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Geology

Geologic Units

The formations, groups and supergroups exposed along the rivers and routes — read bottom-up, oldest first, the way a canyon wall reads.

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By Period — Oldest First

54 units, read the way the wall reads.

Cambrian

  • Bright Angel Shaleformation
    Bright Angel Shale is the green-and-red bench of the Grand Canyon — the soft, fossil-rich Cambrian shale that erodes back into the broad, flat Tonto Platform, the great mid-canyon terrace where trails and campsites string along above the Inner Gorge. Between the Tapeats cliff below and the Muav cliff above, its glauconite-green and iron-red mudstones record a shallow Cambrian sea over a drowning coast, and hold the canyon's richest Cambrian fossils — trilobites and brachiopods by the score. Where a broad green terrace opens halfway down the canyon, you are on the Bright Angel.
  • Lodore Formationformation
    Lodore Formation is the Cambrian floor of the Uinta canyons — a white-to-red quartzitic sandstone that rests directly on the ancient Uinta Mountain Group across the Great Unconformity, the leading edge of the first sea to wash over this deep basement. It gave its name to the Gates of Lodore, where the Green River abandons open country and plunges into the heart of the range. Above it, most of the Paleozoic is simply gone: the Ordovician, Silurian, and Devonian are missing, so Mississippian limestone lies almost directly on these Cambrian beds. Where a hard white-and-red sandstone forms the lower walls at the Gates of Lodore, you are on the Lodore.
  • Muav Limestoneformation
    Muav Limestone is the gray Cambrian ceiling of the Tonto Group — a mottled, thin-bedded limestone and dolomite that caps the broad green bench of the inner Grand Canyon and records the deepest reach of the first great sea to flood the continent. Above the recessive Bright Angel slope, its cliffs and ledges mark where the Cambrian shoreline had drowned the land completely, laid down in repeated pulses of a westward-deepening shelf. Over it lies one of the canyon's great gaps — the whole Ordovician and Silurian missing — before the Devonian Temple Butte and Mississippian Redwall. Where a gray limestone cliff tops the green Tonto bench, you are on the Muav.
  • Tapeats Sandstoneformation
    Tapeats Sandstone is the great floor of the Paleozoic Grand Canyon — the brown, cliff-forming Cambrian sandstone that rests directly on the ancient basement across the Great Unconformity, a single bedding plane that skips more than a billion years of missing time. Coarse and cross-bedded, riddled with the vertical burrows of the first shelly animals, it marks the leading edge of the sea that finally drowned the old Precambrian land. Where its cliff meets the dark rock of the Inner Gorge you are standing on one of the most famous gaps in the geologic record. Where a brown sandstone cliff caps the black basement, you are on the Tapeats.
  • Tonto Groupgroup
    The Tonto Group is the base of the Grand Canyon's great Paleozoic staircase — the Cambrian trio of brown Tapeats cliff, green Bright Angel platform, and gray Muav cliff that records the first sea to bury the ancient basement. Deposited as the Cambrian shoreline marched eastward across a drowning continent (the Sauk transgression), it rests on the Great Unconformity — the mile-deep gap where Paleozoic sediment lies directly on billion-year-old rock. Its middle shale erodes into the broad Tonto Platform, the terrace that carries the canyon's trails. Where sandstone, shale, and limestone stack in a cliff-bench-cliff sequence above the Inner Gorge, you are in the Tonto Group.

Devonian

  • Temple Butte Formationformation
    Temple Butte Formation is the Grand Canyon's missing chapter made visible — a thin, easily overlooked Devonian dolomite that, in the eastern canyon, survives only as purple lenses filling ancient channels cut into the Muav below. It marks more than a hundred million years of missing time, the whole span between the Cambrian and the Mississippian, preserved by accident where old tidal channels caught its sediment. In the western canyon it thickens into a continuous ledge that merges with the Redwall above. It holds the armored plates of some of the earliest fish. Where purple dolomite lenses lie in channels atop the gray Muav, you are looking at the Temple Butte.

