of this super sequence of rock units are exposed in the Grand Canyon, the intermediate ones in Zion National Park, and its youngest parts are laid bare in Bryce Canyon area. A small amount of overlap occurs in and around each park.

The rocks exposed in Bryce Canyon are about 100 million years younger than those in nearby Zion National Park, and the rocks exposed in Zion are younger than those in the Grand Canyon to the south. There are, however, shared rock units between all three, creating a supersequence of formations that geologists call the Grand Staircase. Together the formations of the Grand Staircase record nearly 2000 million years of the Earth's history. Bryce Canyon's formations are the youngest known units in the Grand Staircase. Younger rock units, if they ever existed, have been removed by erosion.

In the Cretaceous, a shallow seaway spread into the interior of North America from the Gulf of Mexico in the south into Utah and later to the Arctic Ocean in the far north. Geologists call this shallow sea the Cretaceous Seaway or Western Interior Seaway. The seaway divided North America into two halves: an eastern portion dominated by the already ancient Appalachian Mountains and a western part composed primarily of the still growing Sevier Mountains; As the shoreline moved back and forth, the Bryce area alternated from being part of the Sevier landmass to being under the Cretaceous Seaway. Alternating layers of nonmarine, intertidal, and marine sediments lay on top of each other as a result. Conglomerate, siltstone, and fossil rich sandstone that together are up to 300 feet (91 m) thick mark the arrival of the Cretaceous Seaway. It sits unconformably on much older Jurassic formations that are not exposed in the immediate area (see geology of the Zion and Kolob canyons area for a discussion about these older sediments). Mud and silt were deposited on top of the Dakota Formation as the seaway became deeper and calmer in the area. Its members represent various stages in this process. The cliff forming sandstone of the Tibbet Canyon Member was conformably deposited on top of the Tropic Shale in shallow marine and later near shore environments. Shale and sandstone from the Smoky Hollow Member were deposited on top of its basal layer of coal rich mudstone in coastal swamps and lagoons on the shore of the seaway. While the alternating layers of shale and sandstone mixed with massive coal deposits of the John Henry Member were laid down in swamps, lagoons and fluvial environments, one member, the Drip Tank, is not found in the Bryce Canyon area. Compression from the Laramide event deformed the land in the area to form the up to 5° dipping Bryce Canyon Anticline. All of the Canaan Peak, Pine Hollow, Kaiparowits, and Waheap formations, along with part of the underlying Straight Cliffs, were removed from the anticline's crest by erosion before the Claron Formation was deposited. An angular unconformity therefore exists along the anticline's crest. The park also sits on the western gently dipping flank of the much larger Kaibab uplift, which was also formed as a result of the Laramide. Climate change and cycles caused the lakes in the system to expand and shrink through time. As they did so, they left beds of differing thickness and composition stacked atop one another; Volcanic ash and lava from these flows are found less than 20 mile (32 km) from Bryce Canyon but at least some volcanic material was likely deposited directly in the park area only to be later removed by erosion.

Younger rock units were laid down but were mostly removed by subsequent uplift accelerated erosion. Outcrops of these formations can be found in the northern part of the park and in a few places on the plateau rim. Among these are the thick Oligocene or Miocene aged Boat Mesa Conglomerate and the Pliocene to early Pleistocene aged Sevier River Formation. The Boat Mesa is made mostly of conglomerates with minor amounts of sandstone and some limestone from lakes, representing stream and overbank flood deposits. Long, north–south trending normal faults were either newly created or reactivated from older pre existing faults; a plateau rose on one side of each fault while valleys subsided on the other as the crust was extended in an east–west direction. Headward erosion of one of those tributaries, the Paria River, eroded north northwestward toward what is now Paria Amphitheater. The river took a route roughly parallel to and east of the Paunsaugunt Fault. Erosion from snow and rain that fall directly on the east facing rim of the Paunsaugunt Plateau forms gullies that widen into alcoves and amphitheaters while differential erosion and frost wedging create the hoodoos. Streams on the plateau do not contribute to the formation of alcoves or amphitheaters because they flow away from the rim. Frost wedging exploits and widens the nearly vertical joint planes that divide the Pink Member of the Claron Formation. Internal layers of mudstone, conglomerate and siltstone interrupt the limestone horizontally. These layers are more resistant to attack by carbonic acid and they can therefore act as protective capstones of fins, windows and hoodoos. Many of the more durable hoodoos are capped with a type of magnesium rich limestone called dolomite. Dolomite dissolves at a much slower rate, and consequently protects the weaker limestone underneath. However, the same processes that create hoodoos will also eventually destroy them. In the case of Bryce Canyon, the hoodoos' rate of erosion is 2–4 feet (0.6–1.3 m) every 100 years. As the canyon continues to erode to the west it will eventually capture (in perhaps 3 million years) the watershed of the East Fork of the Sevier River. Once this river flows through Bryce Amphitheater it will dominate the erosional pattern; replacing hoodoos with a V shaped canyon and steep cliff walls typical of the weathering and erosional patterns created by rivers. A foreshadowing of this can be observed in Water Canyon while hiking the Mossy Cave Trail. A diversion canal has been taking a portion of the East Fork of the Sevier River through this section of the park for over 100 years.