Mississippian

  • Madison Limestoneformation
    Madison Limestone is the gray Mississippian cliff of the Uinta canyons — a massive, karst-prone carbonate that in Dinosaur country rests, astonishingly, almost directly on Cambrian sandstone, with the whole Ordovician, Silurian, and Devonian missing beneath it. Laid down in a warm, shallow tropical sea that swept back over the craton, it is packed with crinoids and corals and honeycombed with caves. It builds the tilted gray walls of Split Mountain, where the Green River saws across the upturned edge of the uplift. Where a gray, fossil-rich limestone cliff dips steeply at Split Mountain, you are on the Madison.
  • Redwall Limestoneformation
    Redwall Limestone is the great red wall of the Grand Canyon — the sheer, 400-to-800-foot cliff of massive gray limestone, stained blood-red by iron bleeding down from the beds above, that forms the single most formidable barrier in the canyon's mid-section. Laid down in a warm, clear tropical sea, it is honeycombed with caves and karst where ancient groundwater dissolved it, and packed with the crinoids and corals of a Mississippian sea floor. Its unbroken cliff is the reason so few routes reach the river. Where a towering red cliff bars the way down, you are at the Redwall.
  • Surprise Canyon Formationformation
    Surprise Canyon Formation is the Grand Canyon's hidden river system — a discontinuous Late Mississippian unit that exists only where ancient rivers and caves carved deep into the top of the Redwall, then filled with gravel, sea mud, and estuary sand. It is not a layer you can trace along the wall; it appears as isolated, lens-shaped bodies, some of them inside old Redwall caverns, mapping out a buried dendritic drainage across the canyon. Diverse fossils — even fossil logs and shark teeth — pack its lenses. Where an odd purplish pocket fills a channel or cave in the Redwall top, you may have found the Surprise Canyon.

Pennsylvanian

  • Hermosa Groupgroup
    The Hermosa Group is the Pennsylvanian marine foundation of canyon country — a thick three-part package of cyclic sea-floor limestone, buried salt, and gray shale that underlies the red rock everyone comes to see. Deposited in the Paradox Basin beside the rising Ancestral Rockies, it stacks the fossil-rich Honaker Trail limestone over the great Paradox evaporite over the basal Pinkerton Trail limestone. Its salt still moves, shaping the country above; its top limestones wall the lower Cataract Canyon; and where the sea's evaporites are absent along the basin margin, the whole group collapses into a single undivided Hermosa Formation. It is the oldest chapter of the story a Cataract trip descends into.
  • Honaker Trail Formationformation
    The Honaker Trail Formation is the gray, ledge-and-slope limestone that walls the lower Cataract Canyon and coils through the Goosenecks of the San Juan — the fossil-rich top of the Pennsylvanian Hermosa Group. Laid down as a warm, shallow sea advanced and withdrew in cyclic pulses across the Paradox Basin beside the rising Ancestral Rockies, it stacks fossiliferous marine limestone against sandstone and siltstone in rhythmic bands. Its stepped gray benches are the bedrock a Cataract trip runs against below the red rims, and its fusulinid-bearing limestones are what geologists date the whole sequence by. Where the San Juan entrenches tight meanders into gray marine rock, you are in the Honaker Trail.
  • Molas Formationformation
    The Molas Formation is a thin, blood-red seam at the very base of the Pennsylvanian rocks of the Four Corners — a fossil landscape turned to stone. It is a terra rossa: the rusty residual soil, chert rubble, cave-fill, and wind-blown silt (loess) left behind when a whole tropical limestone plain, the Mississippian Leadville Limestone, sat exposed and slowly dissolved over tens of millions of years. The red marks a major gap in time before the Ancestral Rockies began shedding sediment into the rising Paradox Basin. It grades up into the Pinkerton Trail Formation at the base of the Hermosa Group. Where a thin red band of soil and chert rubble caps a buried limestone surface, you are on the Molas.
  • Morgan Formationformation
    Morgan Formation is the banded ledge between the Uinta canyons' great cliffs — a Pennsylvanian alternation of gray marine limestone and red-and-white sandstone that steps back between the massive Weber cliff above and the Mississippian carbonates below. Laid down in a shallow sea that rose and fell in cycles across the ancestral Uinta shelf, it mixes sea-floor limestone with river and dune sand, ledge on ledge. Where a striped limestone-and-sandstone bench separates the Weber cliff from the gray Madison, you are on the Morgan.
  • Paradox Formationformation
    The Paradox Formation is the buried salt that shapes canyon country from below — a Pennsylvanian evaporite of halite, gypsum, and black shale, thousands of feet thick, laid down as a restricted arm of the sea evaporated again and again in the sinking Paradox Basin. Salt flows: over millions of years it has risen into anticlines, collapsed as groundwater dissolved it, and warped the rock above into the domes, grabens, and fins of Canyonlands. Along the Colorado it is the oldest rock the river reaches, exposed in contorted gray-and-white pockets at the head of Cataract Canyon — and its slow tectonics are what steepen Cataract into the Colorado Plateau's most anomalous, fast-dropping reach.
  • Pinkerton Trail Formationformation
    The Pinkerton Trail Formation is the quiet limestone floor of the Hermosa Group — the first marine bench laid down as Pennsylvanian seas flooded north over the red Molas soil at the base of the sequence. Dark-gray, crinoid- and fusulinid-bearing, thin and wedge-shaped, it thickens toward the basin and pinches out toward the shelf. In canyon country it is mostly a subsurface unit: along the Colorado the river bottoms out in the salt of the overlying Paradox, so the Pinkerton Trail stays hidden below. Its best face is far to the east, on the trail near Durango that gave it its name.

Permian

  • Cedar Mesa Sandstoneformation
    The Cedar Mesa Sandstone is the pale, cross-bedded dune rock that stands up as the spires and fins of the Needles — a Permian erg frozen mid-stride at the eastern edge of a great sand sea. White to banded red-and-tan, it weathers into slickrock domes, joint-controlled towers, and the improbable balanced needles that name Canyonlands' Needles District. Deposited as wind-blown dunes alternated with interdune flats and thin desert soils, it intertongues with the red river beds around it — Organ Rock above, the marine Elephant Canyon below. Where the desert stands in banded stone spires, you are on the Cedar Mesa.
  • Coconino Sandstoneformation
    Coconino Sandstone is the great white cliff of the Grand Canyon — a bold band of pale, cross-bedded sandstone, high on the canyon wall, that was once a Sahara-scale desert of wind-blown dunes at the edge of the Permian world. Its sweeping cross-beds are frozen dune faces; its surfaces preserve the trackways of reptiles and arthropods that crossed the sand, with not a single sea creature among them. Well below the rim, its clean white face is one of the canyon's most recognizable lines. Where a sheer white sandstone cliff bands the upper canyon, you are on the Coconino.
  • Cutler Groupgroup
    The Cutler Group is the great Permian red-bed wedge of canyon country — the pile of sandstone, siltstone, and dune rock that erosion shed off the ancestral Uncompahgre mountains and spread across the Colorado Plateau. Near its mountain source it is a single coarse, arkosic red mass — the fluted dark cliffs of Fisher Towers and Professor Valley, mapped as the undivided Cutler Formation. Basinward, in Canyonlands, it fans out into the named formations everyone knows: the marine Elephant Canyon, the spire-forming Cedar Mesa, the red Organ Rock, and the white bench of the White Rim. Divided or undivided, it is the Permian foundation the Mesozoic cliffs stand on.
  • Hermit Formationformation
    Hermit Formation is the red slope beneath the white Coconino cliff — a soft, iron-stained Permian red bed of fine sandstone and siltstone that erodes back into a recessive shelf, marking a low coastal plain of sluggish rivers and drifting dust. Despite the old name 'Hermit Shale,' it holds little true shale; what it holds are fossil ferns and conifers and the tracks of animals that walked its mudflats. Deep mudcracks at its top, filled by the sand of the Coconino above, record the plain drying out as the great dune sea advanced. Where a red slope undercuts the white cliff high in the canyon, you are on the Hermit.
  • Kaibab Limestoneformation
    Kaibab Limestone is the pale gray rim of the Grand Canyon — the cherty, sandy limestone that caps both canyon rims and stretches for miles across the surrounding plateau as the youngest Paleozoic layer, laid down in a warm, shallow Permian sea. Studded with chert nodules weathered from ancient sponges and full of the shells of a vanished sea floor, it forms the flat, forested benchland a visitor stands on to look down into the abyss. Above it, across a great gap, lie the red Triassic beds; below, the white Coconino cliff. Where a gray, chert-flecked limestone caps the rim, you are on the Kaibab.
  • Organ Rock Formationformation
    The Organ Rock Formation is the dark-red slope beneath the White Rim — a Permian red bed of thin siltstone and fine sandstone shed off the ancestral Uncompahgre highland by braided rivers. Recessive and soft, it usually forms the deep-red talus and benches below the resistant White Rim caprock; but where it is indurated and White-Rim-capped, it stands up as the improbable pinnacles of the Land of Standing Rocks and the Chocolate Drops. Above the pale Cedar Mesa and below the white bench, its red band is one of the clearest color lines in canyon country.
  • Park City Formationformation
    Park City Formation is the Permian cap of the Uinta canyons' Paleozoic — a light-gray, cherty, fossil-rich limestone and sandstone that tops the great Weber cliff and marks the last shallow sea before the red Triassic beds. Equivalent to the phosphate-rich Phosphoria Formation to the west, it records tidal flats and a carbonate shelf grading offshore into deeper water. Above it the Paleozoic ends and the Mesozoic begins across an unconformity. Where a gray, cherty, fossiliferous limestone caps the Weber sandstone, you are on the Park City.
  • Toroweap Formationformation
    Toroweap Formation is the middle band of the Grand Canyon's Permian rim rock — a mixed unit of gray limestone, red sandstone, and white gypsum sandwiched between the pale Coconino cliff below and the Kaibab rim above, recording a shallow sea that washed repeatedly across an evaporating coast. It changes dramatically across the canyon: carbonate and gypsum in the west, sandstone in the east, its three members thinning and merging toward Marble Canyon. Its soft, gypsum-rich Woods Ranch Member weathers into a distinct slope just below the rim. Where a slope of red and gray beds separates the white Coconino from the gray Kaibab, you are on the Toroweap.
  • White Rim Sandstoneformation
    The White Rim Sandstone is the pale, resistant bench that gives the Island in the Sky its middle rim — a thin, hard cap of clean Permian quartz sand, bleached white by ancient hydrocarbon-rich fluids, that armors the soft red Organ Rock below. It is the youngest of the Cutler units and the surface the White Rim Road rides for a hundred miles around the mesa. Deposited as coastal dunes and sabkha flats at the edge of an Early Permian sea, its white sandstone is capped in turn by an unconformity — a long gap in the record — beneath the Triassic Moenkopi. Where a white bench holds up the red country between the rivers, you are on the White Rim.

Triassic

  • Chinle Formationformation
    The Chinle Formation is the soft, colorful floor beneath canyon country's great cliffs — a band of purple, gray, and red-brown badlands that slumps below the sheer Wingate wall. Laid down by sluggish Late Triassic rivers, lakes, and floodplains across a low tropical landscape, its bentonitic clays soak up water and creep, undermining the harder sandstone above and helping build the cliff-and-bench architecture of the Colorado Plateau. It is the plateau's great fossil archive — petrified logs, early dinosaurs, and armored reptiles — and its basal Shinarump conglomerate marks a surface scoured into the older Moenkopi. Wherever a Wingate cliff rises out of slumping rainbow slopes, the Chinle is doing the quiet work below.
  • Moenkopi Formationformation
    The Moenkopi Formation is the chocolate-red apron beneath the Chinle — a Triassic red bed of thin, ripple-marked siltstone and mudstone that records low, muddy coastal flats spreading across the Colorado Plateau after the Permian sea withdrew. Soft and slope-forming, it stripes the country in deep red-brown bands, threaded with marine limestone tongues (the Sinbad, Virgin, and Timpoweap) that mark brief returns of the sea, and printed with reptile trackways. It rests on the Permian below across a long pre-Triassic gap and is scoured above by the Chinle's Shinarump. Where thin chocolate-red slopes rise below the colorful Chinle badlands, you are on the Moenkopi.

Jurassic

  • Carmel Formationformation
    The Carmel Formation is the marine hinge at the base of the San Rafael Group — the band of pale limestone, red mudstone, and white gypsum that records a Middle Jurassic sea flooding back over the great Navajo dune field. It rests on the Navajo across a long erosional gap (the J-2 unconformity) and grades up into the Entrada, its oolitic limestones and contorted gypsum marking shallow, restricted, tidal seas. At Capitol Reef it caps domes as the yellow 'Golden Throne'; across the San Rafael Swell it is the soft, banded floor beneath the Entrada cliffs. Where a red-and-white marine band separates slickrock below from sandstone fins above, you are on the Carmel.
  • Curtis Formationformation
    The Curtis Formation is the green marine ribbon in the San Rafael Group — a distinctive glauconite-tinted, cross-bedded sandstone that records a brief return of the Jurassic sea across the Entrada's drying dunes. Its resistant, greenish-gray ledges cap the goblins of Goblin Valley and ledge out across the San Rafael Swell, sharply truncating the red Entrada below along the J-3 unconformity. Marine fossils and the green iron mineral glauconite mark it unmistakably as sea-laid. Where a hard green-gray sandstone ledge cuts across red Entrada, you are on the Curtis.
  • Entrada Sandstoneformation
    The Entrada Sandstone is the fin-and-arch rock of canyon country — the Middle Jurassic sandstone whose smooth, salmon-and-white walls erode into the slender fins and spans of Arches National Park. Deposited as coastal dunes, tidal flats, and sabkhas along the edge of a shallow Jurassic sea, it hardened into a cliff-forming middle (the Slick Rock) sandwiched between softer, earthier beds that weather back and undercut it — the recipe that makes arches. It sits on the marine Carmel and is truncated above by the glauconite-green Curtis. Where sandstone stands in freestanding fins and arches, you are on the Entrada.
  • Kayenta Formationformation
    The Kayenta Formation is the ledge-maker of the Glen Canyon Group — a band of dark red, maroon, and lavender river sandstones that steps back between two great cliffs: the sheer Wingate below and the pale Navajo slickrock above. Deposited by shifting Early Jurassic streams crossing an arid plain, its harder, thin-bedded channel sandstones armor and protect the Wingate, which is why the two so often stand together across canyon country. Its slabs record abundant dinosaur trackways, and near its top it interfingers with the encroaching Navajo dune sea — the desert slowly burying the rivers. Spot the ledgy maroon stair-step between two massive walls and you are looking at the Kayenta.
  • Morrison Formationformation
    The Morrison Formation is the rainbow-banded dinosaur graveyard of the Colorado Plateau — the Late Jurassic floodplain of gray, green, and maroon mudstone, threaded with river sandstones, that holds the richest trove of Jurassic dinosaurs on Earth. Its soft bentonitic clays (volcanic ash weathered to popcorn-textured mud) erode into low, banded badlands, while its channel sandstones (Salt Wash, Brushy Basin) stand out as ledges and host the bones. This is the rock of the Dinosaur National Monument quarry — Allosaurus, Stegosaurus, Apatosaurus, Diplodocus. It rests on the San Rafael Group and is capped, across a long gap, by the Cretaceous Cedar Mountain and Dakota. Where soft green-and-maroon badlands hold dinosaur bone, you are on the Morrison.
  • Navajo Sandstoneformation
    The Navajo Sandstone is the great pale dome-rock of the Colorado Plateau — the white-to-salmon slickrock that swells into the beehives and big walls of Zion and the bald domes above Glen Canyon. It is a fossil desert: one of the largest sand seas the planet has known, its wind-driven dunes frozen mid-migration as sweeping cross-beds tens of feet tall. Well-sorted, frosted quartz sand cemented into cliff and dome, it bleaches white where groundwater has stripped its iron and flushes red where the iron remains — a color story written across canyon country. Where the rock rounds into bald, cross-bedded slickrock and climbs into sheer light walls, you are on the Navajo.
  • San Rafael Groupgroup
    The San Rafael Group is the Middle Jurassic middle story of canyon country — the four-formation package of marine limestone, dune-and-tidal sandstone, green sea-sand, and striped mudflat that sits between the pale Navajo slickrock below and the dinosaur-bearing Morrison above. Named for the great San Rafael Swell where its whole stack stands exposed, it records a Jurassic seaway advancing and retreating across the plateau again and again: the marine Carmel, the eolian-and-tidal Entrada that builds the arches, the glauconite-green Curtis, and the tidal-flat Summerville. It rests on the Glen Canyon Group across the J-2 unconformity and is capped by the J-5 gap beneath the Morrison. Where the country stacks arch-rock over red-and-white marine bands, you are in the San Rafael Group.
  • Summerville Formationformation
    The Summerville Formation is the thin, rhythmic tidal-flat unit that caps the San Rafael Group — a chocolate-brown alternation of gypsiferous mudstone and pale sandstone that records a Middle Jurassic coast drying out for the last time before the Morrison rivers arrived. Its evenly banded, thin beds weather into striped badland slopes across the San Rafael Swell and Cathedral Valley, where thin resistant sandstones stand out as ledges and seams. It rests on the marine Curtis and is beveled off above by the sub-Morrison (J-5) unconformity. Where thin chocolate-and-tan bands stripe a low slope below the Morrison, you are on the Summerville.

Cretaceous

  • Cedar Mountain Formationformation
    The Cedar Mountain Formation is the drab, easily-overlooked bench that hides the Colorado Plateau's Early Cretaceous dinosaurs. Sitting above the rainbow Morrison and below the Dakota, it is a package of gray-green to purple smectitic (swelling-clay) mudstone that erodes into low badlands, with a distinctive chert-pebble conglomerate — the Buckhorn — at its base that marks tens of millions of years of missing time. Deposited on a low, seasonally-dry plain draining east off the rising Sevier mountains, it holds the world of Utahraptor and the armored Gastonia. Where a nondescript greenish-gray slope sits between the Morrison badlands and the cliffs above, you are on the Cedar Mountain.
  • Dakota Sandstoneformation
    The Dakota Sandstone is the resistant Cretaceous cap that rims the Colorado Plateau's high country — a tan, quartz-rich sandstone, coal-bearing and conglomeratic at its base, that marks the first advance of the Western Interior Seaway across a long-eroded Jurassic landscape. Thin but hard, it erodes into small cliffs and hogbacks and holds the country's canyon rims and mesa edges, layered with carbonaceous shale, coal, and some of the earliest flowering-plant leaves. It rests on the Cedar Mountain Formation (or directly on the Morrison) across the sub-Dakota unconformity and grades up into the gray Mancos sea-shale. On the plateau, modern USGS work reassigns much of it to the Naturita Formation, though 'Dakota' remains the name on the map. Where a hard tan ledge caps a gray-shale valley, you are on the Dakota.
  • Mancos Shaleformation
    The Mancos Shale is the great gray sea-floor of the Colorado Plateau — a thick, dark, easily eroded marine shale laid down on the bottom of the Cretaceous Western Interior Seaway as it drowned the continent's middle. Recessive and lead-gray, it weathers into vast popcorn-textured badlands and 'blue desert' valleys, thickening westward toward the seaway's axis and threaded with deltaic sandstone tongues (Ferron, Emery) that prograded from the rising Sevier mountains. It rests on the transgressive Dakota below and grades up into the shoreline sandstones of the Mesaverde. Where a broad gray badland valley opens below the cliffs, you are on the Mancos.
  • Mesaverde Groupgroup
    The Mesaverde Group is the great striped wall of the Book Cliffs — the stacked sandstone, gray shale, and black coal of a Late Cretaceous coastline turned on edge. As the Western Interior Seaway pulled back to the east, rivers and deltas off the rising Sevier mountains built beaches, lagoons, and coal swamps that piled into a thick, eastward-marching wedge: cliff-forming shoreface sandstones (Star Point, Castlegate) over coal-bearing coastal-plain shales (Blackhawk, Neslen). It grades up out of the marine Mancos Shale and forms the rim of Desolation and Gray Canyons. Where a long banded cliff of sandstone ledges and black coal seams caps the gray Mancos badlands, you are looking at the Mesaverde.

Paleogene

  • Green River Formationformation
    Green River Formation is the drowned world of the Uinta Basin — pale, finely layered lake beds of marlstone, limestone, and oil shale that record Lake Uinta, one of a chain of great Eocene lakes that stood here for millions of years. Its thin, even laminations are seasonal layers of the lake floor, and among them lie some of the finest fossil fish, insects, and leaves on Earth. It also holds the richest oil-shale deposits in the world. In Desolation and Gray canyons the Green River beds form the pale, ledgy middle walls above the red Wasatch. Where the canyon walls turn to fine, even-bedded buff-and-gray rock full of fossils, you are on the Green River.
  • Uinta Formationformation
    Uinta Formation is the rim of the basin — drab and reddish river beds that cap the Uinta Basin Tertiary, laid down by streams that rebuilt floodplains over the shrinking Eocene lake. It is the type unit of the Uintan age of the mammal timescale, and its badlands have yielded a classic Eocene fauna, including the huge, blunt-horned Uintatherium. It forms the highest walls and the rim above Desolation Canyon, the last chapter of the basin fill before the country was buried and then re-excavated. Where the drab upper walls of Desolation Canyon rise to the rim, you are on the Uinta Formation.
  • Wasatch Formationformation
    Wasatch Formation is the variegated floor of the Tertiary in the Uinta Basin — banded red, purple, and gray river mud and sand that forms the lower badland slopes of Desolation Canyon, laid down by streams on a broad floodplain before the great Eocene lake spread over them. In the basin its red-bed tongue is mapped as the Colton Formation, and it climbs and intertongues upward into the pale Green River lake beds. It rests on the eroded top of the Cretaceous section at the base of the Tertiary. Where the lower walls of Desolation Canyon step up in bands of red and gray mudstone, you are on the Wasatch.

Neogene

  • Browns Park Formationformation
    Browns Park Formation is the soft blanket that once buried the Uintas — loosely cemented, tuff-rich Miocene sand and gravel that filled the valleys and lapped high onto the range after it had been worn down, then was itself partly stripped away as the rivers cut back in. It rests on the beveled edge of the tilted Precambrian basement across a sharp angular unconformity, the record of a landscape twice made. It gives Browns Park its name and its open, rounded country, just above the point where the Green River gathers itself and plunges into the Gates of Lodore. Where loose, pale tuffaceous sandstone forms rounded slopes above the Gates of Lodore, you are on the Browns Park.

Period not yet assigned

Units whose single-period assignment is still unsourced.

  • Elephant Canyon Formationformation
    The Elephant Canyon Formation is the fossil-bearing marine floor at the head of Cataract Canyon — a cyclic stack of gray limestone and sandstone that records the last Pennsylvanian seas giving way to the first Permian ones. Named for the canyon near the Green–Colorado confluence, it sits on the gray Honaker Trail limestone and grades up into the eolian Cedar Mesa, its fusulinids straddling the Pennsylvanian–Permian boundary. Its very name is contested — some geologists fold it into the informal 'lower Cutler beds' — but its marine fossils make it the one clearly datable unit low in the inner gorge. Where the river cuts gray marine rock at the top of Cataract, you are in the Elephant Canyon.
  • Glen Canyon Groupgroup
    The Glen Canyon Group is the great cliff-and-slickrock package of canyon country — the Triassic-to-Jurassic units that build the plateau's most recognizable walls, stacked from the sheer red Wingate at the base, through the ledgy Kayenta, to the pale, dome-forming Navajo at the top (with the Moenave standing in for the Wingate in the southwest). Deposited as the region dried from river floodplains into one of Earth's largest dune seas, its members share a common story of an aridifying Early Mesozoic landscape. Where a Wingate cliff, a Kayenta ledge, and a Navajo dome stack in sequence, you are reading the Glen Canyon Group.
  • Grand Canyon Supergroupsupergroup
    The Grand Canyon Supergroup is the tilted, buried underworld of the canyon — a mile-and-a-half-thick pile of Precambrian sandstone, shale, dolomite, and lava that predates the Paleozoic layer cake by hundreds of millions of years. Deposited in seas and basins during the assembly and breakup of the supercontinent Rodinia, it was then faulted, tilted ten to fifteen degrees, and eroded nearly flat — so it survives only in down-dropped fault blocks, appearing as slanting, colorful wedges beneath the horizontal Tapeats. It is the reason the Great Unconformity is sometimes an angular one: where a Supergroup block was preserved, the flat Cambrian rock rests on tilted Precambrian strata. Where colorful slanting beds appear beneath the flat-lying walls, you are looking at the Grand Canyon Supergroup.
  • Moenave Formationformation
    The Moenave Formation is the red-slope-and-cliff couplet at the base of the Glen Canyon Group in southwestern Utah — the earliest-Jurassic river-and-lake beds that stand in for the Wingate cliff where the Wingate itself thins out. Its lower Dinosaur Canyon Member weathers into steep reddish hillsides; its upper Springdale Member forms a blocky sandstone cliff. Laid down by streams, ponds, and the shallow 'Lake Dixie' at the very dawn of the Jurassic, it preserves some of the region's richest early-Jurassic dinosaur tracksites and the little crocodile-cousin Protosuchus. It rests on the Chinle across a ten-million-year gap and grades up into the Kayenta. Around Zion and St. George, it is the red foundation the big walls rise from.
  • Nugget Sandstoneformation
    Nugget Sandstone is the great slickrock of the Dinosaur flanks — a pale, cross-bedded eolian sandstone that is the northern cousin of the Colorado Plateau's Navajo, the frozen dunes of the same Early Jurassic desert carried up into Uinta country. It arches over the tilted Mesozoic beds around Island Park and Rainbow Park, where the Green River leaves the Precambrian core and swings out through younger rock. Between the red Chinle below and the Carmel above, it is a clean band of wind-blown sand turned to stone. Where smooth, cross-bedded pale sandstone rolls across the Dinosaur flanks, you are on the Nugget.
  • Supai Groupgroup
    The Supai Group is the great red terrace of the middle Grand Canyon — a thick, cyclic stack of red-bed sandstone, siltstone, and limestone that steps back from the inner gorge as the broad Esplanade Platform, recording a coastal plain that oscillated between desert dunes and shallow sea again and again. Four stacked formations, each a lower cliff over an upper slope, ring the canyon in ledgy red bands between the Redwall cliff below and the Hermit slope above. Marine to the west, terrestrial to the east, it captures a whole world shifting with sea level. Where broad red ledges and the Esplanade bench wrap the middle canyon, you are in the Supai.
  • Uinta Mountain Groupgroup
    The Uinta Mountain Group is the deep red heart of the Uintas — a staggering pile of Neoproterozoic quartzite, four to seven miles thick, that makes up the core of the only major mountain range in the Lower 48 that runs east-west. Deposited as sand, gravel, and mud in a fault-bounded basin as the supercontinent Rodinia began to break apart nearly 750 million years ago, it was later inverted and arched into the Uinta uplift. Its hard red-and-white quartzite forms the range's high peaks and the sheer walls of the Gates of Lodore, where the Green River drives straight into the mountain. Where the river cuts red quartzite at the bottom of a mile-deep canyon, you are in the Uinta Mountain Group.
  • Uncompahgre Uplift Basementcomplex
    The Uncompahgre Uplift Basement is the black heart of the Colorado River's canyons on the Utah–Colorado line — a Paleoproterozoic mass of dark gneiss, schist, and amphibolite, cut by granite and pegmatite, that the river exposes in erosional windows through the Uncompahgre uplift. Roughly 1.7 billion years old, it is the oldest rock on the run: it forms the polished black inner gorge of Westwater Canyon and the Black Rocks of Ruby Canyon, where hard crystalline rock refuses to erode and squeezes the river into steep bedrock rapids. Triassic beds rest directly on its beveled surface across the Great Unconformity, a gap of roughly 1.5 billion years. It is correlative in age with the Grand Canyon's Vishnu Schist and Zoroaster Granite, but a separate, informally unnamed body. Where the river runs beneath dark, polished crystalline walls, you are on the basement.
  • Vishnu Schistcomplex
    Vishnu Schist is the ancient crystalline floor of the Grand Canyon — the dark, contorted metamorphic rock of the Inner Gorge, nearly 1.8 billion years old, that forms the steep black walls the Colorado River carves through at the bottom of the canyon. Once submarine sediments and volcanic rock, it was buried deep, cooked and crumpled during the collision of island arcs onto early North America, and shot through with pale granite. It is the crystalline basement beneath the Great Unconformity — the oldest rock the river touches, and the same kind of rock exposed a day's drive north at Westwater. Where the canyon narrows into steep, dark, polished walls at river level, you are in the Vishnu.
  • Weber Sandstoneformation
    Weber Sandstone is the great cross-bedded cliff of the Uinta canyons — a thousand feet of pale, wind-blown Pennsylvanian-Permian sand, hardened locally to quartzite, that forms the towering walls of Split Mountain and Whirlpool Canyon where the Green River cuts clean through the uplift. A frozen dune sea with ponded carbonates in the low spots between dunes, it is the principal cliff-former of Dinosaur country, printed here and there with the tracks of early land animals. Where the river runs beneath sheer, sweeping cross-bedded walls in the heart of the Uintas, you are on the Weber.
  • Wingate Sandstoneformation
    The Wingate Sandstone is the defining cliff-former of the Colorado Plateau — the sheer red wall that runs unbroken for hundreds of miles across canyon country. Deposited by enormous wind-blown dune fields around the Triassic–Jurassic transition, the Wingate hardened into a uniform fine-grained sandstone that fractures vertically and weathers into the iconic vertical faces seen at Capitol Reef, Canyonlands, the San Rafael Swell, and Colorado National Monument. It almost always sits on the slope-forming Chinle Formation below and is capped by the protective Kayenta Formation above — a stratigraphic sandwich that has preserved Wingate cliffs across most of the plateau.
  • Zoroaster Granitesuite
    Zoroaster Granite is the pale fire in the dark basement — the light-colored granite and pegmatite that shot up through the Vishnu Schist as molten rock nearly 1.7 billion years ago, freezing into dikes and plutons that cut bright veins across the black walls of the Inner Gorge. Born of the same arc collisions that cooked the Vishnu, its cross-cutting sheets are a textbook of relative dating: the granite must be younger than the schist it slices through. Modern work groups these intrusions as the Zoroaster Plutonic Complex. Where pink-and-white bands zigzag across the dark canyon-bottom rock, you are looking at the Zoroaster.